From: John Woodgate
Subject: Re: How does a mixer work?
Date: Wed, 23 Oct 2002 07:45:05 +0100
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Wed, 23 Oct 2002 11:00:15 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
I read in sci.electronics.design that John S. Dyson
wrote (in ) about 'How does a mixer
work?', on Tue, 22 Oct 2002:
>Firstly, the real world signal (as we normally perceive it) is in the time
>Any normal means of spectrum analysis, or fourier transform tends to make
>the assumption that the function is non-varying in character.
No, you are using the wrong term and that is very confusing. The
function is not assumed to be non-varying, but IS assumed to be
repetitive over time - the pattern of variation repeats.
>normal linear transforms (where spectrum analysis is allied to the fourier
>kind of transform) don't generally work well WRT nonlinear operations (or
>with signals that have time varying characteristics.)
Again, this is confused and misleading. You can certainly, for example,
do Fourier analysis on a half-wave or full-wave rectified sine wave,
which is the result of a non-linear process.
> Spectrum analysis or
>fourier transforms can still be useful in varying situations, but the
>needs to be interpreted slightly differently, and with some caveats.
You need to define 'varying situations'. Your discussion of passing a
Morse signal through a very narrow filter, so narrow that the output at
carrier frequency is almost constant shows that the 'interpretation' may
need to be dramatically rather than slightly different.
Some of the statements in your long post are correct and some are
misleading at best.
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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