Radar System Engineering

SEC, 59] ML’LTIPLE-TARGET F-M RANGE JIEALSCRE.llEIVT 147
frequency Iowerthan the desired signals, arethus discriminated against.
Roughly speaking, the response should rise linearly with frequency, since,
on a voltage basis, the incoming signals go down at about this rate in
range and thus in the equivalent frequency. This frequency distortion
serves a second purpose in that it makes the output signal less dependent
on altitude so that the limiter does not ha~-e to \vork over such a large
range of amplitude. The complete dc~ice is illustrated in Fig, 5.13.
The effect of doppler shift is to raise or lower the dotted curve (Fig.
5.12) corresponding to the returned signal. With the proportions here
used and with the aircraft in reasonably level flight, the doppler frequency
is almost always less than the beat frequency due to altitude; consequently
the end result is that there are somewhat fewer cycles of beat
frequency in one half of the modulating cycle, and somewhat more in
the other, the total number per cycle remaining the same. The difference
in numbers of cycles in the two halves of the modulation period is then
a measure of the doppler frequency. This difference can be measured
in various ways, and has been used, along with the altitude information,
in various dcvelop:nents of the device here described.
5.9. Multiple-Target F-m Range Measurement.—In the absence of
clutter and doppler shift almost the same methods can be used with a
plurality of targets as with a single target. In this case linear frequency
variation with time is almost essential, and the triangular form of Fig.
5.12 is probably most convenient, though a saw-tooth variation might
be used. The detector output then contains a number of frequencies,
one corresponding to each target range present. Preferably these frequencies
are detected by some device such as a Frahm vibrating-reed
frequency meter, which indicates all frequencies simultaneously. If,
however, time is no object, a device that scans the frequency range may
be used. A variable frequency can be added to the signal frequency and
observations made when the sum frequency falls in the pass band of a
resonant circuit. l“ariants of this idea sweep either j, or Aj and observe
when the target frequency falls in the pass band of a resonant circuit.
Any of these scanning devices greatly increases the time required to
obtain the desired information. Thus if we have a frequency band f
which is to be split into n pieces, the time required for such a device as a
Frahm meter to respond is of the order n/f, whereas if the n frequency
intervals are observed in sequence the time is n2/f.
The general design procedure is essentially the same as that described
in Sec. 5.8. First we decide on the allowable range error & and determine
from this the total frequency swing Aj by means of Eq, (7):