Megger CFL510F User Manual

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Finding faults with voice and data communications

Stephen Jenkins

Product Manager

For locating faults rapidly on voice and data communications installations, a time domain reflectomete r
(TDR) is potentially an invaluable tool. Yet all too often, TDRs fail to live up to their potential, largely as a
result of the limitations inherent in older instruments.

The latest generation of TDRs has effectively removed these limitations, making the new instruments
more useful – and easier to use – than ever.

Auto set up is a real time saver for occasional and experienced users alike.

Let’s take a look at some of the shortcomings that have often been associated with TDRs in the past, and
how these have been addressed. The first and possibly most significant issue is the location of near-end
faults. Put simply, for faults close to the instrument, the signal reflected from the fault often arrived at the
instrument before the launch pulse had finished and, therefore, could not be detected.

This is particularly problematic as the majority of faults on communications networks near-end faults. In
principle, the remedy is straightforward – make the launch pulse shorter, so that it no longer masks the
return signal from a nearby fault. It has taken recent advances in technology to make this possible,
however, and the best of the new generation of TDRs can work with launch pulses as short as 2 nS.

In practical terms, this means that the new instruments can locate faults as close as 0.1 m (around 4
inches) from the point of connection. Given that TDRs are almost always used with test leads that are a
foot or more long, the near-end fault location problem has been comprehensively solved! The next
problem commonly encountered with older TDRs is complicated set up. What is the cable impedance?
What is the best pulse width to use for a particular operating range? How should the gain be set?
Experienced users will have the answers, but for others, and particularly for those who use T DRs only
infrequently, the multiplicity of options can be daunting.

The solution is auto set-up, where the TDR itself determines the cable impedance, and sets the optimum
gain and pulse width for each range. Not only is this ideal for ensuring that novice users get the best from
the instrument, it’s also a big time saver for experienced users. Naturally, the best instruments also

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provide a manual operating mode for tackling unusual or complex situations where fine-tuning the
operation of the TDR can help to wring the last ounce of performance from it.

Interpreting the trace produced by the TDR is another issue that sometimes poses challenges. One very
effective solution is to compare the trace from a known good pair and the faulted pair but in the past,
instruments offering dual-channel operation with a trace comparison facility have typically been bulky an d
expensive.

A slightly different but equally effective approach is to provide a
single-channel instrument with the facility for storing a trace on
screen. This means that the instrument can first be used on the
good pair and the resulting trace stored. It can then be used on
the faulted pair, and the new trace compared with the first one,
which is still on the screen. This arrangement allows all the
benefits of visual trace comparison to be provided in a compact
and cost-effective single-channel instrument. Another issue that
can be a problem is the quality of the display used in the TDR.
Getting good results involves interpreting the fine detail of the
traces and setting the cursor accurately. This can only be done if
the display has good resolution, particularly in a handheld
instrument where the overall display dimensions are necessarily
limited. Fortunately, very high resolution displays are now
available at reasonable prices and the best TDRs use these.

The conclusion is clear: the best of the new generation TDRs, of
which Megger’s CFL510G is an excellent example, offer greatly
enhanced usability and performance, allowing these remarkably

useful instruments to live up to their full potential, even in the
hands of inexperienced users. Auto set up is a real time saver for occasional and experienced users
alike.

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