What is FM: Frequency Modulation Tutorial

- 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.

Since its first introduction the use of frequency modulation, FM has grown enormously. Now wideband FM is still regarded as a very high quality transmission medium for high quality broadcasting. FM, frequency modulation is also widely used for communications where it is resilient to variations in signal strength.

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.

The concept of frequency modulation, showing how a varying modulation signal varies the frequency of the carrier
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.

Note on FM Demodulation:

In order to be able to extract any information being carried by an FM signal, it is necessary to pass it through a demodulator. The output from this stage provides the information that was carried by the FM signal. There are many methods of achieving this, using circuits that employ a variety of different techniques.

Read more about FM demodulation

Narrowband FM, NBFM, & 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.

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.

FM pre-emphasis & 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 50µsS. For this the break frequency f1 is 3183 Hz. For broadcasting in North America values of 75µs 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 losing 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 in a 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.

By Ian Poole

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