ve noise and possibly interference signals are present, the design of the demodulator low-pass filters will be governed by considera- tions additional to minimum correlation function degradation, which, as we noted above, requires their bandwidth to be large. In particular, minimiza- tion of the demodulator square-law noise output requires that these same bandwidths be chosen small. thus, as is characteristic of square-law demod- ulation systems, a tradeoff exists between signal X signal and signal X noise plus noise X noise degradations. In summary, then, for SFH of an information-bearing carrier, the low-pass filter bandwidths must be commensurate with the information modulation bandwidth and therefore the demodulator output signal-to-noise ratio will be set by this bandwidth. For FFH of the same information-modulated car- rier, the low-pass filter bandwidths must be large enough to accommodate R(P)IS BT = 1 Figure 3.3. Filter effects on the FH correlation function. Th So ( )dt 2 sin 2 fIF+ RECEIVED SIGNAL FILTER FREQUENCY SYNTHESIZER The Th df GENERATOR EPOCH STEP SEARCH THRESHOLD CONTROL COMPARISON (CLOCK) END SEARCH Figure 3.4. A serial search FH acquisition system with integrate-and-dump-type demodulator. FH Acquisition Techniques the hop rate (which is larger than the modulation bandwidth) and can there- fore be set to maximize the demodulator output signal-to-noise ratio. One way of avoiding the degradation of the correlation curve peak in the FFH cse is to replace the low-pass filters in the non-coherent demodulator with integrate-and-dump circuits as in Figure 3.4. Since for the rectangular bursts of signal correlation appearing at the demodulator input, the integrate-and-dumps act as matched filters, then the demodulator output samples have an average value (3.6) or for of = 0, R(7)=s(1-17). (3.7) It is interesting at this point to note that this very same function of time and frequency offset as expressed by (3.6) will again be significant later on in the chapter when we study the effects of time and 