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Open in Coax

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Subject: Open in Coax
From: (Richard L. King)
Date: Fri, 11 Oct 1996 09:18:34 -0500
>You can build a Time Domain Reflectometer that will work reasonably well if
>also have access to an O-scope.  I do not have the information with me here
>work, but perhaps someone on the reflector can fill in the blanks.

>Basically you build a circuit that produces a 1 Mhz squarewave.  With a
>Tee, feed the squarewave to the coax under test and sample the wave with an

>O-scope on the Tee.  A good length of coax will display a flat trace on the

>scope.  An impedance bump will show up on the trace as an abrupt up or down   

Many Oscilloscopes have a gating pulse available that you can use in lieu of
building the squarewave generator. Look for a coax port on the side or back
of the scope that is labeled as "A gating pulse" or "B gating pulse". Run a
very short piece of coax from the gating pulse output to a T-connector at
the scope input. Also connect the coax under inspection at this T-connector.
Now you can sync on the gating pulse and watch the rise of the pulse down
the coax line and see the bad spots.

I have found that it is helpful to dampen the pulse at the scope input with
a 50 ohm resistor to ground. This sometimes makes the waveform more manageable.

Good luck.

73, Richard

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