Frequency Modulation, FM Detection / Demodulation / Discrimination

In order to use the modulation on an FM signal, it is necessary to extract the modulation, i.e. demodulate or detect the signal.

Frequency Modulation Tutorial Includes:
Frequency modulation, FM     Modulation index & deviation ratio     FM sidebands, bandwidth     FM demodulation     FM slope detector     FM ratio detector     Foster Seeley detector     PLL FM demodulator     Quadrature demodulator     MSK     GMSK    

Modulation formats:     Modulation types & techniques     Amplitude modulation     Phase modulation     Quadrature amplitude modulation    

FM demodulation is also called FM detection and sometimes the phrase FM discrimination can be used.

FM demodulation basics

In any radio that is designed to receive frequency modulated signals there is some form of FM demodulator or detector.

This circuit takes in frequency modulated RF signals and takes the modulation from the signal to output only the modulation that had been applied at the transmitter.

The principle behind the demodulation / detection of frequency modulated, FM signals.
FM demodulation principle

In order to be able to demodulate FM it is necessary for the radio receiver to convert the frequency variations into voltage variations.

It is necessary to have a response that is as linear as possible over the required bandwidth.

The typical response curve for a frequency demodulator known as the S curve
Frequency demodulator S response curve

The response that is normally seen for an FM demodulator / FM detector is known as an "S" curve for obvious reasons. There is a linear portion at the centre of the response curve and towards the edge the response becomes very distorted.

Types of FM demodulator

There are several types of FM detector / demodulator that can be used. Some types were more popular in the days when radios were made from discrete devices, but nowadays the PLL based detector and quadrature / coincidence detectors are the most widely used as they lend themselves to being incorporated into integrated circuits very easily and they do not require many, if any adjustments.

  • Slope detection:   This is a very simple form of FM demodulation and it relies on the selectivity of the receiver itself to provide the demodulation. It is not particularly effective and is not used except when the receiver does not have an FM capability.     . . . . Read more about the FM slope detector.
  • Ratio detector:   This type of detector was one that was widely used when discrete components were used in transistor radios.     . . . . Read more about the FM ratio detector.
  • Foster Seeley FM :   In the days when radio used discrete components, this was the other main contender for the FM demodulator in radios.     . . . . Read more about the Foster Seeley FM discriminator.
  • Phase locked loop demodulator:   It is possible to use a phase locked loop to demodulate FM. The PLL FM detector provides excellent performance and does not require many, if any adjustments in manufacture.     . . . . Read more about the PLL FM demodulator.
  • Quadrature detector:   The quadrature FM detector is now widely used in FM radio ICs. It is easy to implement and provides excellent levels of performance.     . . . . Read more about the Quadrature FM demodulator.

These FM demodulators are used in different applications. Although the PLL FM detector and the quadrature detectors are most widely used, the Foster Seeley and ratio FM detectors are still used on some occasions.

More Essential Radio Topics:
Radio Signals     Modulation types & techniques     Amplitude modulation     Frequency modulation     RF mixing     Phase locked loops     Frequency synthesizers     Passive intermodulation     RF attenuators     RF filters     Radio receiver types     Superhet radio     Radio receiver selectivity     Radio receiver sensitivity     Receiver strong signal handling    
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