Angle Modulation

Angle modulation is a name given to forms of modulation that are based on altering the angle or phase of a sinusoidal carrier. Using angle modulation there is no change in the amplitude of the carrier.

The two forms of modulation that fall into the angle modulation category are frequency modulation and phase modulation.

Both types of angle modulation, namely, frequency modulation and phase modulation are linked because frequency is the derivative of phase, i.e. frequency is the rate of change of phase.

Another way of looking at the link between the two types of modulation is that a frequency modulated signal can be generated by first integrating the modulating waveform and then using the result as the input to a phase modulator. Conversely, a phase modulated signal can be generated by first differentiating the modulating signal and then using the result as the input to a frequency modulator.

Signal bandwidth

One key element of any signal is the bandwidth it occupies. This is important because it defines the channel bandwidth required, and hence the number of channels that can be accommodated within a given segment of radio spectrum. With pressure on the radio spectrum increasing, the radio signal bandwidth is an important feature of any type of radio emission or transmission.

The bandwidth is governed by two major features:

- The type of modulation: Some forms of modulation use their bandwidth

more effectively than others. Accordingly, where spectrum usage is of importance, this alone may dictate the choice of modulation.

- The bandwidth of the modulating signal: A law called Shannon's law

determines the minimum bandwidth through which a signal can be transmitted. In general, the wider the bandwidth of the modulating signal, the wider the bandwidth required.

Modulating signal type

Apart from the form of modulation itself, the type of signal being used to modulate the carrier also has a bearing on the signal. Analogue and data are two very different forms of modulating signal and need to be treated differently. While different formats of actual modulation may be used, the type of signal being applied via the modulator also have a bearing on the signal.

Signals for high quality stereo broadcasting will be treated differently to signals that provide digital telemetry for example. As a result, it is often important to know the signal type that needs to be carried by the RF carrier.

By Ian Poole