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TDR risetimes

The following statement was recently posted:

> As I understand telephone cable fault finding, the rise times of the
> voltage pulses are normally long to allow large propagation distances.

I don't know if it was an oversight, but the rise times are not "long to
allow large propagation distances." You never want a long rise time. I
think what he meant was that the pulse length was long, the rise time
would still be short. The shorter the rise time the better the resolution
for detecting small discontinuities. However it isn't just the initial rise
time, but the rise time at the point on the cable that is being examined.
If the cable is poor quality or very long, then the rise time will be
degraded and even if the initial rise time was short the results will be
the same as if the rise time had been long.

The length of the pulse determines the length of cable which can be
examined. For fine resolution on a long cable you need a fast rise time,
good quality cable and a long pulse length. The half sine type of TDR
pulsers would not be useful when you are observing small changes in
impedance along the cable.

Russell Anderson         Campbell Scientific, Inc.
(801) 750-9697               815 W. 1800 N.  
(801) 750-9639 FAX           Logan, UT 84321
russell@csius.com