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From: Don Pearce
Subject: Re: How does a mixer work?
Date: Fri, 18 Oct 2002 11:38:52 +0430
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On Fri, 18 Oct 2002 07:58:23 +0100, "Kevin Aylward"
>Having reread the original post, I still think my approach was the best
>for the question:-)
>In that context, I don't see any reason at all to introduce a physical
>embodiment explanation of mixing.
>e.g. "How does a mixer work?" "why it needs a non-linear element", "what
>does multiply mean in terms of mixing"
>None of this is asking for a circuit explanation. It wants the theory.
>The theory is that multiplying two sin waves generates sum and
>difference frequencies, and that a non-linear transfer functions can
>generate this multiplication feature. A circuit explanation actually
>says *nothing* at all. It does *not* *explain* *why* sum and difference
>frequencies are generated in a non-linear circuit. Only the math will do
I would suggest you take a look at the two inputs and the output of a
double balanced mixer. Take a single sweep freeze, and look at the
waveforms. The whole thing becomes beautifully clear - you can see the
sum and difference frequencies as well as the absence of LO and RF
frequencies. You should be able to do this in your own Spice, but it
is actually rather better viewed in MathCad, where you can manipulate
the numbers more easily.
Having got that far, and understood what is really happening, it is
easier to look into circuits with more minor non-linearities to see
why there is still some - albeit reduced - mixing action going on.
You can always write your own non-linear transfer function and
multiply a pair of sine waves by it to see what happens.
Viewing it in circuit terms this way provides - I believe - a far
better understanding of what is actually happening than merely
reproducing an equation and saying - "look, sum and difference come
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