Frequency Modulation, FM Modulation Index & Deviation Ratio

The Modulation Index and Deviation Ratio are two key parameters for any FM, frequency modulation signal.

Two key parameters of any frequency modulated signal are the modulation index and the deviation ratio.These two parameters describe some of the basic characteristics of a given FM signal - the modulation index providing a measure of what is effectively the level of modulation and the deviation ratio a measure of the deviation relative to the modulating frequency.

Frequency modulation index

The frequency modulation index is the equivalent of the modulation index for AM , but obviously related to FM. In view of the differences between the two forms of modulation, the FM modulation index is measured in a different way.

The FM modulation index is equal to the ratio of the frequency deviation to the modulating frequency.

To give an example of the FM modulation index, take the example where a signal has a deviation of ±5kHz, and the modulating frequency is 1kHz, then the modulation index for this particular instance is 5 / 1 = 5.

FM deviation ratio

One of the issues with the modulation index is that it will vary according to the instantaneous values of deviation and modulating frequency.

On typical audio transmissions, both the frequency deviation and modulating frequency will vary. The frequency deviation will vary according to the level of the audio at that moment. Also the modulating frequency will vary as normal audio consists of a variety of frequencies, which vary to give the speech or music, etc.

For many applications it is more useful to have a figure for the maximum permissible values.

Accordingly the FM deviation ratio can be defined as: the ratio of the maximum carrier frequency deviation to the highest audio modulating frequency.

One common example of the FM deviation ratio can be seen by taking the figures for a typical FM broadcast station. Fir these stations the maximum frequency deviation is ±75 kHz, and the maximum audio frequency fort he modulation is 15 kHz.

Using the formula above, this means that the deviation ratio is 75 / 15 = 5.

FM bandwidth & modulation index.

Frequency modulation is used in a variety of applications. Different levels of deviation are used in different applications. For broadcast FM transmissions the aim is to be able to transmit high quality audio and to achieve this high levels of deviation are used and the bandwidth is wide. For communications purposes, quality is not the issue, but bandwith is more important. Accordingly deviation levels are less and the bandwidth is much smaller.

This has given rise to classifications of narrow band FM and wide band FM. These can be related to the modulation index and deviation ratio.

• Wideband FM:   Wideband FM is typical used for signals where the FM modulation index is above 0.5. For these signals the sidebands beyond the first two terms are not insignificant. Broadcast FM stations use wideband FM which enables them to transmit high quality audio, as well as other facilities like stereo, and other facilities like RDS, etc..
• Narrowband FM:   Narrow band FM, NBFM, is used for signals where the deviation is small enough that the terms in the Bessel function is small and the sidebands are negligible. For this the FM modulation index must be less than 0.5, although a figure of 0.2 is often used. For NBFM the audio or data bandwidth is small, but this is acceptable for this type of communication.