FM Ratio Detector / Discriminator Circuit

The Ratio detector or discriminator was widely used for FM demodulation before the introduction of integrated circuit demodulators and it is still found in some radios today.

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Ratio detector or discriminator was widely used for FM demodulation within radio sets using discrete components. It was capable of providing a good level of performance.

In recent years the Ratio detector has been less widely used. The main reason for this is that it requires the use of wound inductors and these are expensive to manufacture. Other types of FM demodulator have overtaken them, mainly as a result of the fact that the other FM demodulator configurations lend themselves more easily to being incorporated into integrated circuits.

Ratio FM detector basics

When circuits employing discrete components were more widely used, the Ratio and Foster-Seeley detectors were widely used. Of these the ratio detector was the most popular as it offers a better level of amplitude modulation rejection of amplitude modulation. This enables it to provide a greater level of noise immunity as most noise is amplitude noise, and it also enables the circuit to operate satisfactorily with lower levels of limiting in the preceding IF stages of the receiver.

FM Ratio detector / demodulator / discriminator circuit — FM ratio discriminator / 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 degrees 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.

The capacitors C1 and C2 filter any remaining RF signal which may appear across the resistors. The capacitor C4 and R3 also act as filters ensuring no RF reaches the audio section 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 ratio detector

Simple to construct using discrete components
Offers good level of performance and reasonable linearity

Disadvantages of Ratio FM discriminator:

High cost of transformer
Typically lends itself to use in only circuits using discrete components and not integrated within an IC

As a result of its advantages and disadvantages the ratio detector is not widely used these days. Techniques that do not require the use of a transformer with its associated costs and those that can be more easily incorporated within an IC tend to be used.