 the same energy content on (0, TM). For this reason, other types of detectors that are more feature- dependent have been developed. One such configuration is the hop rate detector shown in Figure 1.15. For this receiver, the input band is subdivided into two sub-bands (upper and lower bands), each WS/12 wide. The output of each BPF is square-law detected and the results are differenced. When an FH signal appears at the receiver input, on the average, it spends 50 per- cent of its time in each sub-band, transitioning randomly at multiples of the hop period. Therefore, the differenced signal appears as (in the absence of noise) a two-level or bipolar waveform. Highpass filtering to remove any direct voltage components and lowpass filtering with a bandwidth on the order of the hop rate yields a narrowband lowpass signal. This signal is delayed by Th/2, and the signal and its delay are multiplied. The result is a process that contains a strong periodic component at the hop rate which, in turn, is detected by, say, a narrowband spectrum analyzer. Although this receiver is capable of making a very accurate measurement of the hop rate, it does SO at a large penalty in SIN because it is basically a fourth-law device. (For the widely used system parameters of this section, the SIN would have to be increased by 16 dB in order to achieve an equivalent detectability SNR.) As a result, the FH hop rate detector of Figure 1.15 offers the interceptor no advantage whatsoever in terms of basic detectabil- ity. Other rate detection (e.g., PN chip rate or TH rate) receivers may also be realized, but they likewise provide no gain over the wideband energy detector in terms of basic detectability. Based upon our previous discussion, the implementation of the optimum multichannel receiver of Figure 1.4 is thought to be impractical. There is, DELAY FREQUENCY DETECTOR Low Probability of Intercept Communications however, a particular mechanization which might make the multichannel detector somewhat more tractable, namely, the digital spec