FM Demodulation Using Dynamic Control Action of a Band Pass Filter

Abstract

This paper audits FM demodulation using discriminator circuits, and the FM–AM conversion using a conventional single-tuned, double-tuned and staggered-tuned circuit is discussed. The demodulation capabilities of the double- and staggered-tuned circuits are audited in the time domain. It is observed that the conventional single- and double-tuned circuit outputs are highly distorted, in particular, the single-tuned circuit output is rich in second harmonics, and a balanced slope detection technique is proposed which reduces the output THD. A novel method of dynamically controlling the center frequency of the tuned circuit is proposed, and an excellent demodulation capability with a reduction in output THD by controlling the feedback loop gain is reported. This dynamically controlling action is applied to a conventional Foster–Seeley discriminator, and it is found that for higher values of feedback gain, the output THD decreases. It has been established that by judiciously controlling the center frequency of the tuned circuit, the effective modulation index has been reduced. The maximum permissible value of input modulation by the dynamic BPF has been derived. Experimental findings coupled with analytical results are presented and are shown to be in good agreement.