Configuration for Two Received Frequencies

The two radiation fields f[ and are again mixed in a heterodyne detector with a strong, coherent, and polarized LO signal (at frequency yielding two electrical beat signals at and I/2 —/lI, along with a dc component which 

Since the output of the nonlinear device is essentially independent of the Doppler shift as well as the LO frequencies, variations in these quantities have little effect on the system output. In many instances, therefore, the necessity for a stable and tunable LO may be eliminated. Again, the reception in this system is angle independent. 

The SNR at the output (o) of the three-frequency nonlinear heterodyne system (SNR)q is given by [see (7.76) and (7.107b)] 

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