 Techniques with Active Correlation Although our primary interest in this chapter is with slow or fast FH/MFSK, it is perhaps more instructive to first discuss the most basic non-coherent FH acquisition configuration, which, although best suited to analog information transmitted as an amplitude modulation, or no information modulation at all, nevertheless represents the simplest illustration of the above-mentioned acquisition process dichotomy. As such, consider the single (fixed) dwell time serial search acquisition system illustrated in Figure 3.1. In this scheme, the received FH signal plus noise is correlated in a wideband mixer with the local hop sequence produced by an FH synthesizer driven by a PN generator WIDEBAND MIXER 2 sin 2w f IF f RECEIVED FILTER SIGNAL FREQUENCY SYNTHESIZER PN GENERATOR NON-COHERENT DEMODULATOR EPOCH STEP SEARCH THRESHOLD CONTROL COMPARISON (CLOCK) END SEARCH Figure 3.1. A single dwell time serial search FH acquisition system. FH Acquisition Techniques whose epoch is controlled in accordance with the decision to continue the search. The result of this correlation is passed through an IF filter followed by an energy detector implemented here as a baseband non-coherent demodulator. (The bandwidths of the IF filter and demodulator low-pass filters are chosen based upon considerations to be discussed shortly.) Post- detection integration of the energy detector output produces a signal whose mean value is nominally zero when the two hop sequences are misaligned and non-zero when they are either partially or fully aligned. Thus, compar- ing this signal with a preset threshold allows a decision to be made as to whether or not FH acquisition has been achieved, or equivalently whether or not to step the PN code epoch and continue the search. A similar serial search technique for acquiring PN sequences was dis- cussed in great detail in Chapter 1. Thus, our discussion here will be, in com- parison, brief and merely serve to highlight the essential differences between the two systems. To begi