Foster-Seeley discriminator

Foster-Seeley discriminator

The Foster-Seeley discriminator [D.E. Foster and S.W. Seeley, "Automatic tuning, simplified circuits, and design practice," "Proceedings of the Institute of Radio Engineers", vol. 25, no. 3, part 1, pages 289-313 (March 1937).] [U.S. patent 2,121,103; issued 21 June 1938.] is a common type of FM detector circuit, invented in 1936 by Dudley E. Foster [Dudley E. Foster: biographical information and photo: "Proceedings of the Institute of Radio Engineers", vol. 29, page 571 (October 1941). Available on-line at:] and Stuart William Seeley.It uses a tuned RF transformer to convert frequency changes into amplitude changes. A transformer, tuned to the carrier frequency, is connected to two rectifier diodes. The circuit resembles a full-wave bridge rectifier. If the input equals the carrier frequency, the two halves of the tuned transformer circuit produce the same rectified voltage and the output is zero. As the frequency of the carrier changes, the balance between the two halves of the transformer secondary changes, and the result is a voltage proportional to the frequency deviation of the carrier.

Foster-Seeley discriminators are sensitive to both frequency and amplitude variations, unlike some detectors. Therefore a limiter amplifier stage must be used before the detector, to remove amplitude variations in the signal which would be detected as noise. The limiter acts as a Class A amplifier at lower amplitudes; at higher amplitudes it acts like a Class C amplifier, which clips off the peaks.

Other types of FM detectors are
*Slope detector
*Ratio detector
*Quadrature detector
*Phase-locked loop detector.


External links

* [ schematic and operation]

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