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).
$h={\frac {d}{f_{max}}}$
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 h > 1, the bandwidth can be approximated with $f_{b}=2*(d+f_{max})$ (d = frequency deviation)

For signals with a modulation index h < 0.5, the bandwidth can be approximated with $f_{b}=2*f_{max}$

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