For a radio signal to carry information it must be varied or modulated - there are various different types of modulation, each best in different situations.

Radio signals must be modulated or changed in some way to enable them to carry any form of information. From analogue to digital, and from amplitude to frequency and phase, some aspect of the signal must be changed or modulated in line with the signal that needs to be carried.

There are various different ways in which the signal can be modulated, using one of a variety of different techniques, and hence there are different types of modulation. Each type of modulation has its own characteristics. Different types of modulation are applicable for different occasions. Some types and techniques are better in some situations than others.

As a result it is important to understand what the different types of modulation and the different techniques that can be used. In this way the best form of modulation can be chosen for any given situation.

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Basic types of modulation

There are three main techniques used to modulate a radio frequency carrier or signal signal:

Amplitude modulation AM:   As the name implies, this form of modulation involves modulating the amplitude or intensity of the signal.
Amplitude Modulation, AM Amplitude modulation was the first type of modulation to be used to broadcast sound. Today other forms of modulation are being increasingly used; but amplitude modulation is still in widespread use.
Frequency modulation, FM:   This form of modulation varies the frequency in line with the modulating signal.
Frequency Modulation, FM Frequency modulation has the advantage that it is possible to limit amplitude noise on the signal because it is only the frequency variations that carry the required information. This can be achieved by passing the signal through a stage that runs into limiting, thereby removing the amplitude variations which may be the result of noise and general signal variations. If there is sufficient signal to run a stage into limiting, any signal strength variations will not change the level of the demodulated audio, assuming audio is being carried. As a result is form of modulation has been used for many applications including high quality analogue sound broadcasting.

Phase modulation, PM:   As the name indicates, phase modulation varies the phase of the carrier in line with the modulating signal.
Phase modulation and frequency modulation have many similarities and are linked - one is the differential of the other. However phase modulation lends itself to data transmissions, and as a result its use has grown rapidly over recent years.

Each type of modulation has its own advantages and disadvantages, and accordingly they are all used in different radio communications applications.

In addition to the three main basic forms of modulation or modulation techniques, there are many variants of each type. Again these modulation techniques are used in a variety of applications, some for analogue applications, and others for digital applications.

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.

Modulation combinations

It is possible to use forms of modulation that combine both amplitude and angle modulation components. In this way enhancements in performance can be gained.

Quadrature amplitude modulation, QAM:   Using this form of information amplitude and phase information are used to carry the signal. Data is modulated onto In-phase and Quadrature elements of the signal: I & Q and the constellation forms a number of points in the two planes.
Amplitude & Phase Sift Keying, APSK:   Using APSK, the constellation can be arranged to optimize the peak to average power ratio and fewer amplitude levels cab be set when compared to QAM. This enables RF power amplifiers to operate more efficiently.
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 uses 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

When choosing the type of modulation to be used it is necessary to look at the advantages and disadvantages of each type of modulation. AM and FM are widely used for analogue sound transmission, whereas phase shift keying and quadrature amplitude modulation are often used for transmitting digital data.

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Source: electronics-notes