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calculated (e.g., 13.8 m - 4 m = 9.8
m). The noticeable drop in measured
amplitude to the right of the 50-
ohm load signifies the end of the
cable. As this reflection measurement
represents two-way signal paths,
FieldFox properly adjusts the marker
values and x-axis formatting to the
appropriate one-way lengths.
Now, consider a coaxial cable that
has been damaged in two areas
(Figures 4A and 4B). Fault A is a
bend in the cable that has exceeded
the manufacturer’s specification for
minimum bend radius of 1-inch. The
bend at Fault A is well below this radius,
creating an undesired reflection from
this part of the cable. Fault B is a cut
through the outer conductor of the
cable. The braided shield has been
partially removed, exposing the inner
dielectric of the coax. Both faults can
be examined using the DTF and TDR
modes on FieldFox; however, only the
TDR measurement will characterize
fault type.
Figure 4C shows the measurement for
the damaged cable with FieldFox in
TDR mode. As can be seen from the
TDR response, the cable impedance
is generally 50-ohms across most
of the time-domain response until
a discontinuity is encountered.
Discontinuities occur at the input
connector, the bend at fault A, the cut
at fault B, and the 50-ohm termination
at the end.
Of all discontinuities on the cable,
the cut at fault B has the largest
mismatch. This is demonstrated by
the magnitude of the associated peak.
The cut on the TDR response has a
single peak in the positive direction,
indicating an inductive mismatch.
This is typical for cuts in the outer
conductor of a coaxial cable. Generally
speaking, if the cable terminated
in a load with a resistance smaller
than the characteristic impedance,
the TDR response would show a
step in the negative direction. If the
load resistance were larger than the
characteristic impedance, the TDR
response would show a step in the
positive direction.
Summary
Cable measurement in the field can
be tricky. Determining that a cable
is faulty is just the first step in the
process. Engineers and technicians
then need to identify the fault’s
physical location and its cause.
This is accomplished using various
time-domain techniques. FieldFox’s
comprehensive suite of cable test
measurements, including the new
TDR and ERTA options provide the
ideal solution for testing any cable
system in the field. FieldFox’s DTF
and TDR time-domain measurements
identify fault locations and causes
in coaxial cables, while its bandpass
measurement finds the physical
location of faults in a waveguide.
Using FieldFox, today’s engineers and
technicians now have a faster, easier
way to test cable systems in the field.
About the Author
Tom Hoppin is an application
consultant on contract to Keysight
Technologies. Tom started his career
as an electronic aviation technician
in the U.S. Marine Corps. He joined
Hewlett-Packard after his service
ended in 1973. Through the years he
has held a number of engineering and
management roles at HP/Agilent and
now Keysight, focused on test system
design and spectrum analysis. Tom
retired in 2009. He has since returned
to Keysight as an application specialist
for its RF and Microwave Handheld
Analyzers.
Figure 3. Shown here is a FieldFox DTF
measurement for two connected coaxial cables
terminated in a 50-ohm load.
Figure 4. The top left-most and right-most images, A and
B, respectively, depict damage in a coaxial cable. The
bottom image, 2C, is the result of a TDR measurement of
the damaged cable using the FieldFox TDR option. FieldFox
can identify various types of discontinuities in TDR mode,
including: R > Z0, R < Z0, inductive, and capacitive
65 l New-Tech Magazine Europe