A General Theory for Frequency Discriminators Containing Null Networks

The characteristics of the ring demodulator and of null networks have suggested a generalized approach to the design of discriminators containing null networks. In the past, discriminators using null networks have been designed about various types of demodulators in which a carrier signal is necessary, the design fundamental being that the phase difference between the signal and carrier voltages applied to the demodulator should change by 180 degrees at the null frequency and be a multiple of pi radians immediately above and below the null frequency. However, since the phase difference has not been an even number of pi radians as the frequency departs from the null region, the output of the discriminator becomes sensitive to the amplitudes of both the signal and carrier voltages. The generalized theory presented in this paper permits the design of discriminators containing null networks where, neglecting second-order effects, coupling capacitors, and the like, the phase difference is maintained a multiple of pi radians at all frequencies. The result is an inherently wide-band discriminator which can be constructed largely from unbalanced networks and which can be accurately calibrated. It is further possible to design discriminators of arbitrary sensitivities to the point where, neglecting noise, the discriminator behaves more as a frequency switch. The mechanization of the theory involves phase-equalizing networks in both the signal and carrier input systems to the demodulator. The general theory is concerned with admissible network functions consistent with the desire to maintain phase differences at a multiple of pi radians.