FM Slope Detector / Demodulator

One of the easiest methods of demodulating / detecting FM frequency modulation is to use the slope of a tuned circuit: simple but not very effective.

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FM slope detection is a concept that can used be recover the modulation from an FM signal. It is simple because it can be used with virtually any radio even if it is not equipped , with an FM capability. It simply uses the selectivity of the radio, or the slope of a tuned circuit.

Whilst FM slope detection is very simple it is not particularly effective and therefore is not normally used. However the basic concept is useful to understand before moving on to look at much more effective methods of FM detection or demodulation. .

FM slope detector basics

Slope detection is the very simplest form of frequency modulation, FM demodulation. An FM slope detector consists of a tuned circuit where the centre frequency is tuned to a frequency slightly offset from the carrier of the signal. In this way the frequency modulated signal sits on the slope of the response curve, giving rise to the name of FM slope detector.

As the frequency of the signals varies up and down in frequency according to its modulation, so the signal moves up and down the slope of the tuned circuit response curve. This causes the amplitude of the signal to vary in line with the frequency variations. In fact at this point the signal has both frequency and amplitude variations.

FM slope detection concept
FM slope detection concept

It can be seen from the diagram that changes in the slope of the filter, reflect into the linearity of the demodulation process. The linearity is very dependent not only on the filter slope as it falls away, but also the tuning of the receiver - it is necessary to tune the receiver off frequency and to a pint where the filter characteristic is relatively linear.

The final stage in the process is to demodulate the amplitude modulation and this can be achieved using a simple diode circuit. One of the most obvious disadvantages of this simple approach is the fact that both amplitude and frequency variations in the incoming signal appear at the output. However the amplitude variations can be removed by placing a limiter before the detector.

A variety of FM slope detector circuits may be used, but the one below shows one possible circuit with the applicable waveforms. The input signal is a frequency modulated signal. It is applied to the tuned transformer (T1, C1, C2 combination) which is offset from the centre carrier frequency. This converts the incoming signal from just FM to one that has amplitude modulation superimposed upon the signal.

FM slope detector circuit operation
FM slope detector circuit showing the signal waveforms

This amplitude signal is applied to a simple diode detector circuit, D1. Here the diode provides the rectification, while C3 removes any unwanted high frequency components, and R1 provides a load.

FM slope detection advantages & disadvantages

The FM slope detector is not widely used. However the concept is useful to understand as its effects may arise in some circuits.

Advantages of FM slope detectors

  • Simple - can be used to provide FM demodulation when only an AM detector is present.
  • Enables FM to be detected without any additional circuitry.

Disadvantages of FM slope detectors

  • Not linear as the output is dependent upon the curve of a filter.
  • Not particularly effective as it requires the signal to be centred on the falling response of a filter. This means that the signal cannot be received at its maximum signal strength.
  • Both frequency and amplitude variations are demodulated and this means that much higher levels of noise and interference are experienced.

From the list of FM slope detector advantages and disadvantages, it can quickly be seen that this form of FM demodulation has many drawbacks. As a result, any radio receiver equipped for frequency modulation reception will include a much better FM detector / demodulator circuit that will provide much higher levels of performance.

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