FM Ratio Detector: FM ratio discriminator

The ratio FM detector is an easily used form of FM demodulator very appropriate for discrete components.

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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.

Typical transistor radio that might use a FM ratio detector
Typical transistor radio that uses a ratio FM detector

FM ratio detector basics

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.

FM ratio detector / demodulator / discriminator circuit
FM ratio detector circuit

The operation of the ratio detector centres around a frequency sensitive phase shift network with a transformer and the diodes that are effectively in series with one another. When a steady carrier is applied to the circuit the diodes act to produce a steady voltage across the resistors R1 and R2, and the capacitor C3 charges up as a result.

The transformer enables the circuit to detect changes in the frequency of the incoming signal. It has three windings. The primary and secondary act in the normal way to produce a signal at the output. The third winding is un-tuned and the coupling between the primary and the third winding is very tight, and this means that the phasing between signals in these two windings is the same.

The primary and secondary windings are tuned and lightly coupled. This means that there is a phase difference of 90° between the signals in these windings at the centre frequency. If the signal moves away from the centre frequency the phase difference will change. In turn the phase difference between the secondary and third windings also varies. When this occurs the voltage will subtract from one side of the secondary and add to the other causing an imbalance across the resistors R1 and R2. As a result this causes a current to flow in the third winding and the modulation to appear at the output.

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 discriminator
  • Higher 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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