Q18. What is the frequency range of magnetron oscillators? Q19. What two forms of instability are common in magnetrons? Q20. What is the effect on magnetron operation if the magnetic field strength is too high? Q21. What is the typical frequency range about the center frequency of a tunable magnetron?
POWER-AMPLIFIER TRANSMITTERS are used in many recently developed radar sets. This type of transmitter was developed because of the need for more stable operation of the moving target indicator (mti). In a magnetron transmitting system, the high-power magnetron oscillator has a tendency to drift in frequency because of temperature variations, changes in the modulating pulse, and various other effects.
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Frequency drift is compensated for, in part, by the use of automatic frequency control (afc) circuits designed to control the frequency of the local oscillator in the receiver system. This, however, does not completely eliminate the undesirable effects of frequency drift on mti operation.
The power-amplifier transmitter system does the same thing as the keyed-oscillator transmitter but with fewer stability problems. It generates, shapes, and amplifies pulses of rf energy for transmission.
Figure 2-10 is a block diagram of a typical power-amplifier transmitter system. In this transmitter system a multicavity klystron tube amplifies lower-powered rf pulses that have been generated and shaped in other stages. CROSSED-FIELD AMPLIFIERS (AMPLITRONS) are used in radar systems with a wide band of transmitted frequencies because they are stable over a wider frequency range. A crossed-field amplifier transmitter is discussed later in this section.
In figure 2-10, the power-amplifier chain input signals are generated by heterodyning (mixing) two frequencies. That is, two different frequencies are fed to a mixer stage (mixer amplifier) and the resultant, either the sum or difference frequency, may be selected as the output. (The operation of mixer circuits is explained in more detail in the section on receivers.) The low-power pulse is then amplified by intermediate power amplifier stages and applied to the klystron power-amplifier. The klystron power- amplifier concentrates the rf output energy into a very narrow frequency spectrum. This concentration makes the system more sensitive to smaller targets. In addition the detection range of all targets is increased.
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To examine the operation of the transmitter, we will trace the signal through the entire circuit. The local oscillator shown at the left of figure 2-10 is a very stable rf oscillator that produces two cw rf outputs. As shown, the cw output is sent to the receiver system; the cw output is also one of the two rf signals fed to the mixer amplifier by way of the two BUFFER AMPLIFIER STAGES. The buffer amplifiers raise the power level of the signal and also isolate the local oscillator.
The COHERENT OSCILLATOR (COHO) is triggered by the system trigger and produces as its output an rf pulse. This rf pulse is fed directly to the mixer amplifier.
The mixer-amplifier stage receives three signals: the coherent rf pulse, the local oscillator cw rf signal, and a dc modulating pulse from the low-voltage modulator. The coherent and local oscillator signals are mixed to produce sum and difference frequency signals. Either of these may be selected as the output. The modulator pulse serves the same purpose as in the keyed-oscillator transmitter, because it determines the pulse width and power level. The mixer stage functions only during the modulator pulse time. Thus the mixer amplifier produces an output of rf pulses in which the frequency may be either the sum or difference of the coherent and local oscillator signals.
The mixer-amplifier feeds the pulses of rf energy to an intermediate power amplifier. This amplifier stage is similar to the buffer-amplifier stage except that it is a pulsed amplifier. That is, the pulsed amplifier has operating power only during the time the modulator pulse from the low-voltage modulator is applied to the stage. The amplified output signal is fed to a second intermediate power amplifier that operates in the same manner as the first.
From the second intermediate power amplifier, the signal is fed to the KLYSTRON POWER AMPLIFIER. This stage is a multicavity power klystron. The input rf signal is used as the exciter signal for the first cavity. High-voltage modulating pulses from the high-voltage modulator are also applied to the klystron power amplifier. These high-voltage modulating pulses are stepped up across a pulse transformer before being applied to the klystron. All cavities of the klystron are tunable and are tuned for maximum output at the desired frequency.
Provisions are made in this type of transmitter to adjust the starting time of the modulating pulses applied to the coherent oscillator, mixer amplifier, intermediate power amplifiers, and klystron power- amplifier. By this means the various modulator pulses are made to occur at the same time.
This transmitter produces output rf pulses of constant power and minimum frequency modulation and ensures good performance.
Q22. What is the primary advantage of power-amplifier transmitters over keyed-oscillator transmitters?
Q23. In the power amplifier shown in figure 2-10, what two signals are mixed to produce the output signal?
Q24. What type of klystron is used as the final stage of a power-amplifier transmitter? Figure 2-11 is a block diagram of a power-amplifier transmitter that uses a FREQUENCY
SYNTHESIZER to produce the transmitted frequency rather than the heterodyning mixer. The frequency synthesizer allows the transmitter to radiate a large number of discrete frequencies over a relatively wide band. Such a system is commonly used with frequency-scan search radars that must transmit many different frequencies to achieve elevation coverage and to compensate for the roll and pitch of a ship.
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A typical frequency synthesizer consists of a bank of oscillators producing different fixed frequencies. The outputs of a relatively few fixed oscillators can be mixed in various combinations to produce a wide range of frequencies. In mti systems the selected oscillator frequencies are mixed with a coherent oscillator frequency to provide a stable reference for the mti circuits. The frequency synthesizer also produces the local oscillator signals for the receiver system. Because the transmitted pulse changes frequency on each transmission, the local oscillator signal to the receiver must also change and be included in the transmitted frequency. A system of this type is frequency-programmed by select gates from the synchronizer.
The detailed operation of frequency synthesizers is beyond the scope of this manual but may be found in the technical manuals for most frequency scan radar systems.
The first rf amplifier receives the pulses of the selected frequency from the synthesizer and a modulator pulse (from the first stage modulator) at the same time. The rf pulse is usually slightly wider than the modulator pulse which prevents the amplifier tube from pulsing when no rf energy is present. Most pulsed rf amplifiers will oscillate at an undesired frequency if pulsed without an rf input. The output of the first rf amplifier is an amplified rf pulse that is the same width as the first stage modulator pulse. The second stage modulator is designed to produce a pulse slightly narrower than the first stage modulator pulse; this also prevents the amplifier from pulsing when no rf is present. Therefore, the second stage amplifier receives a modulator pulse a short time after the first stage rf arrives at the input. As shown in figure 2-11, the same procedure is repeated in the third and final stage.
The amplifiers in this type of power-amplifier transmitter must be broad-band microwave amplifiers that amplify the input signals without frequency distortion. Typically, the first stage and the second stage are traveling-wave tubes (twt) and the final stage is a crossed-field amplifier. Recent technological
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advances in the field of solid-state microwave amplifiers have produced solid-state amplifiers with enough output power to be used as the first stage in some systems. Transmitters with more than three stages usually use crossed-field amplifiers in the third and any additional stages. Both traveling-wave tubes and crossed-field amplifiers have a very flat amplification response over a relatively wide frequency range.
Crossed-field amplifiers have another advantage when used as the final stages of a transmitter; that is, the design of the crossed-field amplifier allows rf energy to pass through the tube virtually unaffected when the tube is not pulsed. When no pulse is present, the tube acts as a section of waveguide. Therefore, if less than maximum output power is desired, the final and preceding cross-field amplifier stages can be shut off as needed. This feature also allows a transmitter to operate at reduced power, even when the final crossed-field amplifier is defective.
Q25. What transmitter component allows the radiation of a large number of discrete frequencies over a wide band?
Q26. What is the result of pulsing a pulsed rf amplifier when no rf is present?
Whenever a single antenna is used for both transmitting and receiving, as in a radar system, problems arise. Switching the antenna between the transmit and receive modes presents one problem; ensuring that maximum use is made of the available energy is another. The simplest solution is to use a switch to transfer the antenna connection from the receiver to the transmitter during the transmitted pulse and back to the receiver during the return (echo) pulse. No practical mechanical switches are available that can open and close in a few microseconds. Therefore, ELECTRONIC SWITCHES must be used. Switching systems of this type are called DUPLEXERS.
In selecting a switch for this task, you must remember that protection of the receiver input circuit is as important as are output power considerations. At frequencies where amplifiers may be used, amplifier tubes can be chosen to withstand large input powers without damage. However, the input circuit of the receiver is easily damaged by large applied signals and must be carefully protected.
An effective radar duplexing system must meet the following four requirements:
1. During the period of transmission, the switch must connect the antenna to the transmitter and disconnect it from the receiver.
2. The receiver must be thoroughly isolated from the transmitter during the transmission of the high- power pulse to avoid damage to sensitive receiver components.
3. After transmission, the switch must rapidly disconnect the transmitter and connect the receiver to the antenna. For targets close to the radar to be seen, the action of the switch must be extremely rapid.
4. The switch should absorb an absolute minimum of power both during transmission and reception.
Therefore, a radar duplexer is the microwave equivalent of a fast, low-loss, single-pole, double- throw switch. The devices developed for this purpose are similar to spark gaps in which high-current microwave discharges furnish low-impedance paths. A duplexer usually contains two switching tubes
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(spark gaps) connected in a microwave circuit with three terminal transmission lines, one each for the transmitter, receiver, and antenna. As shown in views A and B of figure 2-12, these circuits may be connected in parallel or in series. Both systems will be discussed in detail in this section. One tube is called the TRANSMIT-RECEIVER TUBE, or TR TUBE; the other is called the ANTITRANSMIT- RECEIVE TUBE, or ATR TUBE. The tr tube has the primary function of disconnecting the receiver, and the atr tube of disconnecting the transmitter.
The overall action of the tr and atr circuits depends upon the impedance characteristics of the quarter-wavelength section of transmission line. A quarter-wavelength, or an odd multiple of the quarter- wavelength, transmission line presents opposite impedance values at the ends; one end of the line appears as a short and the other end appears as an open.
The type of spark gap used as a tr tube may vary. It may be one that is simply formed by two electrodes placed across the transmission line; or it may be one enclosed in an evacuated glass envelope with special features to improve operation. The requirements of the spark gap are (1) high impedance prior to the arc and (2) very low impedance during arc time. At the end of the transmitted pulse the arc
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should be extinguished as rapidly as possible. Extinguishing the arc stops any loss caused by the arc and permits signals from nearby targets to reach the receiver.
The simple gap formed in air has a resistance during conduction of from 30 to 50 ohms. This is usually too high for use with any but an open-wire transmission line. The time required for the air surrounding the gap to completely deionize after the pulse voltage has been removed is about 10 microseconds. During this time the gap acts as an increasing resistance across the transmission line to which it is connected. However, in a tr system using an air gap, the echo signals reaching the receiver beyond the gap will be permitted to increase to half their proper magnitude 3 microseconds after the pulse voltage has been removed. This interval is known as RECOVERY TIME.
Tr tubes are usually conventional spark gaps enclosed in partially evacuated, sealed glass envelopes, as shown in figure 2-13. The arc is formed as electrons are conducted through the ionized gas or vapor. You may lower the magnitude of voltage necessary to break down a gap by reducing the pressure of the gas that surrounds the electrodes. Optimum pressure achieves the most efficient tr operation. You can reduce the recovery time, or DEIONIZATION TIME, of the gap by introducing water vapor into the tr tube. A tr tube containing water vapor at a pressure of 1 millimeter of mercury will recover in 0.5 microseconds. It is important for a tr tube to have a short recovery time to reduce the range at which targets near the radar can be detected. If, for example, echo signals reflected from nearby objects return to the radar before the tr tube has recovered, those signals will be unable to enter the receiver.
Tr tubes used at microwave frequencies are built to fit into, and become a part of, a resonant cavity. You may increase the speed with which the gap breaks down after the transmitter fires by placing a voltage across the gap electrodes. This potential is known as KEEP-ALIVE VOLTAGE and ranges from 100 volts to 1,000 volts. A glow discharge is maintained between the electrodes. (The term GLOW DISCHARGE refers to the discharge of electricity through a gas-filled electron tube. This is distinguished by a cathode glow and voltage drop much higher than the gas-ionization voltage in the cathode vicinity.) This action provides for rapid ionization when the transmitter pulse arrives.
Failure of the tr tube is primarily caused by two factors. The first and most common cause of failure is the gradual buildup of metal particles that have been dislodged from the electrodes. Such metal bits become spattered on the inside of the glass envelope. These particles act as small, conducting areas and tend to lower the Q of the resonant cavity and dissipate power. If the tube continues in use for too long a
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period in this condition, the particles will form a detuning wall within the cavity and eventually prevent the tube from functioning. A second cause of failure is the absorption of gas within the enclosure by the metal electrodes. This results in a gradual reduction of pressure within the tube to a point where gap breakdown becomes very difficult. The final result is that extremely strong signals (from the transmitter) are coupled to the receiver. Because both types of failures develop gradually, the tr tube periodically must be checked carefully to determine performance level.
Q27. What type of switches are used as duplexers? Q28. What tube in a duplexer has the primary function of disconnecting the receiver? Q29. How may the tr tube ionization speed be increased?
The atr tube is usually a simpler device than a tr tube. An atr tube might use a pure inert gas, such as argon, because recovery time generally is not a vital factor. Furthermore, a priming agent, such as keep- alive voltage, is not needed. The absence of either a chemically active gas or a keep-alive voltage results in atr tubes having longer useful lives than tr tubes.
Tr and atr tubes may contain radioactive material. Handle with care to avoid breakage and possible contamination.
There are two basic tr-atr duplexer configurations. They are the parallel-connected and the series- connected duplexer systems. The following paragraphs describe the operation of both systems.
First, let’s consider a PARALLEL-CONNECTED DUPLEXER system, as shown in figure 2-14. The tr spark gap shown in figure 2-14 is located in the receiver coupling line one-quarter wavelength from the T-junction. A half-wavelength, closed-end section of transmission line, called a STUB, is shunted across the main transmission line. An atr spark gap is located in this line one-quarter wavelength from the main transmission line and one-quarter wavelength from the closed end of the stub. As shown in the figure, antenna impedance, line impedance, and transmitter output impedance, when transmitting, are all equal. The action of the circuit during transmission is shown in figure 2-15.
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During the transmitting pulse, an arc appears across both spark gaps and causes the tr and atr circuits to act as shorted (closed-end) quarter-wave stubs. The circuits then reflect an open circuit to the tr and atr circuit connections to the main transmission line. None of the transmitted energy can pass through these reflected opens into the atr stub or into the receiver. Therefore, all of the transmitted energy is directed to the antenna.
During reception, as shown in figure 2-16, the amplitude of the received echo is not sufficient to cause an arc across either spark gap. Under this condition, the atr circuit now acts as a half-wave transmission line terminated in a short-circuit. This is reflected as an open circuit at the receiver T-junction, three-quarter wavelengths away. The received echo sees an open circuit in the direction of the transmitter. However, the receiver input impedance is matched to the transmission line impedance so that the entire received signal will go to the receiver with a minimum amount of loss.
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In the SERIES-CONNECTED DUPLEXER SYSTEM, shown in figure 2-17, the tr spark gap is located one-half wavelength from the receiver T-junction. The atr spark gap is located one-half wavelength from the transmission line and three-quarters wavelength from the receiver T-junction. During transmission, the tr and atr gaps fire in the series-connected duplexer system, as shown in figure 2-18. This causes a short-circuit to be reflected at the series connection to the main transmission line one- half wavelength away. The transmitted pulse "sees" a low impedance path in the direction of the antenna and does not go into the atr stub or the receiver.
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During reception, neither spark gap is fired, as shown in figure 2-19. The atr acts as a half-wave stub terminated in an open. This open is reflected as a short-circuit at the T-junction three-quarters of a wavelength away. Consequently, the received signal sees a low impedance path to the receiver, and none of the received signal is lost in the transmitting circuit.
Duplexers are constructed in many forms, such as RESONANT-CAVITY COAXIAL SYSTEMS, WAVEGUIDE SYSTEMS, and HYBRID RINGS. Since waveguide and hybrid-ring duplexers are most common in radar systems, those will be discussed in this section.
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WAVEGUIDE DUPLEXERS usually consist of tr tubes and atr tubes housed in a resonant cavity and attached to a waveguide system in some manner. Resonant-cavity tr tubes may be applied to waveguides, either directly or indirectly, to obtain switching action. The indirect method uses a coaxial line system, and then couples the coaxial line into the waveguide that feeds the antenna. If large losses are incurred by the use of a coaxial line, the resonant cavity can be coupled directly to the waveguide. Figure 2-20 shows a direct method of cavity tr switching in a waveguide system. The waveguide terminates in the antenna at one end and in a shorting plate at the other. The magnetron uses a voltage probe to excite the waveguide. The transmitted pulse travels up the guide and moves into the tr box through a slot. The cavity builds up a strong electric field across the gap, breaks it down, and detunes the cavity. This action effectively seals the opening and passes the pulse energy to the antenna.
The signals received during the resting time travel down the guide to the magnetron and the shorting end plate where they are reflected. The slot coupling the waveguide to the cavity is located at a point where the standing-wave magnetic field produced by reflections in the waveguide is maximum. The maximum magnetic field, therefore, energizes the cavity. The echo signals are not strong enough to cause an arc, and the cavity field is undisturbed by the gap. Therefore, the cavity field couples rf energy into the receiver coaxial line and provides maximum energy transfer.
The cavity tr switch can also be applied to branch lines of the waveguide, as shown in figure 2-21. The magnetron is coupled to the guide by a voltage probe to produce proper excitation.
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Maximum use of the received signals is ensured by an atr tube. The transmitted pulse travels from the magnetron to the atr branch where part of the energy is diverted into the gap. A slot (S) is placed across the waveguide one-half wavelength from the main guide, and passes the rf energy through it and into the cavity. The cavity builds up the electric field that breaks down the gap, detunes the cavity, and, as a result, effectively closes the slot. One-half wavelength away, this action effectively closes the entrance to the atr branch and limits the amount of energy entering the atr branch to a small value.
Most of the energy is, therefore, directed down the guide to the antenna. Upon reaching the receiver branch, the same effect is produced by the tr tube in the receiver line. Because the energy entering both openings is effectively limited by the gaps, maximum energy is transferred between the magnetron and the antenna.
During the resting time, the atr spark gap is not broken down by the received signals. The received signal sets up standing waves within the cavity that cause it to resonate. At resonance, the low impedance of the atr cavity is reflected as a high impedance at the entrance to the transmitter waveguide (three- quarter wavelength away). This ensures that the maximum received signal will enter the receiver branch. Hybrid Ring Duplexer
The HYBRID RING is used as a duplexer in high-power radar systems. It is very effective in isolating the receiver during transmission. A simplified version of the hybrid-ring duplexer is shown in views A and B of figure 2-22. The operation of the duplexer, in terms of the E field distribution during transmission and reception, is illustrated in views C and D. The H lines, though present, have been omitted to simplify the explanation.
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During transmission the E field from the transmitter enters arm 3 and divides into two fields 180 degrees out of phase. One field moves clockwise around the ring and the other moves counterclockwise. The two fields must be 180 degrees out of phase at the entrance of an arm to propagate any energy down the arm. The field moving clockwise from arm 3 ionizes the tr tube in arm 4, and the energy is blocked from the receiver. The tr tube reflects a high impedance equivalent to an open circuit. This high impedance prevents any energy from entering the receiver - even though the two fields are out of phase at the entrance to arm 4. The field moving counterclockwise from arm 3 ionizes the tr tube in arm 2, which reflects a short circuit back to the ring junction. No energy is sent to the receiver, however, because the fields arriving at arm 2 are in phase. The clockwise and counterclockwise fields arrive at arm 1 out of phase by 180 degrees. They are then propagated through the arm to the antenna.
During reception, the relatively weak field from the antenna enters arm 1 and divides at the junction into two out-of-phase components. Neither field is sufficient to fire the tr tubes in arms 2 and 4; since the fields arrive at these arms out of phase, energy is propagated to the receiver. The energy arriving at arm 3 is in phase and will not be coupled to the transmitter. Since the operation of the arms of a hybrid ring is the same as the operation of E-type waveguide T-junctions, you may find it helpful to review NEETS, Module 11, Microwave Principles.
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Q30. The actions of the tr and atr circuits depend on the impedance characteristics of what length of transmission line?
Q31. During which of the transmit or receive cycles are both the tr and atr tubes of a parallel- connected duplexer ionized (arcing)?
Q32. In a series-connected duplexer, what tube (tr or atr), if any, fires during the receive cycle? Q33. To propagate energy down an arm of a hybrid ring duplexer, the two fields at the junction of the
arm and the ring must have what phase relationship?
The energy that a distant object reflects back to the antenna in a radar system is a very small fraction of the original transmitted energy. The echoes return as pulses of rf energy of the same nature as those sent out by the transmitter. However, the power of a return pulse is measured in fractions of microwatts instead of in kilowatts, and the voltage arriving at the antenna is in the range of microvolts instead of kilovolts. The radar receiver collects those pulses and provides a visual display of object information.
Information about the position of the object is present visually when the reception of an echo causes the movement or appearance of a spot of light on a cathode-ray tube (crt). The crt requires a signal on the order of at least several volts for proper operation and will not respond to the high frequencies within a return pulse. Therefore, a receiver amplifier and detector must be used that are capable of producing a visible indication on the cathode-ray tube under the following conditions: (1) when the input signal to the amplifier is in the form of pulses of extremely high-frequency, (2) the amplitude of the pulses is in the microvolt range, and (3) the pulses last for only a few microseconds.
The radar receiver evolved directly from the simple radio receiver. The radar receiver operates on exactly the same principles as the radio receiver. However, the overall requirements and limitations of a radar receiver differ somewhat from those of a radio receiver because of the higher frequencies involved and the greater sensitivity desired.
In studying the radar receiver, we will first examine the overall requirements of a radar receiver. Second, we will examine a typical radar receiver that satisfies these requirements. Finally, we will discuss the individual components of the receiver.
RADAR RECEIVER REQUIREMENTS The following characteristics determine the design requirements of an effective radar receiver:
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The word NOISE is a carryover from sound-communications equipment terminology. Noise voltages in sound equipment produce actual noise in the loudspeaker output. In radar, noise voltages result in erratic, random deflection or intensity of the indicator sweep that can mask small return signals.
Were it not for noise, the maximum range at which an object would be detectable by radar could be extended almost infinitely. Objects at great range return exceedingly small echoes. However, without noise, almost any signal could be amplified to a usable level if enough stages were added to the receiver. Because of noise, the signal detection limit or sensitivity level of a receiver is reached when the signal level falls below the noise level to such an extent as to be obscured. A simple increase of amplification is of no help because both signal and noise are amplified at the same rate.
In the radar portion of the rf spectrum, external sources of noise interference are usually negligible; consequently, the sensitivity that can be achieved in a radar receiver is usually determined by the noise produced in the receiver. Not only must noise be kept down, but everything possible must be done to minimize attenuation of the video signal (echo) before it is amplified. Gain
The GAIN of a radar receiver must be very high. This is because the strength of the signal at the antenna is at a level of microvolts and the required output to the indicator is several volts. The gain of a radar receiver is roughly in the range of 106 to 10 8. FEEDBACK, or REGENERATION, is one of the most serious difficulties in the design of an amplifier with such high gain. Special precautions must be taken to avoid feedback. Such precautions include careful shielding, decoupling (isolation) between voltage supplies for the different tubes, and amplification at different frequencies in separate groups of stages.
The radar receiver requires a limited tuning range to compensate for transmitter and local oscillator frequency changes because of variations in temperature and loading. Microwave radar receivers usually use automatic frequency control (afc) for this purpose. Distortion
If distortion occurs in the receiver, the time interval between the transmitted pulse and the received pulse changes, thereby affecting range accuracy.
BLOCKING refers to a condition of the receiver in which the voltage pulse at the receiver input is too large. As a result, for a short time after the pulse, the receiver is insensitive or blocked to signals below a certain level. This condition results from one or more of the amplifier stages in the receiver being overdriven. After a strong pulse, the receiver may be biased to a point at which it will not amplify small signals. Recovery after blocking may be only a fraction of a microsecond, or it may take several hundred microseconds, depending upon the point in the receiver at which blocking occurs. To detect a weak echo immediately following a strong one, the receiver must have a short BLOCKING RECOVERY TIME. The blocking itself must be minimized as much as possible. If a portion of the transmitted pulse leaks into the receiver input, then the receiver may be blocked and not show small, nearby objects. In most receivers, blocking is minimized from this cause by a duplexer. The duplexer protects the receiver by isolating it during the transmitted pulse.
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The SUPERHETERODYNE receiver is almost always used in microwave radar systems. A typical superheterodyne radar receiver is shown in figure 2-23. A receiver of this type meets all the requirements listed above. Signals from the antenna enter the receiver via the duplexer. A low-noise rf amplifier is usually the first stage of modern radar receivers. Some receivers, however, send the antenna signal directly to the mixer, as shown by the dashed path. The low-noise amplifiers used in modern systems are usually solid-state devices, such as tunnel-diode, parametric, or microwave transistor amplifiers.
The MIXER stage is often called the FIRST DETECTOR. The function of this stage is to convert the received rf energy to a lower, intermediate frequency (IF) that is easier to amplify and manipulate electronically. The intermediate frequency is usually 30 or 60 megahertz. It is obtained by heterodyning the received signal with a local-oscillator signal in the mixer stage. The mixer stage converts the received signal to the lower IF signal without distorting the data on the received signal.
After conversion to the intermediate frequency, the signal is amplified in several IF AMPLIFIER stages. Most of the gain of the receiver is developed in the IF amplifier stages. The overall bandwidth of the receiver is often determined by the bandwidth of the IF stages.
The output of the IF amplifiers is applied to the SECOND DETECTOR. It is then rectified and passed through one or more stages of amplification in the video amplifier(s). The output stage of the receiver is normally an emitter follower. The low-impedance output of the emitter follower matches the impedance of the cable. The video pulses are coupled through the cable to the indicator for video display on the crt.
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As in all superheterodyne receivers, controlling the frequency of the local oscillator keeps the receiver tuned. Since this tuning is critical, some form of automatic frequency control (afc) is essential to avoid constant manual tuning. Automatic frequency control circuits mix an attenuated portion of the transmitted signal with the local oscillator signal to form an IF signal. This signal is applied to a frequency-sensitive discriminator that produces an output voltage proportional in amplitude and polarity to any change in IF frequency. If the IF signal is at the discriminator center frequency, no discriminator output occurs. The center frequency of the discriminator is essentially a reference frequency for the IF signal. The output of the DISCRIMINATOR provides a control voltage to maintain the local oscillator at the correct frequency.
Different receiving systems may vary in the type of coupling between stages, the type of mixer, the detector, the local oscillator, and the number of stages of amplification at the different frequencies. However, the receiver is always designed to have as little noise as possible. It is also designed to have sufficient gain so that noise, rather than lack of gain, limits the smallest visible signal.
This section will analyze in more detail the operation of the receiver circuits mentioned above. The circuits discussed are usually found in some form in all radar superheterodyne receivers.
LOW-NOISE AMPLIFIERS, sometimes called PREAMPS, are found in most modern radar receivers. As previously mentioned, these amplifiers are usually solid-state microwave amplifiers. The most common types are tunnel diode and parametric amplifiers. These amplifiers are discussed in detail in NEETS, Module 11, Microwave Principles. Some older systems may still use a traveling-wave tube (twt) as a low-noise first stage amplifier. However, the solid-state amplifiers produce lower noise levels and more gain.
Most radar receivers use a 30 or 60 megahertz intermediate frequency. The IF is produced by mixing a local oscillator signal with the incoming signal. The local oscillator is, therefore, essential to efficient operation and must be both tunable and very stable. For example, if the local oscillator frequency is 3,000 megahertz, a frequency change of 0.1 percent will produce a frequency shift of 3 megahertz. This is equal to the bandwidth of most receivers and would greatly decrease receiver gain.
The power output requirement for most local oscillators is small (20 to 50 milliwatts) because most receivers use crystal mixers that require very little power.
The local oscillator output frequency must be tunable over a range of several megahertz in the 4,000- megahertz region. The local oscillator must compensate for any changes in the transmitted frequency and maintain a constant 30 or 60 megahertz difference between the oscillator and the transmitter frequency. A local oscillator that can be tuned by varying the applied voltage is most desirable.
The REFLEX KLYSTRON is often used as a local oscillator because it meets all the requirements mentioned above. The reflex klystron is a very stable microwave oscillator that can be tuned by changing the repeller voltage.
Most radar systems use an automatic frequency control (afc) circuit to control the output of the local oscillator. A block diagram of a typical afc circuit is included in figure 2-23. Note that the afc circuits form a closed loop. This circuit is, in fact, often called the afc loop.
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A sample of the transmitter energy is fed through the afc mixer and an IF amplifier to a discriminator. The output of the discriminator is a dc error voltage that indicates the degree of mistuning between the transmitter and the local oscillator. In this particular example let’s assume that the IF is 30 megahertz. If the output of the mixer is correct, the discriminator will have no output. If the mixer output is above 30 megahertz, the output of the discriminator will be positive dc pulses; if the mixer output is below 30 megahertz, the discriminator output will be negative dc pulses. In either case, this output is fed through an amplifier to the control circuit. The control circuit adjusts the operating frequency of the local oscillator so that no mistuning exists and the IF is 30 megahertz. In this example the local oscillator is a reflex klystron and the control circuit provides he repeller plate voltage for the klystron; thus, the klystron directly controls the local oscillator frequency. In this manner the local oscillator is maintained exactly 30 megahertz below the transmitter frequency.
Q34. What is the greatest limiting factor in a receiver’s detectable range? Q35. What type of receiver is most often used in radar systems? Q36. What IF frequencies are normally used in radar receivers? Q37. Which component of the receiver produces the signal that is mixed with the received signal to
produce the IF signal?
Many older radar receivers do not use a low-noise amplifier as the receiver front end; they simply send the echo signal directly to a crystal mixer stage. A crystal is used rather than an electron-tube diode because, at microwave frequencies, the tube would generate excessive noise. Electron tubes are also limited by the effects of transit time at microwave frequencies. The crystal most commonly used is the point-contact crystal diode; however, recent developments in the field of solid-state microwave devices may soon replace the point-contact diode with devices that produce even less noise. The Schottky-barrier diode is an example of a relatively recent development that produces less noise than the point-contact crystal.
The simplest type of radar mixer is the SINGLE ENDED or UNBALANCED CRYSTAL MIXER, shown in figure 2-24. The mixer illustrated uses a tuned section of coaxial transmission line one-half wavelength long. This section matches the crystal to the signal echo and the local oscillator inputs. Local oscillator injection is accomplished by means of a probe. In the coaxial assembly, the signal is injected by means of a slot. This slot would normally be inserted in the duplexer waveguide assembly and be properly oriented to provide coupling of the returned signal. In this application, the unwanted signals at the output of the mixer (carrier frequency, the local oscillator frequency, and sum of these two signals) are effectively eliminated by a resonant circuit tuned to the intermediate, or difference frequency. One advantage of the unbalanced crystal mixer is its simplicity. It has one major disadvantage; its inability to cancel local oscillator noise. Difficulty in detecting weak signals will exist if noise is allowed to pass through the mixer along with the signal.
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One type of mixer which cancels local oscillator noise is the BALANCED, OR HYBRID, MIXER (sometimes called the MAGIC T). Figure 2-25 shows this type of mixer. In hybrid mixers, crystals are inserted directly into the waveguide. The crystals are located one-quarter wavelength from their respective short-circuited waveguide ends (a point of maximum voltage along a tuned line). The crystals are also connected to a balanced transformer, the secondary of which is tuned to the desired IF. The local oscillator signal is introduced into the waveguide local oscillator arm and distributes itself as shown in view A of figure 2-26. Observe that the local oscillator signal is in phase across the crystals. In view B the echo signal is introduced into the echo signal arm of the waveguide and is out of phase across the crystals. The resulting fields are shown in view C.
2-37
A difference in phase exists between echo signals applied across the two crystals. The signal applied to the crystals from the local oscillator is in phase. Therefore, at some point both signals applied to crystal #1 will be in phase, and the signals applied to crystal #2 will be out of phase. This means that an IF signal of one polarity will be produced across crystal #1 and an IF signal of the opposite polarity will be produced across crystal #2. When these two signals are applied to the balanced output transformer (figure 2-25), they will add. Outputs of the same polarity will cancel across the balanced transformer.
This action eliminates the noise of the local oscillator. Noise components introduced from the local oscillator are in phase across the crystals and are, therefore, cancelled in the balanced transformer. The rf characteristics of the crystals must be nearly equal, or the noise of the local oscillator will not completely cancel. Note that only the noise produced by the local oscillator is canceled. Noise arriving with the echo signal is not affected.
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The IF AMPLIFIER SECTION of a radar receiver determines the gain, signal-to-noise ratio, and effective bandwidth of the receiver. The typical IF amplifier (commonly called an IF strip) usually contains from three to ten amplifier stages. The IF amplifier has the capability to vary both the bandpass and the gain of a receiver. Normally, the bandpass is as narrow as possible without affecting the actual signal energy. When a selection of pulse widths is available, such as short and long pulses, the bandpass must be able to match the bandwidth of the two different signals. Gain must be variable to provide a constant voltage output for input signals of different amplitudes. Figure 2-27 is a block diagram of an IF amplifier that meets these requirements.
The most critical stage of the IF section is the input (first stage). The quality of this stage determines the noise figure of the receiver and the performance of the entire receiving system with respect to detection of small objects at long ranges. Gain and bandwidth are not the only considerations in the design of the first IF stage. A consideration perhaps of more importance is noise generation. Noise generation in this stage must be low. Noise generated in the input IF stage will be amplified by succeeding stages and may exceed the echo signal in strength.
The detector in a microwave receiver serves to convert the IF pulses into video pulses. After amplification, these are applied to the indicator. The simplest form of detector, and the one most commonly used in microwave receivers, is the DIODE DETECTOR.
A diode detector circuit is shown in view A of figure 2-28. The secondary of T1 and C1 form a tuned circuit that is resonant at the intermediate frequency. Should an echo pulse of sufficient amplitude be received, the voltage (ei) developed across the tuned circuit is an IF pulse. Its shape is indicated by the dashed line in view B. Positive excursions of e
i cause no current to flow through the diode. However, negative excursions result in a flow of diode current and a subsequent negative voltage (eo) to be developed across R1 and C2. Between peak negative voltage excursions of the ei wave, capacitor C2 discharges through R1. Thus, the eo waveform is a negative video pulse with sloping edges and
2-39
superimposed IF ripple, as indicated by the solid line in view B. A negative polarity of the output pulse is ordinarily preferred, but a positive pulse may be obtained by reversing the connections of the diode. In view A, inductance L1, in combination with wiring capacitance and C2, forms a low-pass filter. This filter attenuates the IF components in the eo waveform but results in a minimum loss of video high-frequency components.
The video amplifier receives pulses from the detector and amplifies these pulses for application to the indicating device. A video amplifier is fundamentally an RC coupled amplifier that uses high-gain transistors or pentodes. However, a video amplifier must be capable of a relatively wide frequency response. Stray and interelectrode capacitances reduce the high-frequency response of an amplifier, and the reactance of the coupling capacitor diminishes the low-frequency response. These problems are overcome by the use of FREQUENCY COMPENSATION NETWORKS in the video amplifier. The types of frequency compensation networks that may be used in a video amplifier stage are discussed in detail in NEETS, Module 8, Introduction to Amplifiers.
Q38. What receiver circuit actually produces the IF frequency? Q39. The IF amplifiers are connected in what amplifier configuration? Q40. Which receiver component converts the IF pulses to video pulses?
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The performance efficiency of radar receivers is often greatly decreased by interference from one or more of several possible sources. Weather and sea return are the most common of these interference sources, especially for radar systems that operate above 3,000 megahertz. Unfavorable weather conditions can completely mask all radar returns and render the system useless. Electromagnetic interference from external sources, such as the deliberate interference by an enemy, called jamming or electronic counter measures (ECM), can also render a radar system useless. Many special circuits have been designed to help the radar receiver counteract the effects of external interference. These circuits are called VIDEO ENHANCEMENT FEATURES, ANTIJAMMING CIRCUITS, or ELECTRONIC COUNTER- COUNTERMEASURES (ECCM) CIRCUITS. This section will discuss, in general terms, some of the more common video enhancement features associated with radar receivers.
Automatic Gain Control (AGC) Most radar receivers use some means to control the overall gain. This usually involves the gain of
one or more IF amplifier stages. Manual gain control by the operator is the simplest method. Usually, some more complex form of automatic gain control (agc) or instantaneous automatic gain control (iagc) is used during normal operation. Gain control is necessary to adjust the receiver sensitivity for the best reception of signals of widely varying amplitudes. Agc and iagc circuits are designed with, a shut-off feature so that receiver gain may be adjusted manually. In this way, manual gain control can be used to adjust for best reception of a particular signal.
The simplest type of agc adjusts the IF amplifier bias (and gain) according to the average level of the received signal. Agc is not used as frequently as other types of gain control because of the widely varying amplitudes of radar return signals.
With agc, gain is controlled by the largest received signals. When several radar signals are being received simultaneously, the weakest signal may be of greatest interest. Iagc is used more frequently because it adjusts receiver gain for each signal.
The iagc circuit is essentially a wide-band, dc amplifier. It instantaneously controls the gain of the IF amplifier as the radar return signal changes in amplitude. The effect of iagc is to allow full amplification of weak signals and to decrease the amplification of strong signals. The range of iagc is limited, however, by the number of IF stages in which gain is controlled. When only one IF stage is controlled, the range of iagc is limited to approximately 20 dB. When more than one IF stage is controlled, iagc range can be increased to approximately 40 dB.
difficult. The adjustment of receiver gain for best visibility of nearby target return signals is not the best adjustment for distant target return signals. Circuits used to adjust amplifier gain with time, during a single pulse-repetition period, are called stc circuits.
Sensitivity time-control circuits apply a bias voltage that varies with time to the IF amplifiers to control receiver gain. Figure 2-29 shows a typical stc waveform in relation to the transmitted pulse. When the transmitter fires, the stc circuit decreases the receiver gain to zero to prevent the amplification of any leakage energy from the transmitted pulse. At the end of the transmitted pulse, the stc voltage begins to rise, gradually increasing the receiver gain to maximum. The stc voltage effect on receiver gain is usually limited to approximately 50 miles. This is because close-in targets are most likely to saturate the receiver; beyond 50 miles, stc has no affect and the receiver operates normally.
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The combination of stc and iagc circuits results in better overall performance than with either type of gain control alone. Stc decreases the amplitude of nearby target return signals, while iagc decreases the amplitude of larger-than-average return signals. Thus, normal changes of signal amplitudes are adequately compensated for by the combination of iagc and stc.
AGC CIRCUITS and FAST-TIME-CONSTANT CIRCUITS. A gated agc circuit permits signals that occur only in a very short time interval to develop the agc. If large-amplitude pulses from a jamming transmitter arrive at the radar receiver at any time other than during the gating period, the agc does not respond to these jamming pulses.
Without gated agc, a large jamming signal would cause the automatic gain control to follow the interfering signal. This would decrease the target return signal amplitude to an unusable value. Gated agc produces an output signal for only short time periods; therefore, the agc output voltage must be averaged over several cycles to keep the automatic gain control from becoming unstable.
Gated agc does not respond to signals that arrive at times other than during the time of a target return signal. However, it cannot prevent interference that occurs during the gating period. Neither can gating the agc prevent the receiver from overloading because of jamming signal amplitudes far in excess of the target return signal. This is because the desired target is gated to set the receiver gain for a signal of that particular amplitude. As an aid in preventing radar receiver circuits from overloading during the reception of jamming signals, fast-time-constant coupling circuits are used. These circuits connect the video detector output to the video amplifier input circuit.
A fast-time-constant (ftc) circuit is a differentiator circuit located at the input of the first video amplifier. When a large block of video is applied to the ftc circuit, only the leading edge will pass. This is because of the short time constant of the differentiator. A small target will produce the same length of signal on the indicator as a large target because only the leading edge is displayed. The ftc circuit has no effect on receiver gain; and, although it does not eliminate jamming signals, ftc greatly reduces the effect of jamming.
Q41. Which of the two types of automatic gain control, agc or iagc, is most effective in radar use for the Navy?
Q42. Immediately after the transmitter fires, stc reduces the receiver gain to what level?
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Q43. How does ftc affect receiver gain, if at all? SPECIAL RECEIVERS
The basic receiver of a radar system often does not meet all the requirements of the radar system, nor does it always function very well in unfavorable environments. Several special receivers have been developed to enhance target detection in unfavorable environments or to meet the requirements of special transmission or scanning methods. A radar system with a moving target indicator (mti) system or a monopulse scanning system requires a special type of receiver. Other types of special receivers, such as the logarithmic receiver, have been developed to enhance reception during unfavorable conditions. These receivers will be discussed in general terms in this section.
unwanted echoes) and displays only moving target signals. Clutter is the appearance on a radar indicator of confusing, unwanted echoes which interfere with the clear display of desired echoes. Clutter is the result of echoes from land, water, weather, and so forth. The unwanted echoes can consist of GROUND CLUTTER (echoes from surrounding land masses), SEA CLUTTER (echoes from the irregular surface of the sea), or echoes from the clouds and rain. The problem is to find the desired echo in the midst of the clutter. To do this, the mti system must be able to distinguish between fixed and moving targets and then must eliminate only the fixed targets. This is accomplished by phase detection and pulse-to-pulse comparison.
Target echo signals from stationary objects have the same phase relationship from one receiving period to the next. Moving objects produce echo signals that have a different phase relationship from one receiving period to the next. This principle allows the mti system to discriminate between fixed and moving targets.
Signals received from each transmitted pulse are delayed for a period of time exactly equal to the pulse-repetition time. The delayed signals are then combined with the signals received from the next transmitted pulse. This is accomplished in such a manner that the amplitudes subtract from each other as shown in figure 2-30, views Aand B. Since the fixed targets have approximately the same amplitude on each successive pulse, they will be eliminated. The moving target signals, however, are of different amplitudes on each successive pulse and, therefore, do not cancel. The resulting signal is then amplified and presented on the indicators.
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In figure 2-31, 30-megahertz signals from the signal mixer are applied to the 30-megahertz amplifier. The signals are then amplified, limited, and fed to the phase detector. Another 30-megahertz signal, obtained from the coherent oscillator (coho) mixer, is applied as a lock pulse to the coho. The coho lock pulse is originated by the transmitted pulse. It is used to synchronize the coho to a fixed phase relationship with the transmitted frequency at each transmitted pulse. The 30-megahertz, cw reference signal output of the coho is applied, together with the 30-megahertz echo signal, to the phase detector.
2-44
The phase detector produces a video signal. The amplitude of the video signal is determined by the phase difference between the coho reference signal and the IF echo signals. This phase difference is the same as that between the actual transmitted pulse and its echo. The resultant video signal may be either positive or negative. This video output, called coherent video, is applied to the 14-megahertz cw carrier oscillator.
The 14-megahertz cw carrier frequency is amplitude modulated by the phase-detected coherent video. The modulated signal is amplified and applied to two channels. One channel delays the 14-megahertz signal for a period equal to the time between transmitted pulses. The signal is then amplified and detected. The delay required (the period between transmitted pulses) is obtained by using a mercury delay line or a fused-quartz delay line, which operates ultrasonically at 14 megahertz.
The signal to the other channel is amplified and detected with no delay introduced. This channel includes an attenuating network that introduces the same amount of attenuation as does the delay line in the delayed video channel. The resulting nondelayed video signal is combined in opposite polarity with the delayed signal. The amplitude difference, if any, at the comparison point between the two video signals is amplified; because the signal is bipolar, it is made unipolar. The resultant video signal, which represents only moving targets, is sent to the indicator system for display.
An analysis of the mti system operation just described shows that signals from fixed targets produce in the phase detector recurring video signals of the same amplitude and polarity. (Fixed targets have an unchanging phase relationship to their respective transmitted pulses.) Thus, when one video pulse is
2-45
combined with the preceding pulse of opposite polarity, the video signals cancel and are not passed on to the indicator system.
Signals from moving targets, however, will have a varying phase relationship with the transmitted pulse. As a result, the signals from successive receiving periods produce signals of different amplitudes in the phase detector. When such signals are combined, the difference in signal amplitude provides a video signal that is sent to the indicator system for display.
The timing circuits, shown in figure 2-31, are used to accurately control the transmitter pulse- repetition frequency to ensure that the pulse-repetition time remains constant from pulse to pulse. This is necessary, of course, for the pulses arriving at the comparison point to coincide in time and achieve cancellation of fixed targets.
As shown in figure 2-31, a feedback loop is used from the output of the delay channel, through the pickoff amplifier, to the trigger generator and gating multivibrator circuits. The leading edge of the square wave produced by the detected carrier wave in the delayed video channel is differentiated at the pickoff amplifier. It is used to activate the trigger generator and gating multivibrator. The trigger generator sends an amplified trigger pulse to the modulator, causing the radar set to transmit.
The gating multivibrator is also triggered by the negative spike from the differentiated square wave. This stage applies a 2,000-microsecond negative gate to the 14-megahertz oscillator. The oscillator operates for 2,400 microseconds and is then cut off. Because the delay line time is 2,500 microseconds, the 14-megahertz oscillations stop before the initial waves reach the end of the delay line. This wave train, when detected and differentiated, turns the gating multivibrator on, producing another 2,400- microsecond wave train. The 100 microseconds of the delay line is necessary to ensure that the mechanical waves within the line have time to damp out before the next pulse-repetition time. In this manner the pulse-repetition time of the radar set is controlled by the delay of the mercury, or quartz delay line. Because this delay line is also common to the video pulses going to the comparison point, the delayed and the undelayed video pulses will arrive at exactly the same time.
Q44. What type of target has a fixed phase relationship from one receiving period to the next? Q45. What signal is used to synchronize the coherent oscillator to a fixed phase relationship with the
transmitted pulse?
Q46. What is the phase relationship between the delayed and undelayed video? Logarithmic Receiver
The LOGARITHMIC RECEIVER uses a linear logarithmic amplifier, commonly called a LIN-LOG AMPLIFIER, instead of a normal IF amplifier. The lin-log amplifier is a nonsaturating amplifier that does not ordinarily use any special gain-control circuits. The output voltage of the lin-log amplifier is a linear function of the input voltage for low-amplitude signals. It is a logarithmic function for high-amplitude signals. In other words, the range of linear amplification does not end at a definite saturation point, as is the case in normal IF amplifiers. The comparison of the response curves for normal IF and lin-log amplifiers is shown in figure 2-32. The curves show that a continued increase in the input to the lin-log amplifier causes a continued increase in the output, but at a reduced rate. Therefore, a large signal does not saturate the lin-log amplifier; rather, it merely reduces the amplification of a simultaneously applied small signal. A small echo signal can often be detected by the lin-log receiver when a normal receiver would be saturated.
2-46
A typical circuit for obtaining a lin-log response is shown in figure 2-33. If detectors 2 and 3 were not present, the output voltage would be limited by the saturation point of the final IF stage, as it is in a normal IF section. However, when the final stage of the lin-log is saturated, larger signals cause an increase in the output of the next to last stage. This increase is detected by detector 2 and summed with the output of detector 1. This sum produces an increase in the output even though the final stage is saturated. Detector 3 causes the output to continue to increase after the second stage saturates. The overall gain becomes less and less as each stage saturates, but some degree of amplification is still available. The proper choice of IF stage gains and saturation points produces an approximately logarithmic response curve.
Figure 2-34, shows the response curves of the three IF stages in the lin-log amplifier shown in figure 2-33. The responses of the individual stages produce a segmented overall response curve for the receiver.
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The most common of the automatic tracking radars is the MONOPULSE RADAR. The monopulse radar obtains the three target position coordinates of range, bearing, and elevation from a single pulse. The receiver for a monopulse radar must have three separate channels to process range, bearing, and elevation information. The block diagram of a simplified monopulse receiver is shown in figure 2-35.
As in a conventional receiver, each channel of the monopulse receiver converts the return echo to an IF frequency by mixing the returned signal with a common local oscillator signal. The sum of the energy from all four return signals is mixed with the local oscillator signal to produce range IF information. Bearing information is obtained by subtracting the energy from horns B and D from the energy from horns A and C:
2-48
(A + C) - (B + D) and mixing the difference with the local oscillator signal. The result is a bearing IF signal. Elevation
information is obtained in the same way, except the energy from horns C and D is subtracted from the energy from horns A and B:
the bearing nor elevation channels will receive an input signal. If either of the bearing or elevation signals is off the axis, an input to the IF channel is produced. This input is subsequently converted to an IF signal in the appropriate channel.
The major difference between the monopulse receiver and the conventional receiver is the requirement for a dc error voltage output from the bearing and elevation channels. The range channel of a monopulse receiver is sent to a conventional ranging circuit for presentation are on an indicator or for use by a range-tracking circuit. However, since most monopulse radars are automatic tracking radars, the outputs of the bearing and elevation channels must be converted to dc error signals for use by automatic bearing and elevation tracking systems. The dc error voltages are applied to the antenna bearing and elevation positioning servos. These servos reposition the antenna until the errors are nulled.
The phase detectors compare the phase of the bearing and elevation IF with a reference IF from the range channels. This comparison produces the dc error pulses needed to drive the antenna servos. The signals from both the bearing and elevation channels are the result of a summation process. They can be either positive (in-phase) or negative (180-degrees out of phase) when compared to the reference IF signal. For example, if the output of horns A and C is smaller than the output of horns B and D, a negative or 180-degree-out-of-phase signal is produced by the bearing channel (A + C) - (B + D). If output A + C is greater than output B + D, a positive or in-phase signal is produced by the bearing channel. The phase of the bearing and elevation output signals determines the direction in which the antenna moves; the magnitude of the signal determines the amount of movement. Since two signals must be present at the phase detector to produce an output, an error signal occurs only when a return echo is not on the antenna beam axis.
This technique produces an error signal when the target moves off the radiated beam axis in either bearing or elevation. The error signal causes the antenna to move in the proper direction and for the proper duration to cancel the error signal. This method of automatic tracking is commonly used by weapons-control tracking radar systems.
Q47. When a large signal and a small signal are applied to a lin-log amplifier at the same time, what is the effect on the small signal?
Q48. What happens to the overall gain of a lin-log amplifier as each stage saturates? Q49. A monopulse receiver has how many separate channels? Q50. If a target is on the bearing axis of the radiated beam, what is the input to the bearing IF
channel?
Q51. What characteristic of the bearing and elevation output signals determines the direction of antenna movement?
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The following paragraphs summarize the important points of this chapter.
The SYNCHRONIZER is essential to any radar because it controls and times the operation of the entire system. Radar systems may be self-synchronized by triggers from the transmitter or externally synchronized from a master oscillator.
Most modern systems are synchronized by a MASTER OSCILLATOR, which may be a SINEWAVE OSCILLATOR, an ASTABLE MULTIVIBRATOR, or a BLOCKING OSCILLATOR.
Each of these oscillators fulfills the basic requirements of a synchronizer, which must be:
into space by the antenna.
The MODULATOR controls the radar pulse, width and amplitude.
KEYED-OSCILLATOR TRANSMITTERS produce a high-power output pulse by keying a high- power oscillator, such as a MAGNETRON.
POWER-AMPLIFIER TRANSMITTERS amplify a low-level pulse to the desired power level using a series of microwave amplifiers such as TRAVELING-WAVE TUBES or KLYSTRONS.
The DUPLEXER is a device that allows the same antenna to both transmit and receive. Most duplexers use the impedance characteristics of transmission lines and waveguides in conjunction with TR and ATR tubes to route the energy to the correct place. One of the most important functions of the duplexer is isolation of the receiver during transmission.
The RECEIVER detects the very small target return echo and amplifies it to a usable level for display on the indicator.

Some special purpose receivers are the MOVING TARGET INDICATOR and MONOPULSE RECEIVERS.
ANSWERS TO QUESTIONS Q1. THROUGH Q51. A1. Controls system operation and timing.
A2. Timing and control.
A3. Transmitter.
A4. Free-running.
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A5. The master oscillator.
A6. Leakage from the duplexer. A7. Sine-wave oscillator, single-swing blocking oscillator, and master-trigger (astable) multivibrator. A8. It requires additional shaping circuits.
A9. Blocking oscillators.
A10. Keyed oscillator and power-amplifier chain. A11. The modulator.
A12. Steep leading and trailing edges.
A13. Line-pulsed.
A14. Capacitor, artificial transmission line, or pulse-forming network. A15. Pulse width.
A16. Thyratron.
A17. The charging impedance.
A18. 600-30,000 megahertz.
A19. Mode skipping and mode shifting. A20. The magnetron will not oscillate.
A21. – 5 percent.
A22. Frequency stability.
A23. Local oscillator and coherent oscillator.
A24. Multicavity klystron.
A25. Frequency synthesizer.
A26. Oscillations at an undesired frequency. A27. Electronic.
A28. Tr tube.
A29. Apply keep-alive voltage.
A30. Quarter-wavelength section. A31. Transmit.
A32. Neither fires. A33. 180 degrees out of phase.
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A34. Noise.
A35. Superheterodyne.
A36. Thirty or sixty megahertz.
A37. Local oscillator.
A38. Mixer.
A39. Cascade.
A40. Detector.
A41. IAGC.
A42. Zero.
A43. FTC has no effect on receiver gain. A44. Stationary.
A45. Coho lock pulse.
A46. Opposite.
A47. Amplification is reduced. A48. Decreases.
A49. Three.
A50. Zero.
A51. Phase.
3-1
Upon completion of this chapter, the student will be able to:
1. Describe the purpose of the A scope, the range-height indicator (rhi), and the plan position
indicator (ppi).
2. State the relationship between range and sweep speed and length on a radar indicator.
3. Explain the purpose of timing triggers, video, and antenna position inputs to a radar indicator.
4. List the major units of a ppi and describe their functions.
5. Describe the basic operation of sweep deflection and sweep rotation in a ppi.
6. List and describe the operation of the three range measurement circuits.
7. Describe antenna directivity and power gain characteristics.
8. Describe the focusing action of a basic parabolic antenna.
9. Describe the basic radiation patterns of the most common parabolic reflectors.
10. Describe the basic characteristics of horn radiators.
to display the video information generated. This chapter will briefly describe some commonly used
indicators and antenna systems. Antenna systems are described in more detail in NEETS, Module 10,
Introduction to Wave Generation, Transmission Lines, and Antennas, and Module 11, Microwave Principles.
bits per second. From these and other data, such as the orientation of the antenna, the indicator should
radar targets. It should provide size, shape, and insofar as possible, indications of the type of targets. A
cathode-ray tube (crt) fulfills these requirements to an astonishing degree. The cathode-ray tube's
principal shortcoming is that it cannot present a true three-dimensional picture.
ANGLE (or BEARING), and ELEVATION ANGLE. These displays relate the position of a radar target
3-2
to the origin at the antenna. Most radar displays include one or two of these quantities as coordinates of
the crt face.
known as SLANT RANGE. The majority of displays use as one coordinate the value of slant range, its
horizontal projection (GROUND RANGE), or its vertical projection (ALTITUDE). Since slant range is
involved in every radar situation, it inevitably appears in at least one display on every set. Slant range is
the coordinate that is duplicated most often when more than one type of display is used. This is partly
geometrical reasons.
definite time in each pulse cycle. Thus, distances along this range sweep actually measure slant range.
The sweep speed determines the scale factor, which relates the distance on the tube to actual range. The
sweep length is the total distance represented. Distances are expressed in miles (statute or nautical) or
yards. The origin of the range sweep may be on or off the tube face.
dimensional information in the display; that is, range and azimuth, or range and elevation.
A radar indicator, sometimes called a radar repeater, acts as the master timing device in analyzing the return of the video in a radar system. It also provides that capability to various other locations
physically remote from the radar system. Each indicator should have the ability to select the outputs from
any desired radar system aboard the ship. This is usually accomplished by the use of a RADAR
DISTRIBUTION SWITCHBOARD. The switchboard contains a switching arrangement that has inputs
from each radar system aboard ship and provides outputs to each repeater. The radar desired is selected by
means of a selector switch on the repeater. For the repeater to present correct target position data, the
indicator must have the following three inputs from the selected radar:
1. Trigger timing pulses. These pulses ensure that the sweep on the repeater starts from its point of origin each time the radar transmits. As discussed earlier, the repeater displays all targets at their
instant the target's echo is received.
2. The returning echo. The echo, in rf form, is detected (converted to a video signal) by the radar receiver and applied to the repeater.
3. Antenna information. The angular sweep position of a plan position indicator (ppi) repeater must be synchronized to the angular position of the radar antenna to display target bearing (azimuth)
information.
INDICATOR (RHI) SCOPE, and the PLAN POSITION INDICATOR (PPI) SCOPE. The primary
function of these displays will be discussed in this section. However, detailed descriptions will be limited
to the ppi scope, which is the most common display.
strength of the echo. Such a display is normally used in weapons control radar systems. The bearing and
the antenna.
3-3
The A-scope normally uses an electrostatic-deflection crt. The sweep is produced by applying a
sawtooth voltage to the horizontal deflection plates. The electrical length (time duration) of the sawtooth
voltage determines the total amount of range displayed on the crt face.
or step that is superimposed on the sweep. Ranging circuits will be discussed in more detail later in this
section.
to obtain altitude information. The rhi is a two-dimensional presentation indicating target range and
altitude. The sweep originates in the lower left side of the scope. It moves across the scope, to the right, at
an angle that is the same as the angle of transmission of the height-finding radar. The line of sight to the
horizon is indicated by the bottom horizontal line. The area directly overhead is straight up the left side of
the scope. Target echoes are displayed on the scope as vertical PIPS or BLIPS (spots of increased
intensity that indicate a target location). The operator determines altitude by adjusting a movable height
line to the point where it bisects the center of the blip. Target height is then read directly from an altitude
dial or digital readout. Vertical range markers are also provided to estimate target range.
3-4
Q1. What are the three fundamental quantities involved in radar displays?
Q2. What are the required radar inputs for proper indicator operation?
Q3. What coordinates are displayed on an rhi scope?
PLAN POSITION INDICATOR (PPI).
The ppi scope shown in figure 3-3, is by far the most used radar display. It is a polar coordinate
display of the area surrounding the radar platform. Own ship is represented as the origin of the sweep,
which is normally located in the center of the scope, but may be offset from the center on some sets. The
ppi uses a radial sweep pivoting about the center of the presentation. This results in a map-like picture of
the area covered by the radar beam. A long-persistence screen is used so that the display remains visible
until the sweep passes again.
3-6
extending vertically from the sweep origin to the top of the scope. The top of the scope is either true north
in the relative bearing mode).
The basic block diagram, figure 3-4, illustrates the major units of a plan position indicator.
Synchronization of events is particularly important in the presentation system. At the instant a radar
indicator must be activated. These events must be performed to a high degree of accuracy to ensure
accurate range determination. The synchronization of these events is provided by the gate circuit.
GATE CIRCUIT.The gate circuit develops pulses which synchronize the indicator with the transmitter. The gate circuit itself is synchronized by trigger pulses from the synchronizer. It then
provides timing for the intensity gate generator, sweep generator circuit, and the sweep control circuit.
SWEEP CONTROL CIRCUIT.The sweep control circuit converts mechanical bearing information from the antenna into voltages which control sweep circuit azimuth.
SWEEP GENERATOR CIRCUIT.The sweep generator circuit produces currents which deflect an electron beam across the crt. Varying voltages from the sweep control circuit are applied to deflection
coils. Gate voltages determine sweep rate, and therefore, the effective distance (range) covered by each
sweep. Sweep potentials consist of separate north-south and east-west voltages; the amplitudes of these
voltages determine sweep azimuth. The sweep generator is synchronized by an input from the gate circuit.
INTENSITY GATE GENERATOR.The intensity gate generator provides a gate which unblanks the crt during sweep periods. The intensity of the trace appearing on the crt is determined by the dc level
of this gate. This circuit is also synchronized by the gate circuit.
3-7
VIDEO AMPLIFIER.The video amplifier circuit amplifies the video signal from the receiver and applies it to the crt intensity-modulating element (control grid).
POWER SUPPLY.The power supply produces all voltages needed to operate the indicator. It also includes protective devices and metering circuits.
and bearing determination. These circuits are also synchronized by the gate circuit.
electrostatic deflection. Reasons for this choice are (1) increased control of the beam, (2) improved
deflection sensitivity, (3) better beam position accuracy, and (4) simpler construction of the crt.
method of controlling deflection and focusing of the electron beam. Both types employ electron guns and
use electrostatic fields to accelerate and control the flow of electrons. The physical construction of a crt
employing electromagnetic deflection is similar to an electrostatic type. The construction of a crt
employing electromagnetic deflection is shown in figure 3-5.
focus coil, and anode (composed of a special coating). Focusing the electron beam on the face of the
screen is accomplished by the focus coil. A direct current through the windings sets up a strong magnetic
field at the center of the coil. Electrons move precisely along the axis of the tube and pass through the
focusing field with no deflection. This is because they move parallel to the magnetic field at all times.
it that is perpendicular to its direction of motion. A second force on this electron is perpendicular to the
magnetic lines and is, therefore, constantly changing in direction. These forces cause the electron to move
3-8
in a helical or corkscrew path shown in figure 3-6. With the proper velocity of the electron and strength of
electrons at some point on the crt screen. Focusing is accomplished by adjusting the current flow through
the focusing coils.
a set of deflection coils, as shown in figure 3-5. These coils are mounted around the outside surface of the
neck of the crt. Normally, four deflection coils (N, S, E, and W) are used, as shown in figure 3-4. Two coils in series are positioned in a manner that causes the magnetic field produced to be in a vertical plane.
plane. The coils (N-S) which produce a horizontal field are called the VERTICAL DEFLECTION COILS and the coils (E-W) which produce a vertical field are called the HORIZONTAL DEFLECTION COILS. This may be more clearly understood if you recall that an electron beam will be deflected at right angles
to a deflecting field. The deflection coils are illustrated in view A of figure 3-7. View B shows the N-S windings in schematic form.
3-9
Electron deflection in the electromagnetic crt is proportional to the strength of the magnetic fields.
Magnetic field strength depends on current in the coils. The sweep circuits associated with
desired beam deflection.
A sawtooth current is required to produce a linear trace. A deflection coil may be considered
equivalent to the circuit shown in view A of figure 3-8. Because of the inductance of the coil, a
trapezoidal voltage must be applied across the coil to produce a sawtooth of current through it. This is
illustrated in view B. (Refer to NEETS, Module 9, Introduction to Wave-Generation and Wave-Shaping Circuits, for a review of wave shaping.)
Azimuth indication of the ppi requires that the range trace rotate about the center of the screen. A
the crt in synchronization with the antenna motion. This method, however, has the disadvantages of
inaccuracy and maintenance complications inherent to any mechanical gear-train assembly.
rotate the magnetic field. Figure 3-9 illustrates a method of electronically producing a rotating sweep. In
view A, a range sweep current, i, is applied to the vertical deflection coils only. The resulting magnetic
field, represented by !, lies along the axis of these coils. The resulting range trace, shown by the short
straight line, is vertical because the electron beam is deflected perpendicular to the magnetic field. In view
between the axes of the two sets of coils. Because of this shift of the magnetic field, the range trace is
rotated 45 degrees clockwise from its previous position. In view C, the sweep current is applied to the
horizontal deflection coils only, and the range trace lies 90 degrees clockwise from its original position.
as illustrated in views D and E.
3-10
information into electrical signals. These signals, usually provided by synchros, control the amplitudes
and polarities of the sawtooth sweep currents applied to the deflection coils.
Figure 3-10 illustrates the waveforms of current required to produce a rotating range sweep. The
rotation of the antenna. Notice that there is a 90 degree phase difference between the amplitude variations
of the horizontal and vertical waveforms.
3-11
A ppi requires a crt in which the screen is coated with a long-persistence phosphor. This is necessary
because each target reflects energy for only a short period of time during each rotation of the antenna.
of antenna rotation when the target is not reflecting energy.
Q4. What coordinates are presented on a ppi scope?
Q5. What type of deflection is preferred for a crt electron beam?
Q6. Which of the two types of deflection coils (fixed or rotating) is used most often?
The accuracy of target-range data provided by a radar varies with the use of the radar. For example, a
its maximum range. However, an intercept radar, operating in a tracking mode, must supply range data
that is even more accurate; it must be within a few yards of the actual range.
engraved range scale. This overlay enables the operator to estimate the range of targets. In other
applications, electronic range marks are supplied to the indicator. They usually appear as vertical pulses
3-12
on A-scopes and as concentric circles on ppi scopes. The distance between range marks is generally
determined by the type of equipment and its mode of operation.
marker may be used. The range marker is obtained from a range-marker generator and may be a movable
range gate or range step. When a ppi scope is used, a range circle of adjustable diameter may be used to
measure range accurately. In some cases, movement of the range marker is accomplished by adjusting a
calibrated control from which range readings are obtained.
GENERATOR. The range-gate generator, used in conjunction with a blocking oscillator, generates a
movable range gate. The range-marker generator and the range-step generator, used in conjunction with
an astable multivibrator, generate fixed range marks and a movable range step, respectively.
Figure 3-11 shows a simplified block diagram of a typical range-gate generator. The pulse-repetition
thyratron (both in the synchronizer). The output of the trigger thyratron is used to trigger the radar
same instant, referred to as time T0.
sawtooth to the sweep amplifier. The width of the gate and sawtooth is dependent upon the range selected
by the radar operator.
range-gate pulse. The delay of this pulse from time T0 is dependent on either the range of the target when
3-13
tracking (in the search mode). The range-gate triggers the range-strobe multivibrator, from which the
11. This range gate is used to select the target to be tracked. When in the track mode, the range gate
brightens the trace or brackets the blip (depending on the system) to indicate what target is being tracked.
can also be used with ppi presentations. When used with a ppi presentation, the range gate must be
movable in both range and bearing.
The range-gate generator can easily be modified to produce a range strobe instead of a range gate. A
range strobe is simply a single brightened spot that is movable both in range and bearing. In operation, the
operator.
Several types of range-marker generators are in common use. Figure 3-12 shows a simplified version
of a circuit that produces both range markers and the basic system timing triggers. The master oscillator in
this case is a blocking oscillator that operates at a frequency of 80.86 kilohertz. By dividing 80.86
kilohertz into 1 (t = 1/frequency), we find the time required for one cycle of operation is 12.36
microseconds. Thus the blocking oscillator produces pulses 1 radar mile apart. These are fed to the 5:1
divider circuit. Five of the 1-mile marks are required to produce an output from the divider circuit. These
five-mile marks are sent to the indicator for display and to the 10:1 divider circuit. In the latter case, ten of
the five-mile marks are required to produce an output from the 10:1 divider. Thus the output triggers are
50 miles apart. These basic timing triggers are for a radar with a range of fifty miles. The period between
triggers could be extended through the use of additional dividers for use with longer range systems.
Another version of a range-mark generator is shown in figure 3-13. This circuit provides range
marks at 1,000-, 2,000-, or 3,000-yard intervals. Generation of the marks begins with the ringing
oscillator, which is started by a delayed master trigger from the synchronizer. A ringing oscillator
produces a sinusoidal output of a fixed duration and frequency when triggered. The output is
synchronized to the input trigger. In this circuit, the trigger causes the oscillator to produce a 162-
kilohertz signal that lasts for 4 1/2 cycles. The emitter follower isolates the ringing oscillator from the
countdown multivibrator and clips the oscillator output signals. This action allows only the positive half
of each sine wave to reach the multivibrator. The positive triggers from the ringing oscillator are at 1,000-
yard intervals. This input signal results in an output from the countdown multivibrator of 1,000-, 2,000-,
or 3,000-yard range marks, depending on the position of the RANGE MARK SELECT SWITCH.
3-14
The range step is often used to determine target range on an A-scope presentation. The appearance of
a range step on an A-scope is illustrated in figure 3-14.
generator, a negative clipper, a range potentiometer, and a limiting amplifier. The position of the range
step along the indicator's time base is controlled by the range potentiometer. When the range step
readout associated with the potentiometer.
3-15
View B shows the time relationships of the voltage waveforms produced by the range-step generator.
During the sweep gate, the sawtooth generator produces a sawtooth voltage that is sent to the clipper. The
point at which the sawtooth is clipped is controlled by the range potentiometer. The clipped sawtooth is
shaped in the limiting amplifier to produce the output voltage waveform. The portion of the output
display shown in figure 3-14.
Q7. What type of ranging circuit is most often used with a radar that requires extremely accurate range data?
Q8. The range sweep in a range-gate generator is started at the same time as what other pulse?
Q9. Range-marker generators produce pulses based on what radar constant?
Q10. What radar scope uses a range step for range measurement?
3-16
In this section, we will briefly review the requirements of radar antennas. Antenna characteristics are
discussed in detail in NEETS, Module 10, Introduction to Wave-Generation, Transmission Lines, and Antenna and in Module 11, Microwave Principles. A review of these modules would be helpful at this point to prepare you for the following radar antenna discussion.
Antennas fall into two general classes, OMNIDIRECTIONAL and DIRECTIONAL.
Omnidirectional antennas radiate rf energy in all directions simultaneously. They are seldom used with
and in countermeasures receivers for the detection of enemy radar signals. Directional antennas radiate rf
given antenna position. The radiation pattern also contains minor lobes, but these lobes are weak and
normally have little effect on the main radiation pattern. The main lobe may vary in angular width from
one or two degrees in some radars to 15 to 20 degrees in other radars. The width depends on the system's
purpose and the degree of accuracy required.
Directional antennas have two important characteristics, DIRECTIVITY and POWER GAIN. The
directivity of an antenna refers to the degree of sharpness of its beam. If the beam is narrow in either the
horizontal or vertical plane, the antenna is said to have high directivity in that plane. Conversely, if the
beam is broad in either plane, the directivity of the antenna in that plane is low. Thus, if an antenna has a
directivity is low.
required to cover the same range because the power is concentrated. Thus, the other characteristic of an
antenna, power gain, is introduced. This characteristic is directly related to directivity.
Power gain of an antenna is the ratio of its radiated power to that of a reference (basic) dipole. Both
position. A single point of measurement for the power-gain ratio must lie within the radiation field of
each antenna. An antenna with high directivity has a high power gain, and vice versa. The power gain of a
single dipole with no reflector is unity. An array of several dipoles in the same position as the single
depend on the directivity of the array.
position. The angle may be measured either from true north (true bearing), or with respect to the bow of a
ship or nose of an aircraft containing the radar set (relative bearing). The angle at which the echo signal
returns is measured by using the directional characteristics of the radar antenna system. Radar antennas
energy. A pattern produced in this manner permits the beaming of maximum energy in a desired
direction. The transmitting pattern of an antenna system is also its receiving pattern. An antenna can
therefore be used to transmit energy, receive energy, or both. The simplest form of antenna for measuring
azimuth (bearing) is a rotating antenna that produces a single-lobe pattern.
angle or as altitude. If one is known, the other can be calculated from basic trigonometric functions. A
method of determining the angle of elevation or the altitude is shown in figure 3-16. The slant range is
obtained from the radar scope as the distance to the target. The angle of elevation is the angle between the
axis of the radar beam and the earth's surface. The altitude in feet is equal to the slant range in feet
multiplied by the sine of the angle of elevation. For example if the slant range in figure 3-16 is 2,000 feet
3-17
and the angle of elevation is 45 degrees, the altitude is 1,414.2 feet (2,000 × .7071). In some radar equipments that use antennas that may be moved in elevation, altitude determination is automatically
computed.
sharp nor too directive. On the other hand, a PLANE wavefront does not spread out because all of the
wavefront moves forward in the same direction. For a sharply defined radar beam, the need exists to
change the spherical wavefront from the antenna into a plane wavefront. A parabolic reflector is one
means of accomplishing this.
Radio waves behave similarly to light waves. Microwaves travel in straight lines as do light rays.
They may be focused and/or reflected just as light rays can. In figure 3-17, a point-radiation source is
placed at the focal point F. The field leaves this antenna with a spherical wavefront. As each part of the
cause all parts of the field to travel in parallel paths. Because of the shape of a parabolic surface, all paths
from F to the reflector and back to line XY are the same length. Therefore, all parts of the field arrive at
line XY the same time after reflection.
3-18
(dotted lines in figure 3-17) as well as toward the reflector. Energy that is not directed toward the
reflector. This occurrence is prevented by the use of a hemispherical shield (not shown) that directs most
radiation toward the parabolic surface. By this means, direct radiation is eliminated, the beam is made
sharper, and power is concentrated in the beam. Without the shield, some of the radiated field would leave
the radiator directly. Since it would not be reflected, it would not become a part of the main beam and
thus could serve no useful purpose. The same end can be accomplished through the use of a PARASITIC
paraboloid.
revolution, and several minor lobes, as shown in figure 3-18. Very narrow beams are possible with this
type of reflector. View A of figure 3-19 illustrates the parabolic reflector.
View B of figure 3-19 shows a horizontally truncated paraboloid. Since the reflector is parabolic in
the horizontal plane, the energy is focused into a narrow horizontal beam. With the reflector truncated, or
cut, so that it is shortened vertically, the beam spreads out vertically instead of being focused. Since the
beam is wide vertically, it will detect aircraft at different altitudes without changing the tilt of the antenna.
It also works well for surface search radars to overcome the pitch and roll of the ship.
3-19
degrees, as shown in view C. Because the reflector is now parabolic in the vertical plane, the energy is
focused into a narrow beam vertically. With the reflector truncated, or cut, so that it is shortened
horizontally, the beam spreads out horizontally instead of being focused. Such a fan-shaped beam is used
to determine elevation very accurately.
of its shape, is shown in view D of figure 3-19. Since the reflector is narrow in the horizontal plane and
wide in the vertical, it produces a beam that is wide in the horizontal plane and narrow in the vertical. In
shape, the beam resembles a huge beaver tail. This type of antenna system is generally used in height-
finding equipment.
is desired, a cylindrical paraboloidal section approximating a rectangle can be used. View E of figure 3-19
illustrates this antenna. A parabolic cross section is in one dimension only; therefore, the reflector is
directive in one plane only. The cylindrical paraboloid reflector is either fed by a linear array of dipoles, a
slit in the side of a waveguide, or by a thin waveguide radiator. Rather than a single focal point, this type
of reflector has a series of focal points forming a straight line. Placing the radiator, or radiators, along this
focal line produces a directed beam of energy. As the width of the parabolic section is changed, different
beam shapes are obtained. This type of antenna system is used in search and in ground control approach
(gca) systems.
Q11. Which of the two general classes of antennas is most often used with radar?
Q12. The power gain of an antenna is directly related to what other antenna property?
Q13. A parabolic reflector changes a spherical wavefront to what type of wavefront?
3-20
corner, as shown in view F of figure 3-19. This reflector is normally driven by a half-wave radiator
located on a line which bisects the angle formed by the sheet reflectors.
shown in figure 3-20. The broadside array consists of two or more half-wave dipole elements and a flat
reflector. The elements are placed one-half wavelength apart and parallel to each other. Because they are
excited in phase, most of the radiation is perpendicular or broadside to the plane of the elements. The flat
the unidirectional characteristics of the antenna system.
frequencies. Because they do not involve resonant elements, horns have the advantage of being usable
over a wide frequency band.
The operation of a horn as an electromagnetic directing device is analogous to that of acoustic horns.
the wavelength being used.
impedance-matching device and as a directional radiator. Horn radiators may be fed by coaxial or other
types of lines.
Horns are constructed in a variety of shapes as illustrated in figure 3-21. The shape of the horn, along
with the dimensions of the length and mouth, largely determines the field-pattern shape. The ratio of the
horn length to mouth opening size determines the beam angle and thus the directivity. In general, the
larger the opening of the horn, the more directive is the resulting field pattern.
3-21
A waveguide horn, called a FEEDHORN, may be used to feed energy into a parabolic dish. The
directivity of this feedhorn is added to that of the parabolic dish. The resulting pattern is a very narrow
and concentrated beam. In most radars, the feedhorn is covered with a window of polystyrene fiberglass
to prevent moisture and dirt from entering the open end of the waveguide.
path of the beam.  To solve this problem the
feedhorn can be offset from center. This location change takes the feedhorn out of the path of the rf beam
and eliminates the shadow. An offset feedhorn is shown in figure 3-22.
Airborne radar equipment is used for several specific purposes. Some of these are bombing,
navigation, and search. Radar antennas for this equipment are invariably housed inside nonconducting
radomes, not only for protection but also to preserve aerodynamic design. Some of these radomes are
carried outside the fuselage, while others are flush with the skin of the fuselage. In the latter case, the
nonconducting radome. The radar antenna and its radome must operate under a wide variety of
temperature, humidity, and pressure conditions. As a result, mechanical construction and design must
minimize any possibility of failure. Transmission lines are usually hermetically sealed to prevent moisture
3-22
accumulation inside them. Such accumulation would introduce losses. Because the low air pressures
(the pressure is maintained by a small air pump). In some airborne radar equipments, practically all of the
equipment is sealed in an airtight housing, along with the antenna and transmission line. The antenna
radome forms a portion of the housing.
antennas are rigidly attached to the airframe. The weight of the radar antenna, including the rotating
mechanism required for scanning, is kept to a minimum. In addition, the shape of the radome is
constructed so as not to impair the operation of the aircraft.
The airborne radar antenna must have an unobstructed view for most useful operation. Frequently,
the antenna must be able to scan the ground directly under the aircraft and out toward the horizon. To
meet this requirement, the antenna must be mounted below the fuselage. If scanning toward the rear is not
required, the antenna is mounted behind and below the nose of the aircraft. If only forward scanning is
needed, the antenna is mounted in the nose. When an external site is required, a location at the wing tip is
common. A fire-control radar antenna is frequently located near the turret guns or in a special nacelle,
where it can scan toward the rear or sides of the aircraft.
Q14. How many major lobes are produced by a paraboloid reflector?
Q15. What type of radiator normally drives a corner reflector?
Q16. The broadside array consists of a flat reflector and what other elements?
Q17. Horn radiators serve what purpose other than being directional radiators?
The following is a brief summary of the important points of this chapter.
A radar INDICATOR presents the information (video) from the radar receiver in a usable manner. The display usually consists of one or more of the coordinates of range, bearing, and altitude.
The CATHODE-RAY TUBE (crt) is the best available device for displaying the two-dimensional relationship produced by radar coordinates. The most commonly used crt displays are the A-SCOPE, the
RHI, and the PPI. The A-scope presents range information only. The rhi displays range and height
information. The ppi is the most widely used radar display indicator and presents range and bearing.
3-23
The range of a radar contact is determined by special RANGING CIRCUITS. The following three basic types of ranging circuits are used.
RANGE-GATE GENERATORS produce a movable gate that measures range based on elapsed time and can be used on A-scope and ppi displays.
RANGE-MARKER GENERATORS produce fixed interval range marks that can be used to estimate the range to a detected target. Range marks appear as an intensified series of vertical dots on an
rhi and as concentric circles on a ppi.
The RANGE-STEP GENERATOR produces a movable step that is displayed on an A-scope presentation.
RADAR ANTENNAS are usually directional antennas that radiate energy in a one directional lobe or beam. The two most important characteristics of directional antennas are directivity and power gain.
into a desired beam pattern. Other types of antennas used with radar systems are the corner reflector, the
broadside array, and horn radiators.
ANSWERS TO QUESTIONS Q1. THROUGH Q17.
A1. Range, bearing, and elevation.
A2. Triggers, video, and antenna information.
A3. Range and elevation.
A4. Range and bearing.
A5. Electromagnetic.
A6. Fixed.
A7. Range gate or range step.
A8. Transmitter.
A9. The radar mile (12.36 microseconds).
A10. The A scope.
A11. Directional.
A12. Directivity.
A13. Plane.
A14. One.
A15. Half-wave.
A16. Two or more half-wave dipoles.
A17. Waveguide impedance matching devices.
4-1
Upon completion of this chapter, the student will be able to:
1. Interpret the transmitter frequency spectrum in terms of frequency distribution, power output, receiver response, and an acceptable spectrum curve.
2. Describe the methods for measuring the average and peak power outputs of a radar transmitter.
3. Describe the methods of measuring receiver sensitivity.
4. Define receiver bandwidth in terms of the receiver response curve and state the most common methods of measuring tr tube recovery time.
5. List the support systems associated with a typical shipboard radar system and describe the basic function of each.
6. State the general rules for the prevention of personnel exposure to rf radiation and X-ray emissions.
The effectiveness of your radar system depends largely upon the care and attention you give it. An improperly adjusted transmitter, for example, can reduce the accuracy of a perfectly aligned receiver; the entire system then becomes essentially useless. Maintenance, therefore, must encompass the entire system for best operation.
Because of the complexity of most radar systems, trying to detail step-by-step procedures for specific maintenance actions in this chapter is impractical. However, the basic procedures for some maintenance actions that are common to most radar systems will be discussed. Also, an overview of support systems for radars will be presented. This will include electrical power, dry-air systems, and liquid cooling systems. Finally, safety precautions inherent to radars are listed.
The transmitter of a radar is designed to operate within a limited band of frequencies at an optimum power level. Operation at frequencies or power levels outside the assigned band greatly decreases the efficiency of the transmitter and may cause interference with other radars. Therefore, transmitter performance must be monitored closely for both frequency and output power.
4-2
Whether of the fixed-frequency or tunable type, the radar transmitter frequency should be checked periodically. If the transmitter is of the fixed-frequency type and found to be operating outside its normal operating band, the problem is probably a defective part. The defective component must be replaced. If the transmitter is tunable, the transmitter must again be tuned to the assigned frequency.
Each time a radar transmitter generates an rf pulse, it produces electromagnetic energy. You should recall from your study of NEETS, Module 12, Modulation Principles, that the square wave used to modulate the transmitter carrier wave has (1) the fundamental square-wave frequency and (2) an infinite number of odd harmonics of the fundamental square wave frequency. When this square wave is used to modulate the transmitter carrier frequency, both the fundamental and odd harmonic frequencies of the square wave heterodyne with the transmitter carrier frequency. The heterodyning process produces in each transmitted rf pulse the following frequencies:
1. The fundamental carrier frequency
2. The sum and difference frequencies between the carrier and fundamental square-wave frequencies
3. The sum and difference frequencies between the odd harmonics of the square wave and the carrier frequencies
For a complete discussion of this process, you should review module 12.
Actually, the radar energy is distributed more or less symmetrically over a band of frequencies. This frequency distribution of energy is known as the FREQUENCY SPECTRUM. An analysis of frequency spectrum characteristics may be made with a SPECTRUM ANALYZER. The spectrum analyzer presents a graphic display of energy versus frequency. An extensive explanation of spectrum analyzer use can be found in the Electronics Installation and Maintenance Book (EIMB), Test Methods and Practices, NAVSEA 0967-LP-000-0130.
When properly performed and interpreted, a spectrum analysis will reveal misadjustments and troubles that would otherwise be difficult to locate. Therefore, you should be able to perform a spectrum analysis and understand the results.
You may be wondering why we are so interested in the frequency spectrum of an rf pulse. To better understand why, look at the spectrum of a transmitter as compared to the response curve of a receiver in figure 4-1. The receiver's response curve has a broader bandwidth than the transmitted spectrum, which ensures complete coverage. But the receiver responds best to frequencies in the middle of the bandwidth. This causes the receiver response to taper off from both sides of the center frequency until the response passes through the half-power points, as shown on the curve. Usually the receiver response beyond these points is too low to be useful and is not considered. Notice that the spectrum of the transmitter is centered inside the response curve of the receiver, thus yielding maximum efficiency.
4-3
Any frequency, when modulated by another frequency, will produce a base frequency with sideband frequencies (sum and difference). In other words, the output of a pulsed radar will contain more than one frequency. The output frequency spectrum of the pulsed radar transmitter does not consist of just a single frequency that is turned on and off at the pulse-repetition frequency (prf). Consider the spectrum as a base frequency (carrier) that is modulated by short rectangular pulses occurring at the prf of the radar. Two distinct modulating components are present: One component consists of the prf and its associated harmonics; the other component consists of the fundamental and odd-harmonic frequencies that make up the rectangular modulating pulse.
The distribution of power over the radar frequency spectrum depends on the amount of modulation. A pulsed radar spectrum is illustrated in figure 4-2. The vertical lines represent the modulation frequencies produced by the prf and its associated harmonics; the lobes represent the modulation frequencies produced by the fundamental pulse frequency and its associated harmonics. The amplitude of the main lobe falls to zero on each side of the carrier. The side lobes are produced by the odd harmonics of the fundamental pulse frequency. The zero points are produced by the even harmonics of the fundamental pulse frequency. In an ideal spectrum each frequency above the carrier has its counterpart in another frequency below the carrier. These frequencies are equally spaced and have equal power. Therefore, the pattern is symmetrical about the carrier. The main lobe, of course, contains the major portion of the transmitted rf energy.
4-4
A radar transmitter in good condition should produce a spectrum curve similar to the curves shown in view A or B in figure 4-3. Good curves are those in which the two halves are symmetrical and contain deep, well-defined minimum points (minima) on both sides of the main peak.
A curve without well-defined minima, as in the curve shown in view C, indicates that the transmitter output is being frequency modulated during the pulse. This condition may occur when a pulse without sufficiently steep sides or a flat peak is applied to the transmitter. It may also occur when a transmitter tube is unstable or is operated without proper voltage, current, or magnetic field.
An extremely irregular spectrum, as in the curve in view D, is an indication of severe frequency modulation. This condition usually causes trouble with the receiver automatic frequency control (afc) as well as a general loss of signal strength. You can often improve a faulty spectrum by adjusting the transmission line stubs or by replacing the transmitter tube. When the spectrum has two large peaks that are quite far apart, it indicates that the transmitter tube is DOUBLE MODING (shifting from one frequency to another). This could be caused by standing waves in the transmission line or a faulty transmitter tube. Standing waves may be caused by a faulty line connection, a bad antenna rotating joint, or obstructions in the line. (Standing waves are described in NEETS, Module 10, Introduction to Wave Propagation, Transmission Lines, and Antennas.)
4-5
In the case of a good or fair spectrum curve with sharply defined minimum points on both sides of the main lobe, the distance between these two points is proportional to the duration of the transmitted pulse.
The device most commonly used to check the frequency spectrum of a radar transmitter is the spectrum analyzer.
Devices used to determine the basic carrier frequency of a radar transmitter are the ELECTRONIC FREQUENCY COUNTER, the WAVEMETER, and the ECHO BOX. One or more of these devices may be included in a special RADAR TEST SET designed for a specific system or type of radar. Radar test sets quite often consist of several types of test equipment. This combination of test equipments enables both transmitter and receiver performance checks to be carried out with one test instrument. Electronic frequency counters, frequency meters, and wavemeters are discussed in NEETS, Module 16, Introduction to Test Equipment. The echo box is discussed in the next section. The specific equipments and procedures required to measure the frequency of any radar system are found in the associated system technical manuals and related PMS documents.
Q1. The spectrum of a radar transmitter describes what characteristic of the output pulse?
Q2. Where should the transmitter spectrum be located with respect to the receiver response curve?
Q3. The ideal radar spectrum has what relationship to the carrier frequency?
Q4. The display screen of a spectrum analyzer presents a graphic plot of what two signal characteristics?
The ECHO BOX is an important test instrument for indicating the overall radar system performance. The echo-box test results reflect the combined relative effectiveness of the transmitter as a transmitter of energy and the receiver as a receiver of energy.
The echo box, or RESONANCE CHAMBER, basically consists of a resonant cavity, as shown in view A of figure 4-4. You adjust the resonant frequency of the cavity by varying the size of the cavity (the larger the cavity the lower the frequency). A calibrated tuning mechanism controls the position of a plunger and, therefore, the size of the cavity. The tuning mechanism is adjusted for maximum meter deflection, which indicates that the echo box is tuned to the precise transmitted frequency. The tuning mechanism also indicates on a dial (figure 4-5, view A) both the coarse transmitted frequency and a numerical reading. This reading permits the technician to determine the transmitted frequency with greater accuracy by referring to a calibration curve on a chart (figure 4-5, view B).
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Energy is coupled into the cavity from the radar by means of an rf cable connected to the input loop. Energy is coupled out of the cavity to the rectifier and meter by means of the output loop. You can vary the amount of coupling between the echo box and the crystal rectifier by changing the position of the output loop. A schematic diagram of the output circuit is shown in figure 4-4, view B. The energy picked up by the loop is rectified, filtered, and applied to the meter. The method of connecting the echo box in a radar system is shown in figure 4-4, view C.
Some of the energy generated by the radar transmitter is picked up by the echo box by means of the directional coupler. This energy causes oscillations (known as RINGING) within the echo box that persist for some time after the end of the radar pulse, much in the fashion of an echo that persists in a large room after a loud noise. As this echo dies down, a part of it is fed back into the radar receiving system, again by means of the directional coupler. The ringing causes a saturating signal to appear on the radar indicator (figure 4-6). The longer this ringing extends, the better the performance of the radar.
The length of time the echo box should ring under the particular conditions of the test is called the EXPECTED RING TIME. You may determine whether or not the radar is performing well by comparing the expected ring time with the ring time observed.
The ring time to be expected on a good radar depends on the particular type of radar being tested; on the way the echo box is installed - that is, whether a directional coupler or a pickup dipole is used; on the length and type of cable used; on the individual ringing ability of the particular echo box in use; on the frequency of the radar; and on the temperature of the echo box at the time of the test. Corrections are made for all of these factors according to the procedure given in the technical manual for the echo box being used.
You may use an echo box without correction to detect a change in the performance of a radar. You simply log and compare the ring time from day to day. You should recognize that these readings do not
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permit the comparison of a particular radar with a standard of performance; however, you can use the readings to tell whether or not its performance is deteriorating.
Because ring time measurements are the most valuable single feature of the echo box, they must be measured properly. Ring time measurements are made on the A-scope or on the ppi.
In measuring the ring time, you should make sure the echo-box ringing (not some fixed-target echo or block of echoes) is being monitored. You can determine this condition by adjusting the radar gain control and noting if the ring time varies on the scope. The echo box ringing will change in duration; fixed target echoes, however, will not change duration.
To obtain the best results, you should repeat every ring time measurement at least four times; then average the readings. You should take special care to ensure that all readings are accurate. If two or more technicians use the same echo box, they should practice together until their ring time measurements agree.
Because high peak power and radio frequencies are produced by radar transmitters, special procedures are used to measure output power. High peak power is needed in some radar transmitters to produce strong echos at long ranges. Low average power is also desirable because it enables transmitter components to be compact, more reliable, and to remain cooler during operation. Because of these considerations, the lowest possible duty cycle (pw x prf) must be used for best operation. The relationships of peak power, average power, and duty cycle were described in chapter 1. Peak power in a radar is primarily a design consideration. It depends on the interrelationships between average power, pulse width, and pulse-repetition time.
You take power measurements from a radar transmitter by sampling the output power. In one sampling method, you use a pickup horn in front of the antenna. Air losses and weather conditions make the horn placement extremely critical and also affect the accuracy of the sample. A more accurate and convenient method can be used. In this method, you sample the output power through a directional sampling coupler located at the point in the transmitter where a power reading is desired. Power-amplifier transmitters usually have sampling couplers after each stage of amplification.
Some radar sets have built-in power-measuring equipment; others require the use of general purpose test equipment or a special test set. In any case, the measuring instruments are most often referenced to 1 milliwatt; readings are taken in dBm (a discussion of the decibel measurement system was presented in NEETS, Module 11, Microwave Principles).
When taking power measurements, you must allow for power losses. You must add the directional coupler attenuation factor and the loss in the connecting cable to the power meter reading. The sum is the total power reading. For example, the directional coupler has an attenuation factor of 20 dB, the connecting cable has a loss rating of 8 dB, and the reading obtained on the power meter is 21 dBm. Therefore, the transmitter has an output power that is 49 dBm (21 + 20 + 8). Power readings in dBm obtained by the above procedure are normally converted to watts to provide useful information. Although the conversion can be accomplished mathematically, the procedure is relatively complex and is seldom necessary. Most radar systems have a conversion chart, such as the one shown in figure 4-7, attached to the transmitter or the test equipment. As you can see on the chart, 49 dBm is easily converted to 80 watts average power.
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You can convert average power to peak power by dividing average power by the duty cycle of the radar. If the radar in the above example has a duty cycle of 0.001, then the peak power can be calculated with the following formula:
Many radar systems have charts available to convert average power to peak power.
Q5. The peak power of a radar depends on the interrelationship of what other factors?
Q6. Transmitter power readings are most often referenced to what power level?
The performance of a radar receiver is determined by several factors, most of which are established in the design engineering of the equipment. In the paragraphs that follow, factors concerned with
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maintenance are considered. Important factors are (1) receiver sensitivity, which includes noise figure determination and minimum discernible signal (mds) measurement; (2) tr recovery time; and (3) receiver bandwidth.
Many radar systems contain circuits that serve special functions. Three of these special circuits are instantaneous automatic gain control (iagc), sensitivity time control (stc), and fast time constant (ftc). These circuits may be found in combination or alone, depending on the purpose of the radar. When the test methods and procedures about to be described are used, these special functions should not be used. If an automatic frequency control (afc) circuit is included in the radar, it may be permitted to operate during receiver tests. A good way you can check afc circuit operation is to complete the tests specified for manual tuning and then switch to afc. If the afc circuit operation is normal, test indications should not differ.
Insufficient detection range in a radar system can be caused by decreased sensitivity in the radar receiver. This condition results mainly from the great number of adjustments and components associated with the receiver. A decrease of receiver sensitivity has the same effect on range performance as does a decrease of transmitter power. For example, a 6 dB loss of receiver sensitivity shortens the effective range of a radar just as much as a 6 dB loss in transmitter power. Such a drop in transmitter power is evident and is easy to detect. On the other hand, a 6 dB loss in receiver sensitivity, which can easily result from a slight misadjustment in the receiver, is difficult to detect unless accurate measurements are made.
Figure 4-8 shows a comparison of radar system performance versus maximum range. The system performance loss in dB includes both transmitter and receiver losses. You should note that with a loss of 5 dB in both receiver and transmitter (a total of 10 dB), only 55 percent of the maximum range of the system is realized.
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The sensitivity of the radar receiver is a measure of its ability to pick up weak signals. The greater the sensitivity of the receiver, the better the receiver picks up weak signals. You can determine receiver sensitivity by measuring the power level of the MINIMUM DISCERNIBLE SIGNAL (mds). Mds is defined as the weakest signal that produces a visible receiver output (on a scope). Its value is determined by the receiver output noise level (noise tends to obscure weak signals). Because mds measurement depends on the receiver noise level, measuring either mds or noise level (called NOISE FIGURE) will indicate receiver sensitivity.
Many radar systems have built-in receiver sensitivity test circuits. These test circuits indicate the sensitivity of the receiver to the technician or operator.
To measure the mds, you must measure the power of a test pulse in which the level is just sufficient to produce a visible receiver output. If a radar receiver has the mds level specified in the maintenance manual, then the noise figure should also be correct. Therefore, measurement of the mds is a satisfactory substitute for a noise-figure determination and is less complicated.
Because receiver sensitivity readings are taken periodically for comparison purposes, the identical pulse length must be used for each measurement. Maintenance instructions for the radar set usually specify the correct pulse length to be used in receiver sensitivity tests. In most cases, it is the same as the transmitter pulse length.
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Before any measurements of receiver sensitivity can be made, the receiver must be accurately tuned to the transmitter frequency. If the receiver frequency differs from the transmitter frequency, the most likely cause is an improperly adjusted or malfunctioning local oscillator or transmitter frequency drift. Such problems can be caused by heat or aging components. Local oscillator tuning procedures differ widely according to the type of radar system; therefore, you should follow the tuning procedures in the system maintenance manuals.
Two basic methods are used to measure radar receiver sensitivity. One is the PULSE METHOD, in which a pulse of measured amplitude and width is coupled to the receiver. In the second method, you use an fm generator to vary the signal generator output frequency across the receiver bandwidth. This latter method ensures the test signal is within the bandpass of the receiver.
The sensitivity of the receiver is equal to the sum of the reading on the signal generator and the attenuations of the connecting cable and directional coupler. Receiver sensitivity is expressed as a negative dBm; for example, -90 dBm expresses the sensitivity of a receiver that can detect a signal 90 dB less than the 1-milliwatt reference level. A typical receiver sensitivity reading on a modern radar should be in the vicinity of -105 dBm.
Receiver bandwidth is defined as the frequency spread between the half-power points on the receiver response curve. Receiver bandwidth is specified for each radar, but wide variations are often tolerated. If either the bandwidth or the shape of the receiver response curve is not within tolerances, a detailed check of circuit components may be necessary. A considerable change in the value of circuit components is required to alter the response. You should check receiver response after any extensive repair to an IF amplifier.
Figure 4-9 shows a typical response curve of a radar receiver. The half-power points are shown as 3 dB below maximum response. Since the curve is plotted in terms of voltage, these points are also represented by the 70.7 percent voltage points as shown in the figure.
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The time required for tr recovery is determined by the time taken by the tr switch (tube) to deionize after each transmitter pulse. It is usually defined as the time required for the receiver to return to within 6 dB of normal sensitivity after the end of the transmitter pulse. However, some manufacturers use the time required for the sensitivity to return to within 3 dB of normal sensitivity. Tr recovery time is a factor that limits the minimum range of a radar because the radar receiver is unable to receive until the tr switch is deionized. In various radars, the recovery time may differ from less than 1 microsecond to about 20 microseconds.
The primary function of the tr switch is to protect the sensitive crystal detectors from the powerful transmitter pulse. Even the best tr switches allow some power to leak through; but when the switch is functioning properly, leakage power is so small that it does not damage the crystal. However, the useful life of a tr tube is limited because the amount of leakage to the receiver increases with use.
To ensure efficient performance, some technicians make a policy of replacing the tr tube after a certain number of hours of use. A better practice is to measure the tr recovery time at frequent intervals and make a graph or chart. A graph or chart will immediately disclose any change in performance. Figure 4-10 shows how the recovery time is correlated with leakage power. Note that the end of the useful life of the tr tube is indicated by an increase in recovery time.
This method of checking the condition of a tr tube is reliable because recovery time increases before leakage power becomes excessive. In practice, a tr tube is replaced when any sharp increase in recovery time becomes apparent.
Ambient temperature also has an effect on recovery time. The colder a tr tube, the greater its recovery time. When tests are conducted under widely varying temperature conditions, this effect must be considered.
One method you can use in testing a tr tube is to measure the KEEP-ALIVE current. This current keeps the tr tube partially ionized, which makes the firing more instantaneous and thus helps protect the
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receiver crystals. The keep-alive current is normally about 100 microamperes but falls off as the end of the tr tube life approaches. You can also measure the keep-alive voltage between the plate of the tr tube and ground when the voltage source is known to have the correct output. You then record this voltage for use as a reference for future checks. However, these checks are not as reliable as recovery time testing.
Specific procedures for measuring tr leakage and recovery time can be found in the equipment technical manuals.
Q7. A loss of receiver sensitivity has the same effect on range performance as what other loss?
Q8. You determine receiver sensitivity by measuring the power level of what signal?
Q9. When measuring receiver sensitivity, what quantities must you add to the dBm reading obtained on the signal generator or test set?