RECEIVER SPECIAL CIRCUITS

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.

Sensitivity Time Control (STC)

In radar receivers, the wide variation in return signal amplitudes make adjustment of the gain 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.

Figure 2-29. - Stc voltage waveform.

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.

Antijamming Circuits

Among the many circuits used to overcome the effects of jamming, two important ones are GATED 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.

Q.41 Which of the two types of automatic gain control, agc or iagc, is most effective in radar use for the Navy?
Q.42 Immediately after the transmitter fires, stc reduces the receiver gain to what level?
Q.43 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.

Moving Target Indicator (mti) System

The MOVING TARGET INDICATOR (mti) system effectively cancels CLUTTER (caused by fixed 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.

Figure 2-30A. - Fixed target cancellation.

Figure 2-30B. - Fixed target cancellation.

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.

Figure 2-31. - Mti block diagram.

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 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.

Q.44 What type of target has a fixed phase relationship from one receiving period to the next?
Q.45 What signal is used to synchronize the coherent oscillator to a fixed phase relationship with the transmitted pulse?
Q.46 What is the phase relationship between the delayed and undelayed video?