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.

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.

• ere is another impairment that manifests itself like

CPD, but the separation is a little diﬀerent; it is called

transient hum modulation. An RF choke can saturate

with too much current draw and cause the ferrite

material to break down. e same thing can happen

in customer installed passives unless they have voltage

blocking capacitors installed.

Troubleshooting CPD

• Pull a forward pad to see if the return “cleans-up”. is

is deﬁnitely CPD, but very intrusive when doing this

and may disrupt CPD temporarily.

• Try not to disturb anything in this tracking process.

Vibrations and movement can temporarily “break

away” the diode/corrosion causing this CPD.

• Voltage surges can also destroy the diode. At least long

enough to warrant a return visit!

• e test point locations will determine the outcome.

If CPD is on any of the downstream output TPs of an

ampliﬁer, it may be the output seizure screw or con-

nector. Otherwise, continue down that leg. Look for

housing terminators.

• If CPD is on the Fwd input TP and not on the output

TP, it may be the input seizure screw or connector. e

reverse ampliﬁer provides isolation that prevents CPD

from appearing on the output if created on the input.

• It could still be downstream though, because the levels

on the reverse input test point may be too low to see,

which may warrant a pre-amp. Otherwise, attach to

the reverse output and terminate reverse input pads

one at a time to determine the oﬀending reverse input

leg.

• If you view the reverse spectrum from a bi-directional

test point with an analyzer, you could overdrive the

front-end of the analyzer with too much forward

path signal and cause intermodulation within the test

equipment. To see the reverse ingress, the instrument

is in its most sensitive mode. Both forward and reverse

signals are going directly into the mixer input. e

high level forward channels will cause intermodulation

products in the front-end of the meter. is will hap-

pen on any type of analyzer.

• Use a low pass ﬁlter to block all the forward channels.

You could use a diplex ﬁlter, but it’s cumbersome. e

insertion loss may not be calibrated, and it may not be

dc blocked.

• is is why newer units have a built-in, switchable,

lowpass ﬁlter to block out the forward channels.

• It may be advantageous to troubleshoot CPD from the

end-of-line back toward the node. is will eliminate

disturbing the fault until you get there.

Note: Be sure forward input levels to the Stealth headend

transmitter (Tx) are between 4 and 12 dBmV. If levels

are too high, distortions will be created in the Tx,

which appear as CPD when viewing the “Noise”

mode.

Tracking Down Ingress

The first step is to verify it is truly on your network and not

self-induced. Use some type of spectrum analyzer to view

the anomaly. Cross reference with frequency charts that

identify different ingress sources to get a best-guess idea.

Noise and transient ingress above the diplex filter region

is probably laser clipping or induced at the node. You may

also want to view the frequencies below 5 MHz to verify

it’s clean. Noise below 5 MHz could still affect the laser’s

dynamic range.

Listening to Ingress for Identification of the

Source

The second step is to demodulate the ingress, if possible, to

identify the type of ingress. Reverse path ingress is usually

amplitude modulated (AM), but could also be FM. Listen-

ing to the ingress helps to identify the source.

• FM demod for the audio of forward channels and

certain shortwave radio.

• AM demod for most reverse interference and ingress,

such as CB, Ham, and shortwave radio.

• is may give you some insight into the location of the

source or at least the nature of the source. You may be

able to get the call signs of a ham radio operator

or a mile marker from a truck driver using his CB.

is could aid in pinpointing the ingress location.