In order to avoid both these difficulties we would like to have exact, frequency shifting PCC’s, i.e. PCC’s that satisfy our definition, Eq. (2), of phase conjugation where [] A simple example of such a PCC is shown in Figure 5 (after [9]). The servoed phase shifter must be both bilateral (to allow the pilot and conjugate signals to traverse it in opposite directions) and nondispersive. Since the phase shift in any circuit is affected by reflections at both input and output, the phase shifter must be well matched at both co and []. If it is not, the phase shifter will, in effect, be dispersive. Some mechanical phase shifters (e.g., a coaxial line stretcher) meet both the bilateral and nondispersive requirements. Electrical phase shifters, such as diode and ferrite phase shifters, meet them to varying degrees, and may be satisfactory when co’ is close to co. Note that the conjugate signal need not be coherent with the pilot in this PCC; a free running oscillator, which need not even be particularly stable, suffices as the source of the conjugate signal. However, like the reference signal, it must have the same phase at each PCC. A new kind of exact PCC is shown in Figure 6. This one uses a phase locked loop which both conjugates the phase and translates the frequency of the input signal. Mixer M2 is an upconverter while Ml is a downconverter. The conjugate relation, where R = 1/(1 - 2/n), follows immediately from the phase lock condition. The circuit is practical for n > 4. Thus the sequence of frequency translating ratios, R, starts at 2 and converges to 1 as n increases. Since R - 1 has to be greater than some small number in order to obtain the required diplexer isolation,
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