Application Notes: Return Path Troubleshooting
329
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14. The number of sweep points is limited to 500, but inserting too many
points will make the refresh of the sweep display slower. The sweep
update time is dependent on the setup.
History of CPD
CPD is created by non-linear mixing from a diode junction created by
corrosion and dissimilar metal contacts. It’s caused not by just dissimilar
metals, but also by dissimilar metal groups.
There are 4 main groups of metals:
• Magnesium and its alloys
• Cadmium, Zinc, Aluminum and its alloys
• Iron, Lead, Tin, & alloys (except stainless steel)
• Copper, Chromium, Nickel, Silver, Gold, Platinum, Titanium, Cobalt,
Stainless Steel, and Graphite
CPD occurs when second and third order intermods from the forward
channels intermix and create distortions which fall everywhere. CPD will
make CSO/CTB worse for forward performance.
Separation depends on the forward channel plan. NCTA, HRC, and IRC
plans that use NTSC 6 MHz spacing will have beats every 6 MHz. PAL could
have beats every 7 or 8 MHz.
The original culprit was the old feed-through connectors, which used
dissimilar metals: the copper clad, aluminum center conductor and the
stainless steel seizure screw.
Housing terminators are notorious because of higher levels to mix and
intermodulate, not to mention that a few bad varieties were manufactured.
Colder weather makes CPD worse because the diode works better. Electron
funneling is better with heat so there isn’t as much non-linear mixing.
Because of contraction and expansion, CPD could become worse with heat.
Another impairment that manifests itself like CPD, but with separation that is
a little different is called transient hum modulation. An RF choke can
saturate with too much current draw and cause the ferrite material to break
RETURN PATH TROUBLESHOOTING
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