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Application Note

WEBSITE: www.jdsu.com

Using JDSU Equipment to Test and Troubleshoot CPD,

Impulse Noise, and Ingress in the Return Path

History of CPD

• Common Path Distortion (CPD) is created by non-linear mixing from a diode junction created by corrosion

and dissimilar metal contacts. It’s not just dissimilar metals, but dissimilar metal groups. ere are 4 main

groups of metals:

1. Magnesium and its alloys,

2. Cadmium, Zinc, Aluminum and its alloys,

3. Iron, Lead, Tin, & alloys (except stainless steel), and

4. Copper, Chromium, Nickel, Silver, Gold, Platinum, Titanium, Cobalt, Stainless Steel, and Graphite.

• CPD is second and third order intermods from the forward channels intermixing and creating distortions,

which fall everywhere. CPD will make CSO/CTB worse for forward performance.

• Separation depends on forward channel plan. NCTA, HRC, and IRC plans that use NTSC, 6 MHz spacing

will have beats every 6 MHz. PAL could be every 7 or 8 MHz.

• e original culprit was the old feed-through connectors. Dissimilar metals from the copper clad, aluminum center

conductor and the stainless steel seizure screw.

Summary of content (6 pages)

PAGE 1
Application Note Using JDSU Equipment to Test and Troubleshoot CPD, Impulse Noise, and Ingress in the Return Path History of CPD • Common Path Distortion (CPD) is created by non-linear mixing from a diode junction created by corrosion and dissimilar metal contacts. It’s not just dissimilar metals, but dissimilar metal groups. There are 4 main groups of metals: 1. Magnesium and its alloys, 2. Cadmium, Zinc, Aluminum and its alloys, 3. Iron, Lead, Tin, & alloys (except stainless steel), and 4.
PAGE 2
Application Note: Return Path Troubleshooting • Housing terminators are notorious now because of the higher levels to mix and intermodulate, not to mention a few bad varieties that 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.
PAGE 3
Application Note: Return Path Troubleshooting 3 A single source of interference is easy to track down. If it’s constant, just use the “divide and conquer” theory to dissect the system. Observing how it reacts and changes could indicate different sources such as a trucker or home user. A CB level changing quickly or slowly could indicate this source quickly. ing. Sometimes an in-line pad can be attached to decrease the amount of energy tested, which in turn, may create a better match.
PAGE 4
Application Note: Return Path Troubleshooting • The ﬁeld unit has a “noise/ingress” feature, which can be used for troubleshooting. This displays the noise seen in the headend with optimum resolution of 280 kHz. This simpliﬁes reverse troubleshooting and testing of headend reverse noise or ingress. The newer headend unit will transmit or broadcast the ingress from all the return ampliﬁers connected to it back to the ﬁeld unit.
PAGE 5
Application Note: Return Path Troubleshooting 5 Why Measure Cable Modem C/N? Reverse ingress spectrum trace after 6 s. with dwell-time = 20 ms. The “Zero Span” Mode • In this mode, you can view desired-to-undesired ratios and see peak bursts of TDMA data. You can also measure peak digital levels, observe high traﬃc periods & collisions, and see ingress in the data packet without taking the service oﬀ-line.
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Application Note: Return Path Troubleshooting All statements, technical information and recommendations related to the products herein are based upon information believed to be reliable or accurate. However, the accuracy or completeness thereof is not guaranteed, and no responsibility is assumed for any inaccuracies. The user assumes all risks and liability whatsoever in connection with the use of a product or its applications.