Mixing techniques WAS:Re: Add, subtract, multiply

[uzs159 at ibm.rhrz.uni-bonn.de]

<\lurkmodeoff>

Hi to everyone on this list.

To further confuse things i'll contribute my point of
view in this mixing discussion:
(although I hope it will clarify things, so that we 
can get back to the solder iron.)

As far as I know there are several things refered to as "mixing",
first of all, there is simply ADDING two signals, (e.g. by the 
use of an summing opamp)  this is what an audio mixer does.
Like the one used in a stereo set.
When you apply two audio signals of equal amplitude and frequency
and of course phase, you get _exactly_ the doubled amplitude, same
frequency, and phase, at the output compared to the input signal.

Ok, 20 times a 2V signal will on a real mixer never get 40V at the 
output, but thats because our circuits are not _ideal_ mixers.
To put it another way: In its linear operation range (say 500mV)
a _real_ mixer will behave (mostly) like an ideal one.

So when you feed two signals with different frequency and therefore
phase (this will change over the full period) and amplidude,
you get the sum of them both.

Lets say S1= A1*sin(f1*t)
         S2= A2*sin(f2*t)
A1,2 = amplitude of signals
f1,2 = frequency of signals
t= Time

Output = A1 * sin(f1*t) + A2*sin(f2*t)   Not more, not less.

Real mixers are limited in an other way, the reason why an ideal mixer
behaves like that, is that it's a linear device, but thats not exactly 
true for real mixers. The amount of nonlinearity is measured by its 
THD (total harmonic distortion) value.

So when you feed two signals into a nonlinear device, you don't get 
just those two signals back, but the sum and the difference of the 
frequencies, so in the above example you would get 

A1*sin(f1*t)+A2*sin(f2*t)+A3*sin(f1+f2*t)+A4*sin(f1-f2*t)= output

where the Amplitudes of those several signals depends on the nonlinearity
of the device. The higher the distortion, the higher the amplitudes of 
the sum and diff. signals, A1,2 will be somewhat lower than above. 

Usually this is unwanted, so that the designer of the circuit
wants low THD so that the A3,4 are small.

A way to archieve this is to feed two signals into a nonlinear circuit,
like a diode for example: (Just an example, I didn't even tried myself!)

in2   o---\/\/----|------+--O output
in1   o---\/\/----|      |
                  V      -     use germanium diodes like AA118 or eq.
                  -      A
                  |      |     if the distortion is way too high try  
                 -+-    -+-    adding resistors in serial to the diodes.

I think thats what the original poster wanted. (Am I right?)

This process could be called nonlinear mixing, it's often referred to as
"heterodyning".  (Sort of an amplitude modulation)

The third thing to be considered as "mixing" is Ringmodulation,
actually this is multiplication of the values of the input signals.
The reason why this is called mixing, is that it adds and subtracts 
frequencies. Like in the above, but the Input signals do not appear 
at the output.

Stay in tune, Bye!

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