
378
there is a frequency equal to that of b plus that of a is not at all
obvious, but its existence can be shown mathematically, or by actual
experiment—picking it out from the rest by tuning a sharply
resonant circuit to it.
The purpose of the local oscillator can now be seen as a means of
varying the conductance of a part of a circuit carrying the incoming
signal, amplitude-modulating it and thereby creating signals of at
least two new frequencies.
Either of these could be chosen as the i.f. in a superhet, but the
lower or difference frequency is the one usually favoured, because it
is easier to get the desired amplification and selectivit ...