Yes, your friend is right, but there is a grain of truth in what you are
saying also.

A mixer multiplies two signals as your friend says and as said in the
posting by W7EL. Signals can be represented by cosines, and the product of
two cosines is:

cosA * cosB = 0.5(cos(A-B) - cos(A+B) )

See product identities on http://www.swt.edu/slac/math/trigrev/trigrev.html
and let A=2*pi*f1 and B=2*pi*f2.

So while the operation of the mixer is that signals are _multiplied_, the
frequencies will _add_ or subtract.


--
Sverre Holm, LA3ZA
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www.qsl.net/la3za