Page 285 - Fundamentals of Radar Signal Processing

sidelobes of the frequency response of a digital filter, a window function is
applied in the time domain to the impulse response. The goal here is to reduce
sidelobes in range, corresponding to the time domain, so the analogous
approach is to window the receiver frequency response in the frequency
domain.
The matched filter frequency response is H(F) = X*(F), and at least for

larger BT products is approximately rectangular. A modified frequency
response H′(F) can be obtained by multiplying H(F) by a window function
w(F) 6

(4.117)

Figure 4.34a shows a Hamming window function overlaid on the matched filter
frequency response for an LFM waveform with βτ = 100. H′(F) is the product of
these two functions. The resulting impulse response h′(t) is shown in Fig. 4.34b.
The response of both the matched filter and the filter of Fig. 4.34b to the
unwindowed LFM echo are overlaid in Fig. 4.35. The LFM waveform used had
βτ = 100 and the window cutoff was placed at ±β/2 Hz. The peak sidelobe has

dropped 23.7 dB, from 13.5 dB below the mainlobe peak in the unwindowed
case to 37.2 dB below the mainlobe peak for the windowed case. This comes at
a cost of the mainlobe peak gain dropping 5.35 dB and the Rayleigh time (range)
resolution increasing by 93 percent.