The Foster-Seeley discriminator is a common type of FM

Frequency modulation

In telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant...

 detector circuit

Detector (radio)

A detector is a device that recovers information of interest contained in a modulated wave. The term dates from the early days of radio when all transmissions were in Morse code, and it was only necessary to detect the presence of a radio wave using a device such as a coherer without necessarily...

, invented in 1936 by Dudley E. Foster and Stuart William Seeley

Stuart William Seeley

Stuart William Seeley was a noted American electrical engineer, best known for inventing the Foster-Seeley discriminator and SHORAN....

. The circuit was envisioned for automatic frequency control of receivers, but also found application in demodulating an FM signal.
It uses a tuned RF

Radio frequency

Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...

 transformer to convert frequency changes into amplitude changes. A transformer, tuned to the carrier frequency

Carrier wave

In telecommunications, a carrier wave or carrier is a waveform that is modulated with an input signal for the purpose of conveying information. This carrier wave is usually a much higher frequency than the input signal...

, is connected to two rectifier

Rectifier

A rectifier is an electrical device that converts alternating current , which periodically reverses direction, to direct current , which flows in only one direction. The process is known as rectification...

 diode

Diode

In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...

s. The circuit resembles a full-wave bridge rectifier. If the input equals the carrier

Carrier wave

In telecommunications, a carrier wave or carrier is a waveform that is modulated with an input signal for the purpose of conveying information. This carrier wave is usually a much higher frequency than the input signal...

 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

Limiter

In electronics, a limiter is a circuit that allows signals below a specified input power to pass unaffected while attenuating the peaks of stronger signals that exceed this input power....

 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
  Phase-locked loop
  A phase-locked loop or phase lock loop is a control system that generates an output signal whose phase is related to the phase of an input "reference" signal. It is an electronic circuit consisting of a variable frequency oscillator and a phase detector...
  
  detector.