The Foster–Seeley discriminator 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: http://ieeexplore.ieee.org/iel5/10933/35665/01694175.pdf] and
Stuart William Seeley. The Foster–Seeley discriminator was envisioned for
automatic frequency control
In radio equipment, Automatic Frequency Control (AFC), also called Automatic Fine Tuning (AFT), is a method or circuit to automatically keep a resonant circuit tuned to the frequency of an incoming radio signal. It is primarily used in radio ...
of
receivers, but also found application in demodulating an FM signal.
The Foster–Seeley discriminator uses a tuned
RF transformer to convert frequency changes into amplitude changes. A transformer, tuned to the
carrier frequency
In telecommunications, a carrier wave, carrier signal, or just carrier, is a waveform (usually sinusoidal) that is modulated (modified) with an information-bearing signal for the purpose of conveying information. This carrier wave usually has a ...
, is connected to two
rectifier diodes. The circuit resembles a full-wave
bridge rectifier
A diode bridge is a bridge rectifier circuit of four diodes that is used in the process of converting alternating current (AC) from the input terminals to direct current (DC, i.e. fixed polarity) on the output terminals. Its function is to con ...
. 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 input 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
In electronics, a limiter is a circuit that allows signals below a specified input power or level to pass unaffected while attenuating (lowering) the peaks of stronger signals that exceed this threshold. Limiting is a type of dynamic range comp ...
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
In electronics, power amplifier classes are letter symbols applied to different power amplifier types. The class gives a broad indication of an amplifier's characteristics and performance. The classes are related to the time period that the active ...
at lower amplitudes; at higher amplitudes it becomes a saturated amplifier which clips off the peaks and limits the amplitude.
Other types of FM detectors are:
*
Slope detector
*
Ratio detector
The ratio detector is a type of detector circuit, commonly used in radio receivers for demodulating frequency modulated (FM) signal.
The ratio detector is a variant of the Foster–Seeley discriminator, but one diode conducts in an opposite d ...
*
Quadrature detector
In radio, a detector is a device or circuit that extracts information from a modulated radio frequency current or voltage. The term dates from the first three decades of radio (1888-1918). Unlike modern radio stations which transmit sound (an audi ...
*
Phase-locked loop
A phase-locked loop or phase lock loop (PLL) is a control system that generates an output signal whose phase is related to the phase of an input signal. There are several different types; the simplest is an electronic circuit consisting of a ...
detector
Footnotes
External links
schematic and operation
{{DEFAULTSORT:Foster-Seeley discriminator
Communication circuits
Demodulation
de:Diskriminator#Auswertung frequenzmodulierter Signale