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5.8 CPD (Common Path Distortion) ANALYSIS
Test Equipment / Procedure Recommendations
Description of CPD and Causes
CPD (Common Path Distortion) is a form of intermodulation distor-
tion that occurs as the RF spectrum is mixed while passing through a
non-linear junction. This phenomenon results in beats that appear in
the reverse spectrum and impair communication (see figure 5).
CPD has a variety of causes, including corrosion at a dissimilar metals
interface, bad solder connections, and even loose hold-down screws
on amplifier modules or mother boards. The following quotes from
the SCTE-List (used by permission of the authors) provide some
practical insight into causes and methods for combating CPD.
“Other distortion-generating mechanisms include the use of feed-
through connectors and cable with copper-clad aluminum center
conductor. When an active or passive device’s seizing screw cuts
through the cable’s center conductor and penetrates the copper, the
screw will make contact with the aluminum. The result can be a
dissimilar metals interface (depending on the composition of the
seizing screw) that may create a diode effect. This is a good case for
using only pin connectors in the RF distribution plant.
One possible cause of distortion in the reverse path that is not well
understood may be related to the isolation (or lack thereof) between
the downstream amplifier module output and the reverse amplifier
module input in the same housing. Considering the high feeder
levels now available with Mini Bridgers, DAs, and similar multi-
output feeder amps, it is possible that the isolation in the diplex
filter may be insufficient, particularly when single-ended circuitry is
used in the reverse amp module. This would result in the down-
stream spectrum being present at the reverse module input, albeit at
a fairly low level, but which may be enough to cause beats to be
generated in the reverse amp.”
1
“My experiences with CPD have been due to corrosion or oxidation
on connections. So, one thing that should indicate that you are
Summary of Contents for 3SR
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