The Foster–Seeley discriminator is a common type of
FM detector circuit, invented in 1936 by Dudley E. Foster and
Stuart William Seeley. The Foster–Seeley discriminator was envisioned for
automatic frequency control 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 periodic waveform (usually sinusoidal) that conveys information through a process called ''modulation''. One or more of the wave's properties, such as amplitude or fre ...
, is connected to two
rectifier
A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction.
The process is known as ''rectification'', since it "straightens" t ...
diode
A diode is a two-Terminal (electronics), terminal electronic component that conducts electric current primarily in One-way traffic, one direction (asymmetric electrical conductance, conductance). It has low (ideally zero) Electrical resistance ...
s. The circuit resembles a
full bridge rectifier.
The phase of the voltage at the secondary coil depends on whether the carrier is below or above the resonance, resulting in a positive or negative shift, respectively.
The circuit makes use of the near-
phase difference occurring between the voltages in two loosely coupled
resonant circuits at the peak frequency.
Through the coupling capacitor
, the primary voltage is applied to the center tap of the secondary, producing a sum and a difference
on the top and the bottom terminals thereof, respectively.
Depending on the changing phase relationship of the input signal —
(and therefore
) with
, the amplitude of the sum/difference at the upper or lower half, respectively, will be higher, which results in a change of the voltage across the output capacitors. The choke
, sometimes replaced by a resistor, provides a DC path through the rectifier diodes.
The demodulator's bandwidth depends on the Q factor of the resonant circuit; the phase response of the secondary (and therefore, the voltage response of the circuit) to
is an
S-curve.
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 co ...
amplifier stage must be used before the detector, to remove amplitude variations in the signal which would be detected as
noise
Noise is sound, chiefly unwanted, unintentional, or harmful sound considered unpleasant, loud, or disruptive to mental or hearing faculties. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrat ...
. The limiter acts as a
class-A amplifier
In electronics, power amplifier classes are letter symbols applied to different Amplifier#Power_amplifiers, power amplifier types. The class gives a broad indication of an amplifier's Electrical efficiency, efficiency, linearity and other character ...
at lower amplitudes; at higher amplitudes it becomes a saturated amplifier which clips off the peaks and limits the amplitude.
See also
*
Slope detector
*
Ratio detector
*
Quadrature detector
*
Phase-locked loop
A phase-locked loop or phase lock loop (PLL) is a control system that generates an output signal whose phase is fixed relative to the phase of an input signal. Keeping the input and output phase in lockstep also implies keeping the input and ou ...
detector
Footnotes
External links
schematic and operation
{{DEFAULTSORT:Foster-Seeley discriminator
Communication circuits
Demodulation
de:Diskriminator#Auswertung frequenzmodulierter Signale