Measuring the True Length of a Cable Pair with OneCheck and the UltraFED™ IIB

Measuring the True Length of a Cable Pair with OneCheck and the UltraFED™ IIB
Application Note
Measuring the True Length of a Cable Pair with OneCheck
and the UltraFED™ IIB
Signal attenuation (loss) increases with the length
of a cable pair, leading to lower broadband rates.
Therefore, measuring the true length of a cable pair
is essential in assessing whether the pair can support
broadband service. It is also a critical component of
troubleshooting broadband service problems. For
example, service on a pair that is within length limits
but is marginal due to other fault conditions may be
impaired while service on another pair of the same
length does not. Similarly, a shorter pair with the
same fault conditions may not experience issues. To
effectively diagnose and correct problems, a technician
must accurately measure and assess the impact of each
parameter such as loop length, balance, and noise, both
individually and as an aggregate. Failure to do so can
A two-ended test provides accurate fault
result in confusion, wasted time, and misdiagnosis
identification for wideband services
which in turn lead to higher failure rates and repeats.
All too often, plant records are less than 100% accurate and cannot be relied upon to provide accurate loop
length. There are several ways for a technician to determine pair length independent of plant records—and
each method has its own strengths and limitations.
Test Challenge
Time Domain Reflectometer (TDR) — A TDR can be used to see the end of a cable. Depending on the
cable length, the presence of fault conditions, and the technician skill level, assessing the pair length using a
TDR can be simple and straightforward—or, somewhat challenging. One way to simplify the measurement
is to use a far-end device like the JDSU UltraFED IIB to repeatedly open and short the end of the pair while
the TDR is running. This makes the end of the pair very easy to identify and removes all guesswork from the
interpretation. The limitation of using a TDR exclusively is that the length is interpreted rather than directly
measured. In other words, the TDR transmits a pulse and converts the time interval of reflected signals to
distance based on the velocity of propagation (VP) of the signal. If the VP does not match the cable under
test, the resulting distance will be less accurate. VP varies with cable gauge, so there is no selection of the
proper VP in multi-gauge scenarios.
Open Meter — Telco cable is manufactured to a specified capacitance of .083 μF per mile. An open meter
measures the capacitance of the pair to an open at the far-end and, knowing the specified capacitance of
the cable, converts it to distance. Unlike a TDR or ohmmeter, an open measurement is not affected by cable
gauge. The primary limitation of an open measurement is that instances of a bridged tap and water in the
cable add capacitance and therefore make the open measurement appear longer than the true pair length.
Application Note: Measuring the True Length of a Cable Pair with OneCheck and the UltraFED IIB
Ohmmeter — An ohmmeter determines the length of a cable pair by applying a known voltage and current
to a loop that is shorted at the far end to first calculate the resistance. Resistance (R) is calculated using
Ohm’s law (R=E/I), dividing the ohmmeter source voltage (E) by the measured current (I). Cable resistance
is typically .041 ohm per foot for 26-gauge cable at 68° F. Knowing this conversion, the resistance can be
converted to distance. Unlike the OPENS measurement, the distance-to-short (resistive loop length) is
not impacted by water in the cable or a bridged tap. However, cable resistance value can vary depending on
several factors including temperature and cable gauge. A single customer line often has several splices and
sometimes a mix of different cable gauges. The temperature of the pair temperature may vary: inside versus
outside, buried versus aerial, and in-the-sun versus in-the-shade. All these factors can impact the accuracy
of the measurement.
Addressing the Challenge with OneCheck and the UltraFED IIB
The True Length Automated Test
As discussed above, there are multiple methods for measuring loop length and each of these approaches
has advantages and disadvantages. Best practice is to perform all three measurements, compare the
results of each, and analyze the differences. Not only does this combined approach yield the most
accurate assessment of loop length, but it paints a very useful picture (besides just length) of the pair
itself. For example, if the loop lengths measured resistively (distance-to-short) and with the TDR are
very close, but the open meter reads significantly longer, one might suspect the presence of a bridged tap
or water in the cable and look for other indications of those problems. Historically, the largest drawback
to using this combined approach is that performing all three measurements manually takes more time to
set the proper conditions at the far end. Additionally, proper interpretation of the manual test results can
be challenging for those who lack experience. The OneCheck True Length test, used with the UltraFED
IIB, automates this entire test sequence with the press of a button, saving time and taking the guess work
out of interpretation to provide the most accurate true cable length—one that a technician can use with
Test & Measurement Regional Sales
1 855 275-5378
TEL: +1 954 688 5660
FAX: +1 954 345 4668
TEL: +852 2892 0990
FAX: +852 2892 0770
Product specifications and descriptions in this document subject to change without notice. © 2013 JDS Uniphase Corporation
TEL: +49 7121 86 2222
FAX: +49 7121 86 1222
30173480 000 0713 TRUELENGTH.AN.TFS.TM.AE
July 2013
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