FM Demodulation / Detection Tutorial

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.

These advantages mean that FM has been widely used for many broadcasting and mobile applications.

Frequency modulating a signal

Wide band and Narrow band FM

When a signal is frequency modulated, the carrier shifts in frequency in line with the modulation. This is called the deviation. In the same way that the modulation level can be varied for an amplitude modulated signal, the same is true for a frequency modulated one, although there is not a maximum or 100% modulation level as in the case of AM.

The level of modulation is governed by a number of factors. The bandwidth that is available is one. It is also found that signals with a large deviation are able to support higher quality transmissions although they naturally occupy a greater bandwidth. As a result of these conflicting requirements different levels of deviation are used according to the application that is used.

Those with low levels of deviation are called narrow band frequency modulation (NBFM) and typically levels of +/- 3 kHz or more are used dependent upon the bandwidth available. Generally NBFM is used for point to point communications. Much higher levels of deviation are used for broadcasting. This is called wide band FM (WBFM) and for broadcasting deviation of +/- 75 kHz is used.

Receiving FM

In order to be able to receive FM a receiver must be sensitive to the frequency variations of the incoming signals. As already mentioned these may be wide or narrow band. However the set is made insensitive to the amplitude variations. This is achieved by having a high gain IF amplifier. Here the signals are amplified to such a degree that the amplifier runs into limiting. In this way any amplitude variations are removed.

In order to be able to convert the frequency variations into voltage variations, the demodulator must be frequency dependent. The ideal response is a perfectly linear voltage to frequency characteristic. Here it can be seen that the centre frequency is in the middle of the response curve and this is where the un-modulated carrier would be located when the receiver is correctly tuned into the signal. In other words there would be no offset DC voltage present.

The ideal response is not achievable because all systems have a finite bandwidth and as a result a response curve known as an “S” curve is obtained. Outside the bandwidth of the system, the response falls, as would be expected. It can be seen that the frequency variations of the signal are converted into voltage variations which can be amplified by an audio amplifier before being passed into headphones, a loudspeaker, or passed into other electronic circuitry for the appropriate processing.

Characteristic “S” curve of an FM demodulator

To enable the best detection to take place the signal should be centred about the middle of the curve. If it moves off too far then the characteristic becomes less linear and higher levels of distortion result. Often the linear region is designed to extend well beyond the bandwidth of a signal so that this does not occur. In this way the optimum linearity is achieved. Typically the bandwidth of a circuit for receiving VHF FM broadcasts may be about 1 MHz whereas the signal is only 200 kHz wide.

FM demodulators

There are a number of circuits that can be used to demodulate FM. Each type has its own advantages and disadvantages, some being used when receivers used discrete components, and others now that ICs are widely used.

Below is a list of some of the main types of FM demodulator or FM detector. In view of the widespread use of FM, even with the competition from digital modes that are widely used today, FM demodulators are needed in many new designs of electronics equipment.

• Slope FM detector:   This form of detector uses the slope of a tuned circuit to convert the frequency variations into amplitude variations. As the frequency of the FM signal varies, it changes its position on the slope of the tuned circuit, so the amplitude will vary. This signal can then be converted into a baseband signal by using an AM diode detector circuit. Read more about the Slope Detector
• Ratio detector:   This FM demodulator circuit was widely used with discrete components, providing a good level of performance. It was characterised by the transformer with three windings that was required. Read more about the Ratio Detector
• Foster-Seeley FM detector:   Like the Ratio detector the Foster Seeley detector or discriminator was used with discrete components, providing excellent performance for the day in many FM radios. Read more about the Foster-Seeley Detector
• PLL, Phase locked loop FM demodulator:   FM demodulators using phase locked loops, PLLs cna provide high levels of performance. They do not require a costly transformer and can easily be incorporated within FM radio ICs. Read more about the PLL FM Detector
• Quadrature FM demodulator:   This form of FM demodulator is very convenient for use within integrated circuits. It provides high levels of linearity, while not requiring many external components. Read more about the Quadrature FM Detector
• Coincidence FM demodulator:   This form of demodulator has many similarities to the quadrature detector. It uses digital technology and replaces a mixer with a logic NAND gate.

Each of these different types of FM detector or demodulator has its own advantages and disadvantages. These FM demodulators are described in further pages of this tutorial.

By Ian Poole

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