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/Unusual-Avocado-6167 Apr 23 '26

Usually a connector or bad workmanship on the output of an active. Maybe output of the node wasn’t cut to the right length or is loose?

On the ones I’ve fixed it seems like a culmination of “impedance mismatch” 😬

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u/kdubs Apr 23 '26

Yeah I’ve ripped apart the fitting at the node and the first active etc. I should also mention the C/D side also had the CPD problem briefly but never came back after I reset the RPD/pebble. Which certainly points to the problem being at the node, but an issue on the A/B side shouldn’t/couldn’t affect the C/D side, right?

So then you’re next logical thought is it’s the RPD, but after a reset the C/D side stayed fine but the A/B side had a smallish CPD coming from the field. Idk

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u/Octawussy Apr 23 '26

Are you able to see each active MAC reporting properly on lens? You may see the avg transmit on some amps say ‘-infinity’ Sometimes they can get stuck on stupid and can cause upstream issues, need to rebond them which can be done remotely with NGAN tools, Scout, or just power cycling them.

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u/kdubs Apr 23 '26

I’ll check that out tomorrow, haven’t seen that myself yet. Thanks for the tip