of supporting high quality transmissions, but occupy a large amount of bandwidth. Usually 200 kHz is allowed for each wideband FM transmission. For communications purposes less bandwidth is used. Narrow-band FM (NBFM) often uses deviation figures of around +3 kHz or possibly slightly more. FM is used for a number of reasons. 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 60 Newnes Guide to Radio and Communications Technology 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. 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. Max deviation frequency Max modulation frequency For a VHF FM sound br