From: John Woodgate
Subject: Re: Homebrew Microwave Interferometer
Date: Thu, 28 Nov 2002 16:16:52 +0000
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Thu, 28 Nov 2002 18:05:30 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
I read in sci.electronics.design that John Jardine
wrote (in ) about 'Homebrew Microwave Interferometer', on Thu, 28 Nov 2002:
>A 10Ghz 'direct conversion' radar mixer like used on say, the burglar alarm
>detectors is pushing out 3 centimetre radiation.
>A surface vibrating at an audio frequency would (I imagine) be moving to and
>fro in the region of a few microns.
>I'd think that the resulting audio FM at the mixer output (including
>reflection and path losses) would be too low to usefully recover.
A loudspeaker gives a nominally flat frequency response when driven with
a constant voltage. This produces a nominally constant back e.m.f., like
a motor (assuming voice-coil inductance is negligible). A constant back
e.m.f is produced by a constant cone velocity, so the amplitude is
inversely proportional to frequency. If we assume a fairly modest
excursion of 1 mm at 100 Hz, we get 10 microns at 10 kHz. That gives a
deviation of 10 GHz x (10 u)/30 = 3333 Hz.
It does not appear impracticable. But what is measured is the *average*
displacement of the whole cone, which is not all that useful I think,
whereas a laser can scan the surface and show break-up patterns.
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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