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From: phil@ux1.cso.uiuc.edu
Newsgroups: comp.dsp
Subject: Re: Practical DSP problem
Message-ID: <1028600001@ux1.cso.uiuc.edu>
Date: 10 Oct 90 02:11:00 GMT
References: <3575@syma.sussex.ac.uk>
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Nf-ID: #R:syma.sussex.ac.uk:3575:ux1.cso.uiuc.edu:1028600001:000:913
Nf-From: ux1.cso.uiuc.edu!phil    Oct  9 21:11:00 1990


> We don't know the absolute phase of the signal but we do know the
> exact frequency. We are generating two sine waves at frequencies
> f1 and f2 and trying to measure the distortion product at 2f1 - f2.
> The signal is swamped by the two much larger components at f1 and
> f2, and is close to the noise level. 

That might even be better.  If you have f1 and f2 as separate components,
you can generate 2*(f1-f2) directly from them by multiplying the instant
values of the squares of f1 and f2 generating 2*(f1-f2).  No sin/cos
calculations or tables; just some multiplies.  You will need to subtract
out the DC from those multiplications.  If f1 and f2 are of stable amplitude
(and hence no sidebands to crossmodulate) then removing the DC should be
easy.

--Phil Howard, KA9WGN-- | Individual CHOICE is fundamental to a free society
<phil@ux1.cso.uiuc.edu> | no matter what the particular issue is all about.