Decreasing the Threshold in FM by Frequency Feedback

Abstract

The "frequency feedback demodulator" or "frequency compression demodulator" can be used to extend the threshold of signal-to-noise improvement in large index frequency-modulation systems. Previous papers have advanced the argument that the threshold occurs in the usual manner when noise peaks exceed the carrier envelope at the input to the frequency detector of a feedback demodulator. However, correlation between calculated values and carefully measured experimental values has been poor. The calculated values have been incorrect by orders of magnitude in many typical cases. In this paper it is shown that the threshold can also occur because of the feedback action of the demodulator. When this is taken into account, the threshold can be calculated accurately. It is shown that the threshold cannot be improved by the often suggested scheme of inserting a carrier of the proper phase at the input to the frequency detector. The phase-locked loop, a related device, is shown to have a threshold which is equal to or poorer than the threshold of the feedback demodulator for large modulation indices. It is shown that the stability of the feedback loop (and consequently the threshold) of the feedback demodulator is a function of modulation, but that by following the procedure given, the effect can be almost entirely eliminated. The results of this paper allow one to design feedback demodulators for the first time which will extend the threshold in a predictable manner.