Foster Seeley Discriminator or FM Detector

- details of FM demodulators - Foster Seeley discriminator or demodulator and the Ratio demodulator or detector.

The Foster Seeley is a common type of FM detector circuit used mainly within radio sets constructed using discrete components.

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

Invented in 1936 by Dudley E. Foster and Stuart William Seeley, it was widely used until the 1970s when ICs using other techniques that were more easily integrated became more widely available.

Foster-Seeley FM discriminator basics

The Foster Seeley detector or as it is sometimes described the Foster Seeley discriminator has many similarities to the ratio detector. The circuit topology looks very similar, having a transformer and a pair of diodes, but there is no third winding and instead a choke is used.

Foster-Seeley FM Detector / demodulator
The Foster-Seeley discriminator / detector

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 degrees 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 and Foster-Seeley detectors are expensive to manufacture. Wound components like coils are not easy to produce to the required specification and therefore they are comparatively costly. Accordingly these circuits are rarely used in modern equipment.

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 Disadvantages
  • Offers good level of performance and reasonable linearity.
  • Simple to construct using discrete components.
  • Does not easily lend itself to being incorporated within an integrated circuit.
  • High cost of transformer.

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.

By Ian Poole

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