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Re: Time Domain Reflectometer (was Re: internet outages)

<resending to the list (with slight editing to show who wrote what), 
originally from John Hasler>

I (John Hasler) wrote:
> As to TDRs, if you can get by without actually seeing pictures of all
> the impedence bumps 

> you can get by with a fast counter, a high risetime
> pulse generator, and a couple of fast comparators.  Maybe $20 at
> Digikey.

You (Randy Kramer) wrote:
> Can you elaborate a little on how that would work?  

Apply the pulse to the cable under test.  The rising edge triggers the
counter and enables the comparators.  One comparator triggers on the
first rising edge on a return pulse, latching the count and disabling
itself.  The other comparator triggers on the first falling edge on a
return pulse, latching the count in a different register and also
disables itself.

The count latched by the positive comparator represents the distance to
first positive impedance bump encountered by the pulse. An open will
double the applied voltage.  The other will represent the distance to
the first negative bump.  A short will invert the voltage.

You'll want to make the thresholds adjustable, of course, and you will
also want to add range gating so that you can ignore all but a selected
section of the cable.

There are some time to distance converter chips that would be ideal for
this.  You'll want to put the whole thing under the control of a micro,
of course.  It could scan the range gates up and down the cable and
report impedance as a function of distance.
John Hasler 
Elmwood, WI USA