piclist 1999\09\19\223133a >
Thread: reflectometry.. measuring the length of a cable
picon face BY : Mike Keitz email (remove spam text)

On Mon, 20 Sep 1999 11:11:35 +1000 Des Bromilow
<spamBeGoneDes.BromilowKILLspamspamCITEC.COM.AU> writes:

> anyone know how to build such a beastie? Anyone ever built one?

You can buy them in many forms.  But it is within the realm of things
that can be built at home if you really want to.

> I envisage something along the lines of a pulse with a delay which
> is then translated to length, but the detection of the echo,

Generally how they work is to generate a pulse, then sample the voltage
some time later.  I think it was once common to use a "snap diode"
(tunnel diode?) to make the pulse, though now a fast logic gate could do
it.  The sample delay is controlled by an analog one-shot.  This allows
nanosecond delay variations to be easily implemented.  A simple device
would do the measurement repeatedly just taking one sample each pulse.
This avoids the need for rapid conversion and storage of the results.  A
slow A/D converter PIC is plenty of processor for it.  You'd need an
analog "front end" consisting of pulse generator, delay unit, and a fast
sample and hold gate.

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See also: www.piclist.com/techref/index.htm?key=reflectometry+measuring
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