Radar System Engineering

SEC. 16.17] THE RECEIVER 665
~ mph at 10 cm. Full circuit information can be found k a report
by V.A. Olson.’
16.17. The Receiver.—Two types of receiver characteristic have
been discussed in Sec. 168. The lin-log type is necessary in a noncoherent
system, and the limiting type is best for coherent systems.
ThA Lin-log Charoxteristic.-A linear-logarithmic characteristic is
linear for small signals and limits in a specified gradual fashion for large
signals. A method for achieving this response is shown in Fig. 16.29.
Output signals from several successive i-f stages are combined. At low
signal levels, the last stage delivers a linear signal in the normal fashion,
the output from the preceding stages being negligible. As the signal level
increases, the output signal of this last stage reaches a saturation level
If echo signal
[f amplifier
t ! I
1
Reference
signal
Detector Detector Detector
t t t
Video
Video
Video
amplifler amplifier amplifier
I I t
Video output signal
FIG. 16.29.—Circuit to give lin-log response,
above which it cannot rise regardless of further input-signal increases.
At this point the output signal from the preceding stage has become
appreciable; it continues to increase with increasing input signal. NOW,
however, the increase in output-signal level is at a slower rate because the
gain of the last stage is not available. Eventually the second stage
overloads and the third from the last becomes effective. The amplitude
response curve of such a circuit can be adjusted to be accurately
linear-logarithmic. 2
The Limiting Receiver.—The block diagram of a typical limiting
receiver is shown in Fig. 16,30. The arrangement is one that might ,be
appropriate to a system employing a l-psec pulse width and an intermediate
frequency of 30 Me/see. The requirement that the amplitude
response show no inversion above the limit level is one not ordinarily met
1V. A. Oleon, “A Moving Coho Conversion Unit, ” RL Report No. 975, Apr. 3,
1946.
z The design problems involved in this and other MTI receiver circuits are discuaaedin
Vol. 23, Chap. 22.
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