Modulation

Amplitude modulation

In amplitude modulation, the amplitude of the carrier is varied by the incoming modulating signal.

If the modulating signal is an analogue waveform, the amplitude of the carrier will continuously vary, according to the instantaneous value of the modulating signal.

If the modulating signal is a data signal, the amplitude of the carrier will take one of two values, depending on whether the incoming bit is a 1 or a 0.

Amplitude modulation is vulnerable to any noise on the transmission medium, as noise can be seen as a signal by the receiver.

Frequency modulation

In frequency modulation, it is the frequency of the carrier which is varied by the incoming modulating signal.

Again, if the modulating signal is an analogue waveform, the frequency of the carrier is continuously varied depending on the instantaneous value of the amplitude of the incoming signal.

If the modulating signal is a digital waveform, the carrier will take one of two different frequencies, depending if the data is at 1 or 0.

This is known as FSK - Frequency Shift Keying.

At the instant that the carrier changes from one frequency to the other, a considerable amount of spurious frequencies are generated, which can cause considerable interference to other transmission channels. It is therefore normal to filter the incoming data waveform so that the modulating signal changes from 1 to 0 or 0 to 1 quite slowly - this reduces the rate at which the carrier changes frequency, and reduces the spurious transmissions.

Phase modulation

This type of modulation alters the phase of the carrier depending on the state of the input modulating signal.

If the modulating signal is an analogue waveform, then the phase of the carrier is continuously varied depending on the instantaneous value of the amplitude of the incoming signal.

If the modulating signal is a digital data stream, the phase of the carrier takes up certain defined values.

In its simplest form, a two state digital modulating signal causes the carrier to adopt one of two phase states, separated by 180 degrees. This is known as binary phase shift keying - BPSK.

More advanced forms of PSK can use more than two phases states - these are known as N-ary PSK, where N is the number of states.

N = 4, ie 4 states, is quite common - each state represents 2 bits
N = 8 is also used - each state represents 3 bits
N = 16 is less common - each state represents 4 bits
N = 64 is unusual, but has been used on very noise free links - each state represents 6 bits

It is important to distinguish between phase coherent PSK and differential PSK.

In phase coherent PSK, a run of bits which do not change can cause the carrier to have a run where there is no change of phase - in the receiver this can lead to a loss of synchronisation - the receiver does not know how many bits it has received.

This can be prevented by using differential PSK, in which every new bit or set of bits produces a change of phase state, even if the new bit or bits are the same as the last one or ones. This means that the receiver can always detect each new bit or set of bits, and can remain in synchronisation.

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