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From: firstname.lastname@example.org (john jardine)
Subject: Re: How does a mixer work?
Date: 17 Oct 2002 19:23:01 -0700
References: <email@example.com> <firstname.lastname@example.org>
NNTP-Posting-Date: 18 Oct 2002 02:23:01 GMT
email@example.com (Asa Cannell) wrote in message news:<firstname.lastname@example.org>...
> Okay now I get the multiply part, where the LO is driven hard,
> multiply the RF/IF input by 1 or 0. But why do we need non-linear
> elements? I understand if you apply two signals to a linear element
> you just get to signals. But I dont see why a non linear element
> suddenly multiplies. Is there some analogy or other type of circuit
> that uses the non linear aspect of diodes in the same or similar way?
Yeah, it is a problem.
(How come it takes a least 6 hours to get my typing onto Google yet
numerous interesting discussions can come and go in the meanwhile :-)
If you listen to the o/p of an audio amp (fed with a sine wave) that
is over driven and 'clipping' against the supply rails you can easily
hear the extra new distortion frequencies that are being generated.
These are harmonically related to the fundamental frequency and have
been generated by a non linear process. The non linearity in this case
is the linear input signal being flat topped as it tries to push
beyond the supply rails. Its a different signal now and could only be
replicated by adding the original sine wave and number of harmonic
freqencies together using the correct phases and voltages.
The non linear mechanism in that case was a 'static' obstacle and
would always deform each cycle of the incoming signal in *exactly* the
same way. All cycles passing through are clones of each other and only
harmonics of the single input signal are thus being generated.
If the non linear mechanism is caused to vary its strength or effect
in sympathy with a second signal, eg altering the supply rails of the
amp in a cyclic manner then the amount of distortion now varies on a
cycle by cycle basis. This cyclic variation in the distortion of a
waveshape *cannot* be replicated just by adding harmonics to a
fundamental signal. It can only be physically realised by also adding
in a couple of new frequencies, which are actually the sum and the
difference of the 2 driving signals.
Like a fourth dimension, (or FM sidebands), The step by step
visualisation of these new frequencies cannot easily be made (by a
mere mortal anyway) and it is a perfect example of the ability of a
single line of maths to bypass manufacturing a mental model and
provide the insight impossible by other means.
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