Foster Seeley Discriminator: FM detector / demodulator

The Foster Seeley FM discriminator, detector or demodulator enabled audio to be recovered from frequency modulated signals using a relatively simple circuit .

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The Foster Seeley circuit is probably most commonly called the Foster Seeley discriminator. This is really a hang-over from early days of FM, and today the terms detector or probably better demodulator would probably be used.

The Foster Seeley discriminator circuit is characterised by the transformer, choke and diodes used within the circuit that forms the basis of its operation.

This FM demodulator circuit was invented by Dudley E. Foster and Stuart William Seeley in 1936. The Foster Seeley circuit was widely used until the 1970s when ICs using other techniques that were more easily integrated became widely available.

Foster-Seeley FM discriminator basics

The Foster Seeley detector or as it is sometimes described the Foster Seeley discriminator is quite similar to the ratio detector at a first look. It has an RF transformer and a pair of diodes, but there is no third winding - instead a choke is used.

FM Foster Seeley detector / demodulator / discriminator circuit
FM Foster Seeley discriminator / detector circuit

Like the ratio detector, the Foster-Seeley circuit operates using a phase difference between signals. To obtain the different phased signals a connection is made to the primary side of the transformer using a capacitor, and this is taken to the centre tap of the transformer. This gives a signal that is 90° out of phase.

When an un-modulated carrier is applied at the centre frequency, both diodes conduct, to produce equal and opposite voltages across their respective load resistors. These voltages cancel each one another out at the output so that no voltage is present. As the carrier moves off to one side of the centre frequency the balance condition is destroyed, and one diode conducts more than the other. This results in the voltage across one of the resistors being larger than the other, and a resulting voltage at the output corresponding to the modulation on the incoming signal.

The choke is required in the circuit to ensure that no RF signals appear at the output. The capacitors C1 and C2 provide a similar filtering function.

Both the ratio detector and Foster-Seeley detectors are expensive to manufacture. Any wound components like the RF transformers are expensive to manufacture when compared with integrated circuits produced in vast numbers. As a result the Foster Seeley discriminator as well as the ratio detector circuits are rarely used in modern radio receivers as FM demodulators.

Foster-Seeley detector advantages & disadvantages

As with any circuit there are a number of advantages and disadvantages to be considered when choosing between the various techniques available for FM demodulation.

Advantages of Foster-Seeley FM discriminator:

  • Offers good level of performance and reasonable linearity.
  • Simple to construct using discrete components.
  • Provides higher output than the ratio detector
  • Provides a more linear output, i.e. lower distortion than the ratio detector

Disadvantages of Foster-Seeley FM discriminator:

  • Does not easily lend itself to being incorporated within an integrated circuit.
  • High cost of transformer.
  • Narrower bandwidth than the ratio detector

As a result of its advantages and disadvantages the Foster Seeley detector or discriminator is not widely used these days. Its main use was within radios constructed using discrete components.

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