Since and Eq. (4) gives in fulfillment of our requirement that the IF phase be an accurate representation of the pilot signal phase (or phases) uncontaminated by the phase of extraneous signals (the LO phase in this case). Note that [] in Figure 8b must not be exactly half way between [] and [], because then [] would be equal to [] which would produce degeneracy in mixer M2. [] need not be between [] and [], but if it isn’t, mixer M2 must be a downconverter. The limiter in Figure 8b is required to avoid phase offset changes in the mixer M2 produced by changes in its input level. Provided that a two-tone pilot signal is available, the two-tone receiver can always be used in place of the phase-locked receiver of Figure 8a and will usually be cheaper. Moreover, one can carry the two-tone pilot idea a step further by using IF images of the two pilot tones within the PCC-PRR combination itself, and not just in the coherent receiver ahead of it. Such a circuit is shown in Figure 9. The operation of the circuit can be easily followed with the aid of the accompanying phase table (Table 1). Note that the conjugation is exact; i.e. the output phase is the exact conjugate of either pilot signal. Note also that degeneracy and leakage in the mixers are avoided because
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