PSN-L Email List Message

Subject: Re: detector design
From: Tom Leiper twleiper@........
Date: Sun, 9 Sep 2001 05:05:24 -0400

On Sat, 08 Sep 2001 14:17:08 -0230 Ron Thompson
>     I better clarify this, and BTW my job depends on people breaking
> things, someone must keep up the good work :-)
> Vibration magnitude then is determined by the induced voltage
> on the received antenna, and the phase relationship tells us if
> the receiver plate moved to one side or the other, but not how far.
> I believe that this should work at a frequency where the length
> of the coax to the receiving plate should not introduce a
> significant phase shift itself, and within the range of cheaper
> digital IC's.

But, while that might be a clever way to determine direction, I still
think that if you do the math, you'll see that to get a measurable
phase shift even for a rather significant movement of, say, 1mm,
you would need a wavelength of 4 to 10 mm...moving you into the
rarified area of the spectrum above 30 Ghz which, even if you could
produce and detect it, would make coax just a dream since you
would be working with waveguides and cavity tuned elements,
and definitely not with cheap IC's. This argument completely
ignores the additional problem of the interaction of your detector
element with your radiator elements since, in order to receive a
signal of any magnitude, it would have to be tuned to some
extent, and who knows what phase angles would be present in
such a closely coupled environment.

If you really want a super-sensitive and novel way of detection,
why don't you review the archives for a post I did last year which
described a method whereby two oscillators are permeably (and
oppositely) tuned by a moving core, and very sensitive frequency
changes are, in effect, measured with cheap presettable counters.
If fact, I think I even biased the coils with dc feedback to make it
a force-balance instrument at the same time. I have not had the
time to build it, but I know it will work.

And so there is more grist for our ever grinding mill...


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Larry Cochrane <cochrane@..............>