From: John Larkin
Subject: Re: Homebrew Microwave Interferometer
Date: Thu, 28 Nov 2002 14:02:58 -0800
Organization: Posted via Supernews, http://www.supernews.com
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On Thu, 28 Nov 2002 16:16:52 +0000, John Woodgate
>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.
Police speed guns are generally calibrated using a tuning fork.