"W3JDR" a écrit dans le message news:
...
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.


Sure. Just try to suppress ear non linear effects and see how miserable
composers will feel without it and how poor the music will sound to our
"marvellous new ears".


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.


Or, convolving, if the frequency domain, which tells all the story.

Fred.