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April 2001 Issue
A Primer on Common Path Distortion
By , BCE

Although musicians may purposely create distortions to affect sound, random variations that appear in a cable system’s audio or video signals can wreak havoc on the return path.

Common path distortion (CPD) is a series of beats most easily seen in the return spectrum, although these beats also appear simultaneously in the forward spectrum. They have a recognizable signature pattern and are spaced in identical groups or sets at 6-megahertz (MHz) intervals. They usually occur at a passive location common to both the downstream and upstream spectrums, such as taps, terminators or hardline connectors and seizures.

These common points where the two spectrums coexist are vulnerable to CPD. The most common cause of CPD is corrosion at a dissimiliar metals interface. The corrosion—sometimes only a few molecules think—forms a nonlinearity (a crude form of semiconductor diode junction) and causes an electrical mixing or intermodulation of all forward channels. The beat products fall in and out of band, some below the forward channel carriage spectrum. This crude diode can be touchy and is easily disturbed. Its instability produces random variations in its nonlinear characteristic, which can make the second and third order beat contributions appear equal, composite second order (CSO)-dominant or composite triple beat (CTB)-dominant as seen in the CPD screen trace. CPD has reportedly been generated when downstream levels are as low as 0 dBmV (SCTE List, 10/13/98 12:44, Bharat Patel.).

To understand CPD, look no further than the reliable revenue-generator—the forward path. Without the forward carriers creating thousands of distortion beat products, common path distortion could not exist.

Amplifier basics

Although amplifiers are thought of as being close to perfect, they’re not. A perfect amplifier would take an input signal and increase its amplitude (gain) without changing that input signal in any way, shape or form. This doesn’t happen in real life.

All amplifiers introduce some form of distortion to an input signal, which means that the output signal is not the same as the one that went into the amp. The distortion increases when the amplifier is operating in a nonlinear portion of its transfer curve, or its input is being "overdriven." It may be a desirable thing if you are a heavy metal guitarist looking for that particular sound but in most instances, it is something we try to avoid in video and audio signal processing.

Distortion types

Distortion comes in different forms. Intermodulation distortion is the electrical mixing, or beating, of at least two input frequencies that create different frequencies at the amplifier’s output. The two most common types in RF amplifiers are second and third order intermodulation distortion.

Second order intermod distortion produces the second harmonic (2 x F1) and (2 x F2) of each input, the sum (F1 + F2) and the difference (F1 - F2) of the two original input frequencies and the two original input frequencies at the output. Third order intermod distortion produces the third harmonic of each input frequency (3 x F1), (3 x F2) and (3 x F3), the second harmonic of any one frequency plus or minus each of the others (2F1 +/- F2), (2F1 +/- F3), (2F2 +/- F1), (2F2 +/- F3), (2F3 +/- F1) and (2F3 +/- F2), any sum and difference beats of all three input frequencies (F1 +/- F2 +/- F3) and the three original input frequencies at the output. CSO and CTB distortions are the "clustering" of individual beat products on or near forward visual carriers and are usually grouped too close together to be measured individually. CTB requires the channel under measurement be shut off to reveal the beat products underneath the carrier.

Forward path measurements

Forward path measurement distortions are the same as those battled in spring and fall when wide temperature fluctuations and/or a decrease in cable loss with colder temperatures fluctuate line extender input levels, causing picture impairments and service calls. They also are the same ones documented during Federal Communications Commission (FCC) proof-of-performance measurements, that are normally performed using a spectrum analyzer.

Network technicians should become familiar with them, even if they are not directly involved in doing system proofs. The Acterna Stealth meter from Acterna is an example of a field testing device available to many line technicians. It is used for system troubleshooting and has built-in measurement features. You may already have such a meter without knowing about the troubleshooting feature, which may be valuable when tracking a distortion problem.

Figure 1 shows the screen display for second and third order measurements. CTB is measured at zero beat (directly under video carriers), or screen center, while CSO is measured at the four most common points for second order distortions to fall. These are -1.25 MHz, -0.75 MHz, +0.75 MHz and +1.25 MHz in relation to all visual carrier center frequencies.

Additional second order beats also are produced at -0.25 MHz and +0.25 MHz relative to visual carriers that are not shown here. The level of these beats in relation to the visual carrier peak is represented in dBc, or number of decibels (dB) down from visual carrier peak. From the

illustration, F1 (-1.25 MHz) is 66 MHz, F2 (-0.75 MHz) is 66.5 MHz, F3 (+0.75 MHz) is 68 MHz, and F4 (+1.25 MHz) is 68.5 MHz in relation to the visual carrier center frequency of channel 4, which is 67.25 MHz. These four measurement points (referred to as offsets) in Figure 2 are user-programmable by pushing the CSO setup soft key.

Let’s do a few examples for the purpose of illustration (make sure you have your pocket reference book for frequency assignments handy).

• A second order intermod beat at 1.25 MHz below channel 4’s visual carrier would be F1 (241.25 MHz) - F2 (175.25 MHz) = 66 MHz.
• A second order intermod beat at 0.75 MHz above visual carrier of channel 4 would be F1 (145.25 MHz) - F2 (77.25 MHz) = 68 MHz.
• A third order intermod (triple beat) product that falls zero beat (dead on) the visual carrier of channel 4 would be F1 (139.25 MHz) + F2 (301.25 MHz) - F3 (373.25 MHz) = 67.25 MHz.

Now, after referring to Figure 3, take a close look at Figure 4. Note that:

1. The responses of the two wave forms are nearly identical.
2. The center frequency has moved to the return frequency spectrum (36 MHz).
3. The locations of the distortion products remain identical to the forward path in relation to carrier center.
4. Above 50 MHz (forward), zero beat falls exactly on visual carriers (+1.25 MHz offset from the lower edge of each 6-MHz channel) and always is a third order beat.
5. Below 50 MHz (return), zero beat falls exactly on multiples of 6 MHz (6, 12, 18, 24, 30, 36) and is a second order beat .
6. Beat products at +1.25 MHz, +0.75 MHz, +0.25 MHz, -0.25 MHz, -0.75 MHz, and -1.25 MHz (relative to carrier) in the reverse spectrum are second order.

These carriers appear to be related to each other. Take a closer look at what is happening on the return path:

• A second order intermod beat that falls on a multiple of 6 MHz center frequency would be F1 - F2 = (211.25) - (175.25) = 36 MHz
• A third order intermod beat that falls 1.25 MHz above center frequency would be F1 + F2 - F3 = (301.25) + (175.25) - (439.25) = 37.25 MHz
• A third order intermod beat that falls 1.25 MHz below center frequency would be F1 - F2 - F3 = (301.25) - (175.75) - (91.25) = 34.75 MHz
• A third order intermod beat that falls 0.75 MHz above center frequency would be F1 + F2 - (F3 + 4.5) (only using channel 5 or 6) = (83.25) + (55.25) - (101.75) = 36.75 MHz
• A third order intermod beat that falls 0.75 MHz below center frequency would be F1 - F2 - F3 (only using channel 5 or 6) = (109.25) - (83.25) - (61.25) = 35.25 MHz
• A third order intermod beat that falls 0.25 MHz above center frequency would be F1 - F2 - (F3 + 4.5) = (247.25) - (91.25) - (119.75) = 36.25 MHz
• A third order intermod beat that falls 0.25 MHz below center frequency would be (F1 + 4.5) + F2 - F3 = (305.75) + (175.25) - (445.25) = 35.75 MHz

CPD appears to be second and third order intermodulation distortion and resultant beat products of the forward channels fall into the return path spectrum. That is why it was important to show the mathematical relationship between carriers and the repetitive cycle of beats at 6 MHz spacing. Ingress could not cause such a repetitive pattern. This also is supported by the fact that you can have an empty return spectrum (no system carriers) and still observe CPD activity within that same spectrum.

Tracking the beast

Just as distortions in the return spectrum exist, distortions also occur in the forward spectrum. Because the additional beats generated by the little diode mixer coincide in frequency with the beats already occupying the forward spectrum, they come together to produce an overall increase in forward distortion. But measuring forward distortion requires shutting off channels to see the beats hiding underneath the active video, which is not practical.

The best way to view CPD on the forward path is to look just above the highest channel carried on the forward system. Also, try checking pictures on channels 5 and 6 because the beats there fall directly in-channel and could degrade video quality with the increase in forward distortion.

On the upstream side, the return diplex filters within the amplifiers may be helpful in troubleshooting. If your return sweep span is set between 5 MHz and 50 MHz, you will be able to see this filter (low pass) sharply roll off the sweep response at 42 MHz. This will tell you how close you are to the source of the CPD, because the first active device the CPD reaches will band-limit this return signal to 42 MHz and below. (Note: In Figure 5, the sweep span is set between 5 MHz and 65 MHz, and the filter roll off is plainly visible at 42 MHz.). If you can see a flat trace all the way out to 50 MHz, then you are probably near the source because the CPD has not yet passed through an amplifier.

A word of caution here: If you overload the front end of your test equipment, you can create beat products that appear to be CPD but are not. Also, ineffective diplex filters with poor forward and reverse isolation also can seemingly create CPD and have you chasing your tail.

What are the causes?

A variety of sources can create CPD (see Figure 6). Some of the most common include:

• Loose seizure screws
• Loose terminators and drops at tap ports
• Bad drop connectors
• Loose housing connectors and straight splices
• High value terminated taps (check the line terminator)
• Loose hold-down screws on modules or circuit boards
• Anything that allows moisture to enter a device (caps, gaskets, cracks, warping)

Once these frequencies (5 MHz -42 MHz) are inserted onto the return spectrum by the diode mixer effect (caused by corrosion) and reach the next available amplifier station where they are low pass filtered and amplified, their carrier levels may increase considerably and become troublesome.

CPD cures

These beat products and where they fall in the return spectrum are fairly predictable and should raise some concern regarding new services and reverse channel placements. Plan all frequency assignments carefully before loading up the return spectrum.

With the return path so vital to our cable’s future success, it goes without saying that CPD problems must be found, fixed and prevented. The key to accomplishing this is craftsmanship.

Don’t let any major construction or rebuild activity occur without a good quality control program. Every time someone goes up a pole, he or she needs to break (1/4 turn back) and re-tighten all f-fittings on that tap plate, including traps and terminators. This will disturb any corrosion that is beginning to develop and also is a good practice to control ingress.

Visual inspection also is effective. Notice those little things that can cause water ingress, such as forgotten port caps, and correct them. Perhaps putting silicone grease on the threads of all drop fittings followed by a rubber boot, will help deter corrosion. Also, using shrink tubing on all outdoor line connectors will work.

In any event, CPD is going to be with us indefinitely, so understand it and devise effective ways to locate it and troubleshoot solutions.

Nick Romanick, BCE, is a regional NOC analyst with Comcast. He may be reached at .

The author would like to thank Paul Curtis of Comcast for contributing to this article.

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