Up until now we've just considered the creation, transmission and reception of electromagnetic fields. In effect, we've seen how EM power can be carried from place to place. Now we'll look at how EM fields and waves can be used to communicate information. In general, we can represent a sinewave coherent field or wave as
where A, f, and 
are the amplitude, frequency, and phase of the wave. We can send information by modulating one or more of these quantities. Fields or signals modulated in these ways are — for obvious reasons — called AM (Amplitude Modulated), FM (Frequency Modulated), or PM (Phase Modulated) waves. In fact, since EM fields are vector fields we can also modulate the Polarisation of a wave to communicate information. However, polarisation modulation is only normally used for special purposes as the polarisation state of a wave can easily be ‘scrambled’ during free space transmission by effects like unwanted reflections from buildings. (The polarisation state of a field moving along a guide or fibre can also be altered by bends or imperfections in the guide/fibre.)
9.2 An Amplitude Modulation Circuit.
We can define amplitude modulation by saying that a modulating signal , 
, should produce an AM wave of the form
(We'll use ‘S’ instead of ‘V’ or ‘E’ to indicate that we're talking about a modulated quantity and that it can be either a field or a voltage.) 
represents the ‘unmodulated amplitude’ of the wave (i.e. when 
). A +ve value of 
makes the wave's amplitude larger, a -ve value makes its amplitude smaller. Amplitude modulation can be produced in various ways. Here we'll use the example of a Square Law FET as illustrated in figure 9.1. The steady signal, 
is called the carrier. This gives us something to modulate and ‘carry’ the information pattern from place to place. The frequency, f , is called the carrier frequency and 
is the unmodulated carrier amplitude.
A square law n-channel FET (Field Effect Transistor) will pass a drain-source current
where 
is the FET's pinch-off voltage and 
is the gate voltage. This expression is correct provided that we keep 
in the range, 
. In the circuit shown in 9.1 we apply a gate voltage which is a combination of the output from a local oscillator, 
, and a modulation input, 
, which is the information pattern we want to send from place to place. As a result, the gate voltage (assuming the two gate resistors have the same value) will be
Provided we keep this in the required range it will produce a drain-source current
where k and 
are constants whose values depend upon the FET we've chosen. This can be expanded and re-written as
This produces a drain voltage of
where R is the resistance between the FET's drain terminal and the bias voltage, 
By consulting a book on trig we can discover that 
. Hence the last term in expression 9.6 is a combination of a steady current and a fluctuation at the frequency, 
. For simplicity we can arrange that the frequencies with which 
fluctuate are all 
. This means that the first part of the expression consists of a steady current plus some fluctuations at frequencies well below f. We can now use a bandpass filter, designed to only pass frequencies 
to strip away low and high frequencies and obtain an output
which we can re-write in the form
where
i.e. the output is a wave whose unmodulated amplitude is 
and is amplitude modulated by an amount, 
, proportional to the input modulating signal, 
. The circuit therefore behaves as an amplitude modulator.
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