Page 217 - Fundamentals of Radar Signal Processing

FIGURE 3.21 Spectra corresponding to successive signals in the digital I/Q
system of Fig. 3.20. (a) Spectrum of bandpass input signal with information
bandwidth β Hz, (b) result of translation to an IF frequency also equal to β, (c)
one period of spectrum on normalized frequency scale after A/D conversion, (d)

only the upper sideband remains after filtering, (e) a replica of the upper
sideband is centered at DC after decimation. (After Rader, 1984.)

Recall that the goal of quadrature demodulation is to select one sideband of
the bandpass signal and translate it to baseband. Assume that the upper sideband
is to be retained. The next step is therefore to filter the real signal to
eliminate the lower sideband. Since the resulting spectrum will not be
Hermitian, the output signal must be complex; this is shown in Fig. 3.20 as a

one-input, two-output filter. The required frequency response is clear from the
spectrum diagrams in Fig. 3.21; it is