Slope detection for FM demodulation

A look at the simplest FM demodulation technique. It doesn’t give the lowest possible output distortion, it doesn’t reject amplitude distortion effects, but it is simple and can be used at virtually no cost.

Demodulation of frequency modulation (FM) signals can be done in many ways. There are FM discriminators, ratio detectors, quadrature detectors, phase lock loop designs, and even methods of getting down to first principles as shown on here.

However, one more method we can add to the toolkit is slope detection which is perhaps the simplest approach of them all.

Imagine a receiver of some sort which has some sort of bandpass characteristic. Typically, this would be a superheterodyne receiver whose bandpass properties are achieved in the intermediate frequency (IF) amplifier stage(s). We can tune our receiver so that the center frequency of the FM signal appears on one slope of the receiver’s bandpass characteristic meaning off to the side of the characteristic’s peak rather than at that peak itself (Figure 1).

Figure 1 Slope detection method where a bandpass slope below the resonant peak is used to create a slope-induced amplitude modulation where a simple envelope detector can be used to recover the modulation signal.

The figure above shows use of the bandpass slope below the resonant peak, but the slope above the resonant peak could be used just as well.

Whatever frequency deviation the input FM signal may have will result in an output signal in which an amplitude modulation property will have been imparted. A simple envelope detector can then be used to recover the modulation signal.

There will of course be some distortion because the bandpass scale factor versus frequency is not linear, but if that distortion is deemed tolerable, this very simple demodulation technique can work.

John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).

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