Fred,
You're exactly correct! That's why a piano tuner person can strike a tuning
fork and a piano key at the same time and hear the frequency difference as a
low beat note.

As to Ian's comment...I don't think "adding" is the correct term either.
"Multiplying" or "sampling" are more precise terms. A perfect balanced
unity-gain mixer actually uses one of the input signals to sample the other.
On the positive half cycle of the LO, one phase of the RF signal is sampled,
and on the other half cycle of the LO the opposite phase of the RF is
sampled. Mathematically, this is equivalent to multiplying the RF signal by
+1 or -1 on alternating half cycles of the LO.

Joe
W3JDR

"Fred Bartoli"
r_AndThisToo wrote in
message ...

"Ian White, G3SEK" a écrit dans le message news:
...
Roy Lewallen wrote:
Multiplying the two original signals of 1500 and 1955 generates the two
new frequencies of 455 and 3455, for a total of four frequencies after
multiplication. Adding them wouldn't do it.

Part of the confusion is that audio engineers talk about "mixing" where
they actually mean adding. Mixing - as RF engineers use the term - is
precisely what they don't want!



Well, ear is also somewhat non linear. So they are also doing mixing.

Fred.