Synchronous Demodulation of Phase-Reversing Binary Signals, and the Effect of Limiting Action

It is shown that a considerable improvement of SNR in the output is produced in a synchronous demodulator (when the SNR in the input is greater than unity) by interchanging the input terminals with the local oscillator terminals of the modulator circuit when this is of the switching type, or by using a balanced limiter followed by a linear multiplier. Owing to the nonlinear relationship introduced, however, the consequent improvement in error rate in a binary system is much less than the improved SNR might suggest; it is likely to be fairly small and may be nil when the decision device is efficient, but may be fairly large if the decision regarding polarity has to be made at an amplitude which is a substantial fraction of the pulse amplitude. As the performance of the new arrangement, when different, is always better than that of the usual circuit, there seems to be a good case for using it when convenient. Moreover, when it is used, there is little loss of performance with respect to error rate if the binary decision regarding polarity of output pulse is made not at zero amplitude, but at a finite amplitude which may be as great as half-amplitude. Consequently, it is feasible to make a single receiver which will operate almost indiscriminately on ON-OFF or phase-reversing signals.