Actually, for TDR, if you have a good o'scope, you don't even need the generator. Some scopes
have a "calibrate out" jack on the side. That can be used to provide a nice clean square
wave signal to send down the wire. Then you measure it and the reflection coming back
and calculate the distance. If you have 3 wires underground, assuming 2 will be good,
you can use that as the 100 foot reference. Don't forget to include the extra wire inside
the building, not just the part that is underground in your calculations.

John

"John" wrote in message
...
Actually, for TDR, if you have a good o'scope, you don't even need the
generator. Some scopes
have a "calibrate out" jack on the side. That can be used to provide a nice
clean square
wave signal to send down the wire. Then you measure it and the reflection
coming back
and calculate the distance. If you have 3 wires underground, assuming 2 will
be good,
you can use that as the 100 foot reference. Don't forget to include the extra
wire inside
the building, not just the part that is underground in your calculations.


Is the reflection going to be obvious? This is something I've never tried to
see.

I will have to do some experimentation before I visit again.

Bob

"John" wrote in message
...
Actually, for TDR, if you have a good o'scope, you don't even need the generator. Some
scopes
have a "calibrate out" jack on the side. That can be used to provide a nice clean square
wave signal to send down the wire. Then you measure it and the reflection coming back
and calculate the distance. If you have 3 wires underground, assuming 2 will be good,
you can use that as the 100 foot reference. Don't forget to include the extra wire
inside
the building, not just the part that is underground in your calculations.

John

Good idea...

Bob F wrote:
....

Is the reflection going to be obvious? This is something I've never tried to
see.
....

Maybe, maybe not... (How's that for precision? )

Depends on many factors including the cable characteristics itself, the
type of fault, where the fault is located, the rise time and width of
the pulse, etc., etc., etc., ... It takes some skill and experience to
interpret the reflected waveform even w/ a "for purpose" instrument.

Here are a couple of links that have some decent description of some of
the basics. I've never tried TDR on anything except very high impedance
signal cable, so don't really have any idea of what it would look like
for ordinary wiring. My old Tek scope doesn't have fast-enough rise
time in the internal generator for anything of reasonable length so
don't have a way to see how it might look as compared to what I have
seen in the past, unfortunately.

http://www.tscm.com/riprcop.html
http://www.tscm.com/riwavef.html

If you've got some good quality test gear and a sample roll of romex you
could always give it a shot and see if you can figure out the length
of a couple of test sections.

Overall, though, I think either the "binary section" or "rent a locater"
methods will turn out to be more successful than homebrew TDR based on
the level of sophistication it takes in commercial instruments to make a
successful instrument. You might just get lucky though, so can't hurt
to try...

--

"Bob F" wrote in message
. ..

"John" wrote in message
...
Actually, for TDR, if you have a good o'scope, you don't even need the
generator. Some scopes
have a "calibrate out" jack on the side. That can be used to provide a nice
clean square
wave signal to send down the wire. Then you measure it and the reflection
coming back
and calculate the distance. If you have 3 wires underground, assuming 2 will
be good,
you can use that as the 100 foot reference. Don't forget to include the extra
wire inside
the building, not just the part that is underground in your calculations.


Is the reflection going to be obvious? This is something I've never tried to
see.

I will have to do some experimentation before I visit again.

Bob



This should get you started...

http://home.tir.com/~artemus/Growler/

Try Johns suggestion of using the other 2 wires as your reference instead of wading
through calculations. Impedance is listed for some romex on the 'net..

"dpb" wrote in message ...
Bob F wrote:
...

Is the reflection going to be obvious? This is something I've never tried to
see.
...

Maybe, maybe not... (How's that for precision? )

Depends on many factors including the cable characteristics itself, the type
of fault, where the fault is located, the rise time and width of the pulse,
etc., etc., etc., ... It takes some skill and experience to interpret the
reflected waveform even w/ a "for purpose" instrument.

Here are a couple of links that have some decent description of some of the
basics. I've never tried TDR on anything except very high impedance signal
cable, so don't really have any idea of what it would look like for ordinary
wiring. My old Tek scope doesn't have fast-enough rise time in the internal
generator for anything of reasonable length so don't have a way to see how it
might look as compared to what I have seen in the past, unfortunately.

http://www.tscm.com/riprcop.html
http://www.tscm.com/riwavef.html

If you've got some good quality test gear and a sample roll of romex you could
always give it a shot and see if you can figure out the length of a couple of
test sections.

Overall, though, I think either the "binary section" or "rent a locater"
methods will turn out to be more successful than homebrew TDR based on the
level of sophistication it takes in commercial instruments to make a
successful instrument. You might just get lucky though, so can't hurt to
try...

Hey! This is getting fun.

Thanks for the references. It looks like I might need to throw together a pulse
generator for very short pulses.

Bob

"Rick" wrote in message
...

"Bob F" wrote in message
. ..

"John" wrote in message
...
Actually, for TDR, if you have a good o'scope, you don't even need the
generator. Some scopes
have a "calibrate out" jack on the side. That can be used to provide a nice
clean square
wave signal to send down the wire. Then you measure it and the reflection
coming back
and calculate the distance. If you have 3 wires underground, assuming 2
will
be good,
you can use that as the 100 foot reference. Don't forget to include the
extra
wire inside
the building, not just the part that is underground in your calculations.


Is the reflection going to be obvious? This is something I've never tried to
see.

I will have to do some experimentation before I visit again.

Bob



This should get you started...

http://home.tir.com/~artemus/Growler/

Try Johns suggestion of using the other 2 wires as your reference instead of
wading
through calculations. Impedance is listed for some romex on the 'net..

Another great resource. Thanks!

Bob