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Reverse Path Troubleshooting: CPD (Common Path Distortion) Analysis

101

Technicians need to be able to associate seemingly “unconnected”

symptoms. For example, someone notices a minor amount of hum in the

picture of a customer; two blocks away error rates start going up on a data

modem or DMX box. Somewhere in the area you find a connector cracked at

the housing entry which has allowed moisture entry and oxidation to occur.

Note that this can and does happen to F connectors in homes or institutions.

To solve such problems, the source of the moisture ingress needs to be

found and removed, the moisture removed and the oxidation or corrosion

removed. It’s likely to be cheaper, easier and faster to remove and replace

an entire device (tap, directional coupler or amplifier) than to determine and

fix the cause of the moisture ingress. If a bad F connector is found, it is good

practice to also replace the device to which the F connector was attached. In

the case of F connectors, relocation of equipment to be out of the path of

water and/or use of higher cost F connectors with better water seals is

recommended (it’s silly to save 25 cents on an F connector and generate a

50 dollar trouble call). (David Devereaux-Weber, University of Wisconsin -

Madison, (djdevere@facstaff.wisc.edu))

Recommended Setup for CPD Analysis

If the test point is not band limited to the reverse spectrum (in other words,

on the low band side of a diplex filter) a low pass filter may be needed to

eliminate the possibility of instrument related intermodulation products that

mimic CPD. The susceptibility to intermod is dependent on the total number

of carriers present on the input to the instrument. The measurement

technique is similar to using a band pass filter for intermodulation tests

(composite second/third order).

See Also

½ "CSO/CTB Measurements" in Chapter 6 explains how to

make composite second/third order measurements

(page 128).

The objective is to avoid bombarding the RF input of the instrument with a

large number of relatively high level signals (typically 78 channels in a 550

MHz system), while looking for low level signals. The problem is that when

attenuation is removed to see the low level beat products more clearly, the

front end of the instrument may be overloaded with the high level signals on

the same test point, and generate beat products of its own.

You might need a low pass filter when you're connecting the instrument to a

bi-directional amplifier test point—a test point with both forward and reverse

signals present. This phenomenon may present itself when performing a

reverse noise test using the SDA-5500 transmitter. To minimize the

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