This is really a hang-over from early days of FM, and today the terms detector or probably better demodulator would probably be used. The Foster Seeley discriminator circuit is characterised by the transformer, choke and diodes used within the circuit that forms the basis of its operation. This FM demodulator circuit was invented by Dudley E. Foster and Stuart William Seeley in
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This is really a hang-over from early days of FM, and today the terms detector or probably better demodulator would probably be used. The Foster Seeley discriminator circuit is characterised by the transformer, choke and diodes used within the circuit that forms the basis of its operation. This FM demodulator circuit was invented by Dudley E. Foster and Stuart William Seeley in Although it was originally intended as a circuit to provide automatic frequency control, it was more widely used as an FM demodulator, whilst also being able to provide a voltage for automatic frequency control.
The Foster Seeley circuit was widely used until the s when ICs using other techniques that were more easily integrated became widely available. The circuit was widely used for all forms of radio communications applications from broadcasting to two way radio communications. Foster-Seeley FM discriminator basics The Foster Seeley detector or as it is sometimes described the Foster Seeley discriminator is quite similar to the ratio detector at a first look.
It has an RF transformer and a pair of diodes, but there is no third winding - instead a choke is used. The basic operation of the circuit can be explained by looking at the instances when the instantaneous input equals the carrier frequency, the two halves of the tuned transformer circuit produce the same rectified voltage and the output is zero.
If 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. Looking in more detail at the circuit, 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.
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 detector and Foster-Seeley detectors are expensive to manufacture.
Any wound components like the RF transformers are expensive to manufacture when compared with integrated circuits produced in vast numbers. As a result the Foster Seeley discriminator as well as the ratio detector circuits are rarely used in modern radio receivers as FM demodulators.
Foster Seeley circuit for frequency control Prior to the introduction of very stable local oscillators within superhet radios - the universal format for radios receiving FM, local oscillators had a tendency to drift.
Drift was a major factor in domestic radio receivers, although it was present in all radios. When receiving FM signals the drift meant that the incoming FM signal might drift away from being at the centre of the FM detector slope onto the non-linear portions.
This meant that the signal would become distorted. To overcome this, radio receivers would incorporate a facility known as automatic frequency control was implemented. Using this, the DC offset from the FM demodulator is used to tune the receiver local oscillator to bring it back on frequency.
FM demodulator curve produces A DC offset is produced when the centre frequency of the carrier is not on the centre of the demodulator curve. By filtering off the audio, only a DC component remains. Typically a long time constant RC combination is used to achieve this. The time constant of this RC network can be quite long as the drift of the oscillator occurs gradually over a period of seconds, and it must also be longer than that of the lowest frequency of the audio.
AFC circuitry for a superheterodyne radio reciever The filtered voltage is applied to a varactor diode within the local oscillator such that it causes the local oscillator to remain on tune for the FM signal being received.
In this way the receiver can operate so that the signal being received is demodulated within the linear region of the FM demodulator. Essentially the effect of the AFC circuitry is to create a form of negative feedback loop that seeks to keep the centre of the FM signal at the centre of the FM demodulation S curve. It is essentially a frequency locked loop. Most radios used for FM reception that have free running local oscillators incorporate an automatic frequency control, AFC circuit.
It uses only a few components and it provides for a significant improvement in the performance of the receiver, enabling the FM signal to be demodulated with minimum distortion despite the drift of the local oscillator signal. Prior to the widespread introduction of frequency synthesizers, AFC was not always used in radios such as walkie talkies and handhelds radios aimed at for two way radio communications applications as they tended to use crystal controlled oscillators and these did not drift to any major degree.
Hence there was less requirement for an AFC. Advantages of Foster-Seeley FM discriminator: Offers good level of performance and reasonable linearity. Simple to construct using discrete components. Provides higher output than the ratio detector Provides a more linear output, i. High cost of transformer. Narrower bandwidth than the ratio detector The circuit is sensitive to both frequency and amplitude and therefore needs a limiter before it to remove amplitude variations and hence amplitude noise.
Like all circuits, the Foster Seeley FM detector has its own advantages and disadvantages. By understanding these it is possible to mitigate the disadvantages and utilise the advantages to give the best performance. As a result of its advantages and disadvantages the Foster Seeley detector or discriminator is not widely used these days.
FM Detector | Phase Shift Discriminator or Foster Seeley
Foster  and Stuart William Seeley. The circuit was envisioned for automatic frequency control of receivers, but also found application in demodulating an FM signal. 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.
Foster Seeley Discriminator: FM detector / demodulator
This detection should be done efficiently and linearly. Further it is desirable that the detector should be insensitive to amplitude change and should not be too critical in its adjustment and operation. The FM detector may the detection process in two steps: It converts the frequency modulated voltage into corresponding amplitude modulated voltage using one or more tuned circuits and It rectifies this amplitude modulated voltage in linear diode detector to get back the original modulation frequency voltage. These FM detectors may be of the following types: Singles tuned circuit discriminator or slope detector Stagger tuned discriminators or Balanced slope discriminator Phase difference discriminators of the following types: Center tuned discriminator or Foster Seeley discriminator or phase shift discriminator.
Envelope detector[ edit ] A simple envelope detector A simple crystal radio with no tuned circuit can be used to listen to strong AM broadcast signals One major technique is known as envelope detection. The simplest form of envelope detector is the diode detector that consists of a diode connected between the input and output of the circuit, with a resistor and capacitor in parallel from the output of the circuit to the ground to form a low pass filter. If the resistor and capacitor are correctly chosen, the output of this circuit will be a nearly identical voltage-shifted version of the original signal. An early form of envelope detector was the crystal detector , which was used in the crystal set radio receiver.