Agilent Technologies AuroraPresto, User Manual

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Section 3 - Physical Layer Tests
Issue 7 - 05/02 4-25
TDR Overview
Aurora Presto
’s TDR (Time Domain Reflectometer) enables
you to quickly locate the position of a fault - short, break,
low insulation, cross-talk, or any impedance mismatch - in
the copper pair being tested. It also enables you to measure
the total length of an open copper pair.
Aurora Presto
repeatedly transmits a burst of pulses down
the line and measures the time it takes to receive the
reflected signal back from the other end of the line, or from
a fault.
Aurora Presto
compares this measured time with the
PVF (Pulse Velocity Factor) for the line under test, and
calculates the distance between the test position and the
fault in the cable.
Aurora Presto
converts the time to
distance and displays it as a distance reading.
The pulse generated by all TDRs need time and distance to
start. This distance is known as the blind spot. The length
of the blind spot varies with the pulse width. The longer the
pulse width, the larger the blind spot.
You can add a known length of jumper cable (preferably
with the same impedance as the cable being tested) to
eliminate the blind spot.
Aurora Presto
includes the length
of the jumper cable during the TDR test. Remember to
subtract the jumper cable length from the total cable length
when measuring from the connection point.
Any pulse that is transmitted from
Aurora Presto
or any
reflection that is generated as a result of an impedance
mismatch, will reduce in amplitude (energy) as it travels
along the line. This effect is known as signal attenuation.
On long line lengths this can result in a very small signal
being received back at the TDR, which makes the reflection
difficult to display. Increasing the width of the transmitted
pulse or adjusting the receiver gain will make the signal
easier to display.
481801-7.book Page 25 Thursday, May 23, 2002 10:46 AM