The mixer has two input
signals of different
frequencies, f1 and f2.
These inputs are mixed
together in the mixer.
(some books say "beaten"
together, others say
"heterodyned").
f1 and f2 then come out
of the mixer, together
with two new
frequencies.
One of the new
frequencies is the sum
of the two inputs, f1 +
f2.
The other is the
difference between the
two inputs, f1 - f2.
For example, if the
inputs are 1 Mhz and
1.47 MHz then the sum
frequency is 2.47 MHz.
The difference frequency
is 0.47 MHz (470 kHz).
Sometimes, on the radio,
two adjacent stations
will produce an
interfering whistle.
This is because their
frequencies are close
enough to beat together.
The difference between
their frequencies is in
the audio range.
If you have two racks of
equipment, cooled by
fans, the noise produced
by each fan rotating
often beats together to
give a low frequency
beat noise.
Mixers are used as part
of the FREQUENCY CHANGER
in radios.
Understanding mixers
will help you to
understand the
MODULATION process in
A.M. transmitters.
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