IAN LANG ELECTRONICS

Sidebands in FM

<General Theory II Sidebands II>

In the early days of FM it was thought by many that the process did not produce sidebands at all, this was later found to be in error as the deviation in frequency had been confused with the rate of that deviation.  In fact the sidebands produced in FM are infinite, and the way they are produced differs somewhat from those of  AM. The mathematical principles involved are not as simple as those for AM either. Before we can look at the sidebands meaningfully, first we must discuss the terms modulation index and deviation ratio.

Modulation Index and Deviation Ratio

Those of you familiar with AM, or who have read the AM section of this site, will be acquainted with the term modulation index : it is simply the modulating voltage divided by that of the carrier. In FM the meaning is somewhat more esoteric.  In equation terms it is:

in which        is the frequency deviation, fm is the (audio) modulating frequency and  m is the modulating index . As we know, frequency deviation is  determined by the loudness (or more formally amplitude) of  the  modulating signal , and so really what we have here is how loud the signal is divided by how high (or low) pitched it is.  When  both the  deviation of the carrier and frequency of the modulating signal are at the  maximum permissible,  this is known as the deviation ratio. Often the two are synonymous .Again this concept is difficult to grasp and a practical example may serve best.

In the United Kingdom , FM transmissions are regulated to a maximum audio frequency of 15kHz and a maximum carrier deviation of  75 kHz, and this is achieved through design of the circuitry involved in the heterodyning process. As these are maximum (or peak) values, this gives you a deviation ratio of 75/15 or 5.

From the above you may ask yourself why 15kHz?  It is because of the fidelity of sound. As we know, both speech and music do not occupy one frequency only, there are harmonics emanating which gives a voice or an instrument a timbre, and such is the reason an electric guitar, a violin and a piano do not sound the same although all three are playing middle c. The human hearing range ranges from 20-20,000 Hz, though most people have a range within those parameters that falls short of those extremes. 15kHz gives a good reproduction in most circumstances, and it is particularly important when broadcasting  orchestral pieces (think of BBC Radio 3 or Classic FM) which may have some very high frequencies from sopranos, flutes, piccolos etc.

You may also ask yourself why it is important to know the deviation ratio. On the next page we answer that question.