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'reflectometry.. measuring the length of a cable'
1999\09\19@211305 by Des Bromilow

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Anyone got any bright ideas on a device which can be plugged into the end of a single wire (say one core in a power cable) and the device will return a length from the end of where the cable ends.

the use would be for finding the break in a cable, so if you have a 20m cable, and the device says the end of the active (hot) is 20m, and the length of the neutral is 15m, and the length of the earth is 20m, then you would know that the neutral has a break in it 5m from the other end of the cable.

You could also use it to measure (approximate) the length of a spooled cable, or the kink in an underground cable.

anyone know how to build such a beastie? Anyone ever built one?
I envisage something along the lines of a pulse with a delay which is then translated to length, but the detection of the echo, and processing speed may make it harder. Would a PIC be able to keep up?

Any ideas?

Thanks ,
Des Bromilow

1999\09\19@213417 by TIM

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how about something thathas to do with reflected swr...standind wave ratio.?
am modulation? just a thought.....tim

{Original Message removed}

1999\09\19@215531 by Dave VanHorn

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> Anyone got any bright ideas on a device which can be plugged into the end
of a single wire (say one core in a power cable) and the device will return
a length from the end of where the cable ends.

A TDR


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

Tektronix :)

> I envisage something along the lines of a pulse with a delay which is then
translated to length, but the detection of the echo, and processing speed
may make it harder. Would a PIC be able to keep up?

Doubtful indeed, unless the cable is very long, and you don't need any
accuracy.
Most 50 ohm coaxial cable has a V factor of .66, and the pulse goes out and
comes back, so figure .33C as a starting point.

At 300 MHz, a wavelength in free space is 1M, so to get accuracy to the
nearest meter, you'd have to count at 600*.33 or 200 MHz

Even scenix can't play that game.

Other cables are different, basically the higher the Z, the higher the C.

A pic (or any other micro) could control it, but couldn't directly measure
it.

1999\09\19@215733 by Dave VanHorn

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> how about something thathas to do with reflected swr...standind wave
ratio.?
> am modulation? just a thought.....tim

If all you care about is finding the biggest impedance discontinuity, then
you could do a lumped line oscillator, based on the fact that a short at 1/4
wave reflects as an open, and at 3/4 wave as a short.

1999\09\19@220151 by Dan Creagan

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I think you are talking about a time domain reflectometer. They usually
measure in nanoseconds.  Pretty fast for a PIC.  Probably need some sort of
specialty front end and use the PIC to display the results.  A .1 usec pulse
will mask 35 feet of wire with just its pulse width.

They make reasonably inexpensive ones that use Oscilloscopes for displays.

Dan

{Original Message removed}

1999\09\19@220812 by Dennis Plunkett

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At 11:11 20/09/99 +1000, you wrote:
>Anyone got any bright ideas on a device which can be plugged into the end
of a single wire (say one core in a power cable) and the device will return
a length from the end of where the cable ends.
>
>the use would be for finding the break in a cable, so if you have a 20m
cable, and the device says the end of the active (hot) is 20m, and the
length of the neutral is 15m, and the length of the earth is 20m, then you
would know that the neutral has a break in it 5m from the other end of the
cable.
>
>You could also use it to measure (approximate) the length of a spooled
cable, or the kink in an underground cable.
>
>anyone know how to build such a beastie? Anyone ever built one?
>I envisage something along the lines of a pulse with a delay which is then
translated to length, but the detection of the echo, and processing speed
may make it harder. Would a PIC be able to keep up?
>
>Any ideas?
>
>Thanks ,
>Des Bromilow
>
>


Such devices do exist, the will see cable joints as an impedance change and
they work quite well. Try before you buy, have a look at the Tech Rentals
catledog, they have about three or four for the power industry, under cable
relfectometers (Measurement section)

Dennis

1999\09\19@223133 by Mike Keitz

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On Mon, 20 Sep 1999 11:11:35 +1000 Des Bromilow
<spam_OUTDes.BromilowTakeThisOuTspamCITEC.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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1999\09\20@005536 by brooke

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Hello Des:

The TDR approach requires very fast circuitry.
The swept VSWR approach using a scalar network analyzer required doing an FFT an
d is picky in the step size and bandwidth, but it works without a DC path that t
he TDR requires.

The telephone companies use a capactive method that works quite well.  You need
to know the capacitance between a couple of wires for some distance.  The length
of an open pair can then be calculated.  Of course there are already boxes that
do this.  In the case of a short knowing the Ohms/mile you can
calculate the distance to the short.  The surplus ZM-x series LCR bridges contai
n instructions for using these kinds of calculations to find opens and shorts.

I worked on a system that used "modal decomposition".  It measured the standard
unbalanced s-parameters of balanced components, converted them into unbalanced Z
-parameters, and these into balanced Z-parameters, and finally into balanced s-p
arametes.  This was rather expensive, but it could measure the
length of coax and balanced lines with  millimeter resolution.

Have Fun,

Brooke

1999\09\20@113449 by eplus1

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Scenix has a new SX48/52 line (not in production but samples available) that
includes two 16 bit timers. My quick scan of the data sheet indicates that
it has at least one operating mode that can be used with an external clock
(which could possibly be faster than the 50Mhz system clock). An external
200Mhz oscillator that starts when the pulse is sent and stops when the echo
is received would be the only additional circuit necessary. Of course, the
counter may not work at 200Mhz (data sheet doesn't say) and a similar system
could be constructed for a standard PIC with the addition of an external
(large) counter. Maybe someone knows of a many bit, high speed counter than
can be read via serial?

James Newton, webmaster http://get.to/techref
(hint: you can add your own private info to the techref)
.....jamesnewtonKILLspamspam@spam@geocities.com
1-619-652-0593 phone



{Original Message removed}

1999\09\20@122845 by les

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HI
               Is the PicBasic List down? I have received several messages
here in the UK trying to confirm this.


                                       Regards
                                                   Les

1999\09\20@135652 by Harold M Hallikainen

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       I recall seeing a simple TDR as an "Idea for Design" in EDN or
some similar magazine.  It consisted, as I recall, of a pulse generator
that could generate a VERY SHORT pulse, and a series resistor that was
the same as the characteristic impedance of the cable.  In lieu of a very
short pulse, it seems that you could use a longer pulse (perhaps even a
square wave) with a very fast rise time.  You drive the cable with this
and watch the cable input with a scope.  After the end of the rise time,
you should be able to see the reflections.

Harold



Harold Hallikainen
haroldspamKILLspamhallikainen.com
Hallikainen & Friends, Inc.
See the FCC Rules at http://hallikainen.com/FccRules and comments filed
in LPFM proceeding at http://hallikainen.com/lpfm


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1999\09\20@142840 by Thierry Vanmarcke

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> HI
>                 Is the PicBasic List down? I have received several
messages
> here in the UK trying to confirm this.
>
Yes it is, asked Jeff from melabs and James Avey and they did confirm
this...

1999\09\23@160731 by paulb

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Dave VanHorn wrote:

> If all you care about is finding the biggest impedance discontinuity,
> then you could do a lumped line oscillator, based on the fact that a
> short at 1/4 wave reflects as an open, and at 3/4 wave as a short.

 OK, I know I/m picky as ever, but you mean "and at 1/2 wave as a
short".
--
 Cheers,
       Paul B.

1999\09\23@201301 by Dave VanHorn

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> Dave VanHorn wrote:
>
> > If all you care about is finding the biggest impedance discontinuity,
> > then you could do a lumped line oscillator, based on the fact that a
> > short at 1/4 wave reflects as an open, and at 3/4 wave as a short.
>
>   OK, I know I/m picky as ever, but you mean "and at 1/2 wave as a
> short".

User Headspace Error  :)

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