Reverse Path Troubleshooting: CPD (Common Path Distortion) Analysis
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CPD (Common Path Distortion) is a form of intermodulation distortion that
occurs as the RF spectrum is mixed as it passes through a non-linear
junction. This phenomenon results in beat products that appear in the
reverse spectrum and impair communication.
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 motherboards. 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
downstream 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." (Ron Hranac, HSA, (Rhranac@aol.com))
“My experiences with CPD have been due to corrosion or oxidation on
connections. So, one thing that should indicate that you are getting close is
the white powder that is a symptom of oxidation of aluminum housings; often
aluminum darkens when oxidation is taking place. These instances can be
due to port caps left off, missing gaskets or gaskets which have slipped out
of their channel, warped or cracked housings or cracked or broken
connectors. Seals have to be air-tight, otherwise, normal heating/cooling
cycles can bring in moist air which dumps its moisture when the temperature
cools. One of the symptoms associated with CPD is hum in the signal. This
is due to the semi-conductor properties of some of the junk which forms
when connections oxidize. CPD can also be a symptom when coax
experiences annular cracks (although usually the ingress is more noticeable
than the CPD).
CPD (COMMON PATH DISTORTION) ANALYSIS
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