oadcast transmitter the maximum deviation is 75 kHz and the maximum modulation frequency is 15 kHz giving a deviation ratio of 5. Sidebands 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. For small values of modulation index, when using narrow-band FM, an 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. It is also found that the relative Modulation 61 levels of these sidebands change, some rising in level and others falling as the modulation index varies. This makes prediction of the exact levels of all the sidebands more difficult than for AM. If a mathematical calculation of the levels of the sidebands is required, a function known as a Bessel function or series needs to be calculated. Bandwidth In the case of an amplitude modulated signal the bandwidth required is twice the maximum frequency of the modulation. While the same is true for a narrow-band FM signal, the situation is not true for a wide-band 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 X 75) + 15