Radar System Engineering

SEC.5.10] ALTERNATIVE F-M RANGING SYSTEM 149
This ratio depends on target speed, wavelength, repetition rate, and
range accuracy, and might conceivably have almost any value. But in
many cases the value will be too large for this system to be useful,
Specifically, the maximum beat frequency may be written as fr(r/6r) so
that, for example, 1 per cent range accuracy and f, = 10 cps gives
~~ = 1000 CPS, wfich may be compared with a doppler frequency of
894 cps for 10 cm and 100 mph.
6.10. Alternative F-m Ranging System .—Another scheme of the
f-m type which is designed to work on multiple targets and in the presence
of clutter may be understood by reference to Fig. 514. This shows, in
the full curve; a sinusoidal dependence
of the transmitted frequency
on time. The total frequency
swing is made large compared to
the doppler frequency. The received
frequency, indicated by the
dotted line, is a similar curve but
(a) displaced to the right by time
delay resulting from transmission
to the target and back and (b) displaced
vertically by doppler shift.
If now the difference between the
transmitted and received frequencies
is passed through a low-pass
filter whose pass band includes frequencies
as high as the maximum
doppler frequency, there will, in
general, be little signal output because
the difference frequency will
be greater than the doppler frequency
over the majority of the
I
FIG. 5.14.—The full curve in the upper
graph shows the instantaneoustransmitter
frequency as a function of time; the dotted
curve shows the received signal; and the
dashed curve, the beat between the two.
The lower curve showsthe same quantities
when 2r/c = nf,, in which case the beat
frequency is constant at the doppler value.
modulation cycle. If, however, the modulation frequency is adjusted
so that one cycle corresponds to the transmission delay time, the difference
frequency will always just equal the doppler frequent y and so a
large output signal will result. Thus, as the modulation frequency is
varied, output occurs whenever n/f, = 2r/c with n any integer and r
the distance to a target. Ground clutter, having no doppler shift,
gives an output that is periodic with period corresponding to the modulation
frequency. The d-c component is easily removed and, if fD < jr,
the other components are removed by the low-pass filter, which passes
only up to f.. If, therefore, f. < j, ground clutter may, in principle,
be removed.
A variant of this scheme turns the receiver off during one modulation