Radar System Engineering

648 MOVING- TARGET lNDICA TION [SEC. 16+3
the oscillator and amplifier shown in Fig. 16.11 and to send the i-f signal,
together with the reference signal, directly through the delay line. The
amplifier then works at the intermediate frequency used in the receiver.
These changes make the circuit simpler and avoid the problems of carrier
modulation and demodulation. However, the loss of freedom in the
choice of frequency for the delay line and the comparison amplifier is a
disadvantage. Also, the method is not suitable for “back-of-dish” radar
sets, since the i-f signals cannot conveniently be brought out through
slip rings. This method of using a lin-log receiver should be used only
when simplicity and compact ness are of prime import ante. In general,
such a receiver should be used in conjunction with an oscillator and
amplifier (Fig. 16. 11) in the usual way.
An important advantage of the limiting receiver is that the output
video signals have a i-ange of amplitude extending from noise only up to
the limit level. This small dynamic range makes it easy to design the
cancellation circuits. For the lin-log receiver, the dynamic range is 6 to
12 db greater under conditions of equal performance.
For general MTI use, the limiting type of receiver is to be preferred.
However, there is a type of MTI system in which moving targets are
detected in the clutter by the fluctuations which they produce in the
clutter amplitude. The lin-log receiver must be used in such a system,
since the limiting receiver cannot detect amplitude changes.
Some idea of the magnitudes involved in the receiver problem can be
obtained from Table 16.4. The fluctuations due to the wind were
TABLE 164.-FLUCTUATIONS DUE TO W’IND AND SCANNING
h,
cm
9.2
9.2
PRF
500
2000
Beamwidth,
degrees
1
3
Pulses per
beam-
width
Scanning
rate,
rpm
4 21
4 250
Wind effect I Scanning effect
rms fluctuation rms fluctuation
rms total echo’ rms total echo’
db
dh
I
–34
–22
–46 –44
obtained from Table 16,2 for wooded terrain and a wind velocity of
25 mph. The scanning effect was derived from the formula for extended
clutter in Sec. 16.7. It will be noticed that the scanning effect predominates
in the case of the low PRF and narrow beam, whereas the two
effects are about equal for the high PRF and wide beam. The fluctuations
in the first case are roughly 10 per cent, considerably greater than
the fluctuations due to instability of the components. On the other
hand, the fluctuations
in the second case are only about 1 per cent and are
therefore negligible compared with system instability.