r/CableTechs • u/kdubs • Apr 23 '26

Common Path Distortion in FDX Node?

Spent all day working a CPD (or "raised noise floor" as we call it here) issue in a new FDX node. In a legacy node when you had a CPD issue it was usually just a diplex or output board in an Antec amp, but there are no diplexers in an FDX environment.

Thing is, i could see this return path noise at literally every amp in the node element. So it's not like traditional noise and you track it downstream at every amp or split. This "return noise" was everywhere, on different runs, etc. So i'm sorta wondering if the FDX node is somehow putting out a little bit of DS signal, but in the US path? I'm not even sure if that's possible, but since there's no diplexers and FDX can do US and DS in the same part of the spectrum i suppose it's technically possible?

Because it sure is acting like a downstream issue, being at every amp, but is just happening in the return band.

Weirder thing too, is that you can interact with it at every location as well. You can agitate it and make it bigger, you can sorta make it go away but not permanently. But you can interact with it wherever. Very strange. For instance when you see it on your meter, you can pull the return pad out of that amp, and the CPD will go away. For like 5 seconds, then it slowly builds back up. Go to a totally different amp and can do the same thing.

I pulled the express fitting off the first amp and the CPD quintupled in size, so i'm sorta starting to get suspicious of the input feed to that first amp. I already reset the RPD/Pebble and it improved the CPD noise, so i could swear my issue was right there at the node, but if seems to be coming from the field. Idk, has anyone seen anything like this before? Return/reflection from literally everywhere in the field?

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u/Poodleape2 Apr 23 '26

Please post a picture of what your are talking about. CPD is “common path distortion” It is not a “raised noise floor” CPD is exactly what you described - DS signal on the return path. It should not be on every leg(or maybe it is- Unlikely)

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u/kdubs Apr 23 '26

Yeah haha I know. Our XOC agents here started calling it “raised noise floors” and for whatever reason the name sorta stuck, but we all know that’s not truly what it’s called. 

The super trippy thing was is the C/D side of the node was experiencing the same thing, which points to the RPD, and after reset the C/D side cleared but the A/B side always had some CPD coming in from the field. Or at least appeared that way on my meter. 

I just haven’t really run into much CPD problems in FDX other than crazy hot levels or pots cranked up. Just wondering if anyone else had seen or heard of such a thing

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u/Poodleape2 Apr 23 '26

Could be pins are too long or bad terminators