Radar System Engineering

152 C-W RADAR SYSTEMS [SEC.5.11
Now the Fourier resolution of Curve a will contain the repetition
frequency and multiples thereof, whereas Curve b contains f. + nf,
but no multiples off, (unless f~ is a multiple of j,). Thus the first or
“clutter” filter that attenuates j, and multiples thereof, while leaving all
else, will remove all signals due to stationary objects, while leaving those
due to moving objects undisturbed. At
~ULfi
point 2 in Fig. 5.15, therefore, the signals
are like those of Fig. 5’17b, and do not
‘“) ~ Contain thefrequencyjr
The signals are now passed through
l.- 2r/c
a full-wave rectifier, the voltage at Point
(b) ;-~;-m””m
i
3 in Fig. 5.15 being like Fig. 5. 17c.
FI~. 5.16.—Transmitted and received
powers as a function of time.
This curve now contains harmonics
njr ~ mjD.
The various doppler sidebands and most of the noise are now removed
by passage through a filter that passes only j, and its multiples, whereupon
the voltage at Point 4 has a form like that shown in Fig. 5. 17d. Thus
the clutter, the doppler frequency, and much noise have been removed,
while the position of the trai~ing edge
of the pulse, which is a measure of
the range, is undisturbed.
The time delay of the trailing‘+ru-
edge of the wave at d, relative to the
modulating pulse, is now a measure
of target range. This time, or range,
may be displayed on the linear radial
sweep of a PPI tube by differentiating
the voltage and using the result
to intensity-modulate the beam.
A few minor points remain to be
considered. First, the explanation
n n I-1
has assumed that the bandwidth is
large enough to pass square waves
without distortion. Actually, the FIG.5.17.—Curvea showsthe voltage
operation is substantially the same at Point 1(cf. Fig. 5,15)dueto a stationary
target and b showsthat due to a moving
when only a few harmonics of j, are target; the dotted envelope curve correspondsto
the doppler frequency. Curve
passed. Second, the function of the
c is the result of passingb through a fullwave
rectifier, and d is the result of
attenuated square-wave voltage introduced
after Point 1 is to balance filtering c.
out the square-wave voltage due to the d-c component of receiver noise,
as modulated by the gating. Finally, it should be noted that the last
filter can have fairly narrow pass bands, thereby considerably reducing
the noise.