Information Technology Reference
In-Depth Information
The amplified probe can be quite sensitive, as long as the area is electrically
noise-free. However, certain cable areas may generate too much electrical noise for
the amplified probe to function effectively. This is particularly true of “live” tele-
phone circuits, where the residual tone will be faint at best. Many installers carry
both types of probes to the job.
Cable Scanners
The advent of structured wiring standards has increased the need for very sophisti-
cated portable field test sets that thoroughly test a cable run. These test sets are often
called scanners because they use time-domain reflectometry (TDR) techniques to
scan along the cable's length for anomalies. Reflectometry is somewhat akin to a
radar for cable. A pulse is sent along a cable pair and then the cable pair is moni-
tored to see what comes back (and precisely when it does). The type of reflection
that returns is related to whether there is a short, an open, a termination, or a defor-
mity along the cable. Of course, a cable may have several conditions that reflect part
of the pulse. The length of time that it takes for a pulse reflection to return is directly
related to the distance from the test set.
A simple formula defines the distance to the reflection:
d
tkc
/ 2
where
d
distance
k
ratio of the medium's NVP to the speed of light, expressed as a decimal
fraction.
c
speed of light (units/second)
t
time (seconds)
The units of the distance will be the same as the distance units of
c
. In other
words, if
c
is in meters per second, distance will be in meters.
The NVP is the decrease in actual propagation of the pulse from the ideal free-
space speed of light, when traveling through the cable. The NVP is alternately
quoted as a decimal fraction or as a percentage. It ranges from about 0.6
c
to 0.9
c
for electronic cable. Dividing by 2 is required because the time interval is a round
trip, but we are only interested in the one-way distance.
Fortunately, all of these portable scanners are computerized, so they make the
calculations for us. Typical cable scanners are shown in Figs. 15.8 to 15.10.
Scanners can automatically check the wire map for wiring faults, measure NEXT
and attenuation, and check the cable impedance and return loss. Cable scanners also
check for wire length, to ensure it is below the appropriate limits.
