Radar System Engineering

142 C-II’ RADAR SYSTEMS [SEC. 57
}Vhen j, < f~, the system as outlined must be modified by placing
infinite-attenuation filters after the second detector. The filters are
designed to remove j, completely, ~vhile leaving all but closely adjacent
frequencies undisturbed. In principle, this is no different from the
infinite-at tenuat ion filters already present to remove the d-c signals due
to clutter. Practically, somewhat more trouble is involved, since zero
frequency filters are stable because neither the frequency nor the filter
tuning can change, while both possibilities are present for frequencies
other than zero. Usually also, either by accident or design, various
harmonics of j, will be present and these also must be filtered out.
This is the first case n-e have encountered of a general and almost
exactl theorem to the effect that any periodic modulation can produce
10,000
Sooo
60Q0
4000
2000
z
:1000
“-k800
~ 600
40Q
200
100 10 20 40 60 100 200 400 1000
rm,xin miles
510.- -Maximum repetition or modulation fmquenry as a function of maximum
unambiguous range.
in the receiver, whether by direct leakage or reflection from stationary
objects, only output signals that have the same periodicity; these output
signals can theref ore be removed by a sequence of infinit e-attenuat ion
filters tuned to frequency zero and to all harmonics of the modulation
frequency.
Finally, we note that this system will handle only one target. If
more are present the precise behavior depends on the type of phase
meter used and the best that can be done is to choose a type that measures
range to that target which gives the most intense reflection.
If the system is required to handle more than one target at a time,
discrimination may be made either on the basis of range or of doppler
frequency. If range is used, more sidebands are called for, and the
1The theoremwould be exact if the carrierfrequency were an integralmultiple
of the modulation frequency. When this is not so, the deviations are of the order
~,/fo or less and are usually negligible.