FM Demodulation / Detection Tutorial

Radio-Electronics recently had a useful article on FM demodulation.....

FM demodulation or detection involves changing the frequency variations in a signal into amplitude variations at baseband, e.g. audio. There are several techniques and circuits that can be used each with its own advantages and disadvantages.

Frequency modulation is widely used for radio transmissions for a wide variety of applications from broadcasting to general point to point communications.

Frequency modulation, FM offers many advantages, particularly in mobile radio applications where its resistance to fading and interference is a great advantage. It is also widely used for broadcasting on VHF frequencies where it is able to provide a medium for high quality audio transmissions.

In view of its widespread use receivers need to be able to demodulate these transmissions. There is a wide variety of different techniques and circuits that can be used including the Foster-Seeley, and ratio detectors using discreet components, and where integrated circuits are used the phase locked loop and quadrature detectors are more widely used.

What is frequency modulation, FM?

As the name suggests frequency modulation, FM uses changes in frequency to carry the sound or other information that is required to be placed onto the carrier. As shown below it can be seen that as the modulating or base band signal voltage varies, so the frequency of the signal changes in line with it. This type of modulation brings several advantages with it:

• Interference reduction:   When compared to AM, FM offers a marked improvement in interference. In view of the fact that most received noise is amplitude noise, an FM receiver can remove any amplitude sensitivity by driving the IF into limiting.
• Removal of many effects of signal strength variations:   FM is widely used for mobile applications because the amplitude variations do not cause a change in audio level. As the audio is carried by frequency variations rather than amplitude ones, under good signal strength conditions, this does not manifest itself as a change in audio level.
• Transmitter amplifier efficiency:   As the modulation is carried by frequency variations, this means that the transmitter power amplifiers can be made non-linear. These amplifiers can be made to be far more efficient than linear ones, thereby saving valuable battery power - a valuable commodity for mobile or portable equipment.

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