Frequency modulation

Frequency Modulation (FM) is a mode for transmitting voice over the air. It consists of a HF carrier whose frequency is modulated in proportion to the transmitted audio signal. Usually it is used on the VHF/UHF-bands for simplex and repeater operation.

There are two main types of frequency modulation: Narrow Band FM (NBFM), commonly used in ham radio and Wide Band FM (WBFM), used by radio broadcast stations.

Maths

FM has three main characteristics:

• Frequency deviation (The difference between carrier frequency and highest/lowest hf frequency of the signal)
• Modulation index (The ratio between frequency modulation and the maximum modulated frequency)
• Bandwidth (see below)

Frequency deviation

The frequency deviation of a FM signal is the difference between the highest/lowest (doesn't matter because a FM signal is symmetrical) frequency and the carrier frequency. In ham radio it's usually 3 to 5 kHz (NBFM), in radio broadcasting it's ~75 kHz (WBFM).

Modulation index

The modulation index is calculated by dividing the frequency deviation by the highest modulation frequency (the highest frequency in the audio signal). It's useful for exactly calculating the bandwidth.

Bandwidth

With the bandwidth it's a little more difficult: There is no simple formula for calculating the bandwidth of a FM signal. There is a complicated way of calculating it, using a bessel function. But because this is a difficult approach, there are simplified ways of calculating it. Although these formulas are just an approximation, they are much more simple than the exact approach.

For signals with a modulation index greater than 1, the bandwidth can be approximated with fb = 2 * ( frequency deviation + maximum modulated frequency ).

For signals with a modulation index smaller than 0.5, the bandwidth can be approximated with fb = 2 * maximum modulated frequency.

FM in practice

In practice FM is used in many applications:

• It is widely used on VHF/UHF and higher bands, as well as the 10m HF band
• VHF/UHF repeaters also use FM
• Amsats (amateur radio satellites) sometimes use FM
• often the main mode on handheld radios, sometimes accompanied by digital modes like DMR or C4FM