The input signal is converted into a square wave and, together with the VCO
output, forms the two inputs to an Exclusive-OR gate.
Remember that the Exclusive-OR gate provides an output whenever the two inputs
are different in value and zero output whenever they are the same.
Figure 108 shows the situation when the FM input is at its unmodulated carrier
frequency and the VCO output is at the same frequency and 90ฐ out of phase. This
provides an output from the Exclusive-OR gate with an on-off ratio of unity and an
average voltage at the output of half of the peak value (as shown).