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FM Ratio Detector: FM ratio discriminator working and principle

The ratio FM detector, discriminator or demodulator was widely used for FM demodulation for radio receivers that typically used discrete components. Now with radios using integrated circuits other forms of FM demodulator are more applicable.
When used, the FM ratio detector was able to provide good levels of performance with limited number of components.
The FM ratio detector may also be called an FM ratio demodulator or even an FM ratio discriminator.

The two main types of FM detector or demodulator that were used in circuits using discrete components were the ratio detector and the Foster-Seeley FM detector.
Both types were widely used, but the FM ratio detector was the more common because it offered a better level of amplitude modulation rejection of amplitude modulation. This enabled the circuit to provide a greater level of noise immunity as most noise is amplitude noise. It also enabled the FM detector to operate more effectively even with with lower levels of limiting in the preceding IF stages of the receiver.

Ratio detector advantages & disadvantages

As with any circuit there are a number of advantages and disadvantages to be considered when choosing between several options.
Advantages of the ratio FM detector
  • Simple to construct using discrete components.
  • Offers good level of performance and reasonable linearity.
  • Provides a good level of immunity to amplitude noise .
  • Ratio detector has wider bandwidth than Foster Seeley discriminator.
  • Disadvantages of the FM ratio detector
  • High cost of transformer.
  • Typically lends itself to use in only circuits using discrete components and not integrated within an IC.
  • Only 50% output of the Foster-Seeley discriminatorHigher distortion level than Foster-Seeley discriminator.
As a result of its advantages and disadvantages the FM ratio detector is not widely used these days as most radios - even low cost ones use integrated circuits. The ratio detector performs well, but the high cost of the transformer as well as the fact that there are other circuits and approaches that better lend themselves to use in integrated circuits means that this circuit is not widely used today.












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