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.
