TABLE OF CONTENTS Ordinary non-coherent receivers (Chapter 2, Section 2.2.1, Figure 2.10( a)) discard the echo phase information component. The receiver input is a microwave echo superimposed on wideband Gaussian noise. The much-simplified block diagram of Figure 12.9 shows how the receiver mixes the noisy signal down to intermediate frequency, where it is amplified as a pulse of sine waves (at say 50 MHz). A filter restricts bandwidth to minimise noise as far as consistent with acceptance of the major frequency components of the signal pulses. Point S is the filter output and is followed by the demodulator, whose output is point T. As the voltage envelope of the IF signal is preserved, this form of demodulation is called envelope detection. A wide-band video amplifier brings thhe output base-band signal to a convenient level for digitisation and subsequent processing, without affecting the mathematics. A voltage comparator (point U) then makes the initial detection decision whenever the instantaneous baseband voltage exceeds the d.c. threshold, again using the basic circuit of the inset in Figure 12.2. Some of these processes upset the conditions we assumed in Section 12.2 and affect the interplay of P FA, P D and SNR. As before, the signal processor then compares the present and previous single-pulse declarations with the history of activity in that and adjacent cells to deliver the final target declaration to the display. The function of an envelope detector is therefore...
