What is FM, Frequency Modulation

- overview or tutorial about the basics of what is frequency modulation, FM used for modulating a radio signal to carry sound or other information.

Frequency modulation, FM is widely used for a variety of radio communications applications. FM broadcasts on the VHF bands still provide exceptionally high quality audio, and FM is also used for a variety of forms of two way radio communications, and it is especially useful for mobile radio communications, being used in taxis, and many other forms of vehicle.

in view of its widespread use, frequency modulation, FM, is an important form of modulation, despite many forms of digital transmission being used these days.

Fm, frequency modulation has been in use for many years. However its advantages were not immediately apparent. In the early days of wireless, it was thought that a narrower bandwidth was required to reduce noise and interference. As FM did not perform well under these conditions, AM predominated and FM was not used. However, Edwin Armstrong, an American engineer looked at the use of wideband FM for broadcasting and introduced the idea against the trend of the thinking of the time. However since its first introduction the use of frequency modulation, FM has grown enormously.

FM, frequency modulation basics

The most obvious method of applying modulation to a signal is to superimpose the audio signal onto the amplitude of the carrier. However this is by no means the only method which can be employed. It is also possible to vary the frequency of the signal to give frequency modulation or FM. It can be seen below that the frequency of the signal varies as the voltage of the modulating signal changes.

Concept of frequency modulation

The amount by which the signal frequency varies is very important. This is known as the deviation and is normally quoted as the number of kiloHertz deviation. As an example the signal may have a deviation of ±3 kHz. In this case the carrier is made to move up and down by 3 kHz.

Narrowband FM, NBFM, and Wideband FM, WBFM

The level of deviation is important in many aspects. It obviously is important in determining the bandwidth of the overall signal. As a result the deviation used for FM is different between different applications. Broadcast stations in the VHF portion of the frequency spectrum between 88.5 and 108 MHz use large values of deviation, typically ±75 kHz. This is known as wideband FM (WBFM). These signals are capable of supporting high quality transmissions, but occupy a large amount of bandwidth. Usually 200 kHz is allowed for each wideband FM transmission. For radio communications purposes less bandwidth is used. Narrowband FM, NBFM often uses deviation figures of around ±3 kHz or possibly slightly more. As quality is not as important for radio communications applications, the much narrower bandwidth has advantages in terms of radio spectrum efficiency.

FM advantages

Frequency modulation, FM has a number of advantages and this enables it to be used to advantage in many applications. One particular advantage is its resilience to signal level variations. The modulation is carried only as variations in frequency. This means that any signal level variations will not affect the audio output, provided that the signal does not fall to a level where the receiver cannot cope. As a result this makes FM ideal for mobile or portable applications where signal levels are likely to vary considerably. The other advantage of FM is its resilience against noise and interference. It is for this reason that FM is used for high quality broadcast transmissions.

To demodulate an FM signal it is necessary to convert the frequency variations into voltage variations. This is slightly more complicated than demodulating AM, but it is still relatively simple to achieve. Rather than just detecting the amplitude level using a diode, a tuned circuit has to be incorporated so that a different output voltage level is given as the signal changes its frequency. For further information on demodulating FM refer to the "Radio Receivers section of Tutorials area of this website.

Modulation index and deviation ratio

In just the same way that it is useful to know the modulation index of an amplitude modulated signal the same is true for a frequency modulated signal. The modulation index is equal to the ratio of the frequency deviation to the modulating frequency. The modulation index will vary according to the frequency that is modulating the transmitted carrier and the amount of deviation. However when designing a system it is important to know the maximum permissible values. This is given by the deviation ratio and is obtained by inserting the maximum values into the formula for the modulation index.

D = (Max deviation frequency) / (Max modulation frequency)

For a VHF FM sound broadcast transmitter the maximum deviation is 75 kHz and the maximum modulation frequency is 15 kHz giving a deviation ratio of 5.

Sidebands using FM

Any signal that is modulated produces sidebands. In the case of an amplitude modulated signal they are easy to determine, but for frequency modulation the situation is not quite as straightforward. . They are dependent upon the not only the deviation, but also the level of deviation, i.e. the modulation index M. The total spectrum is an infinite series of discrete spectral components expressed by a complex formula using Bessel functions of the first kind.

The total spectrum can be seen to consist of the carrier plus an infinite number of sidebands spreading out on either side of the carrier at integral multiples of the modulating frequency. The relative levels of the sidebands can be obtained by referring to a table of Bessel functions. It can be seen from the image below that the relative levels rise and fall according to the different values of modulation index.

Relative levels of carrier and sidebands for a frequency modulated signal

For small values of modulation index, when using narrow-band FM, and FM signal consists of the carrier and the two sidebands spaced at the modulation frequency either side of the carrier. This looks to be the same as an AM signal, but the difference is that the lower sideband is out of phase by 180 degrees.

As the modulation index increases it is found that other sidebands at twice the modulation frequency start to appear. As the index is increased further other sidebands can also be seen.

Spectra of an FM signal with differing levels of modulation index

Bandwidth of a frequency modulation signal

In the case of an amplitude modulated signal the bandwidth required is twice the maximum frequency of the modulation. Whilst the same is true for a narrowband FM signal, the situation is not true for a wideband FM signal. Here the required bandwidth can be very much larger, with detectable sidebands spreading out over large amounts of the frequency spectrum. Usually it is necessary to limit the bandwidth of a signal so that it does not unduly interfere with stations either side.

While it is possible to limit the bandwidth of an FM signal, this should not introduce any undue distortion. To achieve this it is normally necessary to allow a bandwidth equal to twice the maximum frequency of deviation plus the maximum modulation frequency. In other words for a VHF FM broadcast station this must be (2 x 75) + 15 kHz, i.e. 175 kHz. In view of this a total of 200 kHz is usually allowed, enabling stations to have a small guard band and their centre frequencies on integral numbers of 100 kHz.

Improvement in Signal to Noise Ratio

It has already been mentioned that FM can give a better signal to noise ratio than AM when wide bandwidths are used. The amplitude noise can be removed by limiting the signal to remove it. In fact the greater the deviation the better the noise performance. When comparing an AM signal to an FM one an improvement equal to 3 D2 is obtained where D is the deviation ratio.

Pre-emphasis and de-emphasis when using frequency modulation

An additional improvement in signal to noise ratio can be achieved if the audio signal is pre-emphasised. To achieve this the lower level high frequency sounds are amplified to a greater degree than the lower frequency sounds before they are transmitted. Once at the receiver the signals are passed through a network with the opposite effect to restore a flat frequency response.

To achieve the pre-emphasis the signal is passed through a capacitor-resistor (CR) network. At frequencies above the cut-off frequency the signal increases in level by 6 dB per octave. Similarly at the receiver the response falls by the same amount.

Both the receiver and transmitter networks must match one another. In the UK the CR time constant is chosen to be 50 S. For this the break frequency f1 is 3183 Hz. For broadcasting in North America values of 75 S with a break frequency of 2.1 kHz is used.

Pre-emphasising the audio for an FM signal is effective because the noise output from an FM system is proportional to the audio frequency. In order to reduce the level of this effect, the audio amplifier in the receiver must have a response that falls with frequency. In order to prevent the audio signal from loosing the higher frequencies, the transmitter must increase the level of the higher frequencies to compensate. This can be achieved because the level of the high frequency sounds is usually less than those lower in frequency.

Frequency modulation highlights

Frequency modulation is used ina wide variety or radio communications applications from broadcasting to two way radio communications links as well as mobile radio communications. It possesses many advantages over amplitude modulation and this is the reason for its widespread use. Nowadays, many digital forms of radio communications are being introduced, but despite this the use of frequency modulation, FM will undoubtedly continue for many years to come in many areas of radio communications.