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.

"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.

W3JDR wrote:
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.

Hearing beats does not require non-linear or multiplicative mixing -
please see my separate reply to Fred.


As to Ian's comment...I don't think "adding" is the correct term either.

I was referring to what *audio* engineers call "mixing", which is
nothing else but simple adding or linear combining.

I agree with everything you say below...

"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.

...but the processes you describe are not what a straightforward audio
"mixing" desk does.

The device you describe above, an audio engineer would know as a
"modulator" or a "ring modulator". For example, the LO could be at a low
frequency, to get some kind of throbbing effect. Both RF and audio
engineers would agree, that is true modulation.

The difference is that RF engineers would also call that process
"mixing"... but audio engineers would not because, to in their
professional world, "mixing" means adding or linear combining.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek

"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.

W3JDR wrote:
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.

Hearing beats does not require non-linear or multiplicative mixing -
please see my separate reply to Fred.


As to Ian's comment...I don't think "adding" is the correct term either.

I was referring to what *audio* engineers call "mixing", which is
nothing else but simple adding or linear combining.

I agree with everything you say below...

"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.

...but the processes you describe are not what a straightforward audio
"mixing" desk does.

The device you describe above, an audio engineer would know as a
"modulator" or a "ring modulator". For example, the LO could be at a low
frequency, to get some kind of throbbing effect. Both RF and audio
engineers would agree, that is true modulation.

The difference is that RF engineers would also call that process
"mixing"... but audio engineers would not because, to in their
professional world, "mixing" means adding or linear combining.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek