On 11/10/2010 11:56 AM, Barry wrote:
wrote in message
...
Yeah but in the real world you don't usually need to match a commercial
TDR to find a fault.

Back in the days of ethernet over RG-58 I was able to find lots of
poorly
attached connectors and cables crushed by the electricians that
installed
them (there were no network engineers and installers in those days)
with
a 100 MHz 'scope and a pulse generator made from a 555 timer and a 50
Ohm
line driver.

Can you detect where RG-58 was kinked and then straightened out with the
damaged area being less than 1/2 inch? Likewise can you detect an
abraded shield leaving a 1/4 inch hole in the shielding?

An LM555 has a rise and fall time of 100 nanoseconds. I have no idea of
how much your line driver sharpens the edges of the pulse, but a good
line driver should provide rise and fall times of 5 nanoseconds. The
original TEK 1502 at 140 picoseconds can resolve to about 2 centimeters.
With a risetime of 5 nS, your resolution will be about 70 times worse
(140 cm). Add to this the 100 MHz limitation of the scope (an additional
10 nS), and it becomes far worse.

Yes, you can easily detect shorts and open circuits with your setup, but
you will not be able to detect the 1 to 2 inches or so where the center
conductor has migrated in the foamed dielectric from hanging the cable
over a building edge.

The accuracy of the measurement for fault finding doesn't need to be
much
better than a few feet to be able to find the fault visually once you
know
about where it is and faults at the end are immediately obvious.

If the task is to find {gross} faults, a 'scope and simple pulse
generator works
just fine.

If the task is to certify 6 inches of hard line to GHz to some
mil-spec, you
probably want something more sophisticated.

The kind of fault Spamhog originally described, a 75 ohm cable dropping
to less than 50 ohms over two inches and going back to 75 ohms, will not
be detected by your setup.

73, Barry WA4VZQ



Are you telling all the rest of us that may have inferior equipment that
we shouldn't even try to measure things?

Just asking.

And we aren't all dumb enough not to understand what gives us resolution.

tom
K0TAR