Ratio Detector : Frequency Demodulation

Frequency Modulation (FM) is a widely used modulation technique in modern communication systems due to its superior noise immunity and improved signal quality compared to amplitude modulation (AM). FM signals are widely used in applications such as broadcast radio, satellite communication, and mobile communication. To extract the original information signal from an FM signal, a demodulator is required. There are various FM demodulator such as FM slope detector, balanced slope detector, ratio detector FM detector, pulse averaging discriminator, quadrature detector, phase locked loop etc. One commonly used demodulator is the Ratio Detector FM demodulator. Here, we will delve into the magic of the Ratio Detector FM demodulator, its working principle, advantages, and applications.

Ratio Detector 

Ratio detector is an FM demodulator that is used to recover information signal from FM signal. It is improvement of the Foster-Seeley discriminator FM demodulator wherein amplitude limiter is added to the Foster-Seeley discriminator to eliminate any unwanted amplitude variation. Ratio detector uses transformer coupled tuned circuit to convert instantaneous FM signal frequency into amplitude varying signal. The ratio detector uses pair of AM envelope detectors to convert the instantaneous frequency deviation to amplitude variation signal. The difference between ratio detector and the Foster-Seeley detector is that one of the diodes in ratio detector reversed, a large capacitor is used which is responsible for the amplitude limitation. The linearity of conversion of frequency deviation of the FM signal to amplitude of ratio detector is very good.

Working Principle of Ratio Detector FM demodulator

 The following is circuit diagram of ratio detector for FM demodulation.

ratio detector FM demodulator circuit diagram
The ratio detector consist of tuned RF transformers with center tapped secondary. The primary winding and the capacitor C1 forms the input tuned circuit which is aligned with the carrier frequency of the FM signal. Similarly, the secondary winding with the capacitor C4 forms the tuned circuit at the secondary which is also tuned to carrier frequency. The input primary signal is coupled to the secondary center tap using coupling capacitor CC and RFC(radio frequency choke) is connected from the center tap to the common ground. 

The input FM signal enters into the primary tuned circuit and then is coupled into the secondary tuned circuit. At the secondary 180 degree out of phase signals are produced during the negative and positive half cycle of the input signal. The terminals of the secondary tuned circuit are connected to two separate paths, each path consist of envelope detector circuit, D1, R2 and C2 in one path and D2, R3, C3 on the other. The R2 and C2 in one path and the R3 and C3 on the other path are low pass filters to remove high frequency component, that is the ripples.

The diode D2 is reversed which causes total voltage between the two diodes to be the sum of the output voltage from each diode, that is, Va+Vb. In order to keep this summed voltage,Va+Vb, a constant a large capacitor C5 is placed. The resistors R6 and R7(R6=R7) are used to charging and discharging of this capacitor. This capacitor C5(the resistors R6, R7) plays the role of amplitude limiter in ratio detector. The large electrolytic capacitor and the corresponding resistors, R6 and R7 creates large time constant during charging and discharging and  introducing a delay in the signal path, thereby maintaining the amplitude Va +Vb essentially constant. Thus using the capacitor C5 the voltage Va+Vb is kept more or less constant while their ratio Va/Vb may vary. Hence this FM demodulator circuit is called ratio detector.

The following shows the message signal, the transmitted FM signal and the demodulated or recovered message signal waveform on oscilloscope.

waveform graph ratio detector

Video demonstration of Ratio Detector FM demodulation

In the following video, it is shown how the ratio detector for FM demodulation works.

Advantages of Ratio Detector FM Demodulator

  • Simplicity: The Ratio Detector FM demodulator is relatively simple in its design, consisting of a few basic components such as diodes, resistors, capacitors, and an operational amplifier. This simplicity makes it cost-effective and easy to implement in various communication systems.
  • Wide Frequency Range: The Ratio Detector FM demodulator is capable of demodulating a wide range of FM signal frequencies, making it suitable for various applications such as radio broadcasting, mobile communication, and satellite communication.
  • Good Linearity: The Ratio Detector FM Demodulator exhibits good linearity, which means that it can accurately demodulate FM signals with a wide range of frequency deviation and modulation index, resulting in high-quality demodulated audio or information signals.
  • Noise Immunity: The Ratio Detector FM Demodulator is relatively immune to noise compared to other FM demodulation techniques such as the Foster-Seeley discriminator or the phase-locked loop (PLL) demodulator. Unlike Foster-Seeley discriminator, it does not respond to unwanted amplitude variation. This makes it ideal for applications where noise interference is a concern.

Applications of Ratio Detector FM Demodulator

The Ratio Detector FM demodulator finds applications in various communication systems, including:

  • Broadcast Radio: The Ratio Detector FM demodulator is widely used in broadcast radio receivers to demodulate FM signals and extract the original audio signals for broadcast radio stations.
  • Television Receivers: The Ratio Detector FM demodulator are used in TV receivers to demodulate FM signals.


Overall, the Ratio Detector FM demodulator is a demodulation technique that operates based on the principle of converting instantaneous frequency deviation to amplitude variation such that amplitude variation is maintained constant while the ratio of the instantaneous amplitudes may vary. Its wide frequency range or operation, good linearity, and immunity to noise amplitude variation has made it a popular choice as FM demodulator. In early days it was used in FM radio and TV receivers.

Further Readings and References

[1] Difference and Similarities between FM and PM

[2]  Frequency Modulation(FM) vs Amplitude Modulation(AM)

[3]  FM generation using VCO  

[4] Disadvantage of Slope Detector for FM demodulation

Post a Comment

Previous Post Next Post