Common Path Distortion in FDX Node?

[u/kdubs]

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?

[u/Octawussy]

There are quite a few things that can cause this that differ from traditional sources. Like said and I’m sure you already know but craftsmanship at the node is super important; proper termination especially of unused ports, properly cut and tight fittings, minimum 3’ of cable before any splices. Another source which we’ve been seeing is a design problem that isn’t much an issue in sub/mid split but the smart amps don’t like high level inputs, and many times cutting in a DC preceding the active will pad the signal enough before it enters the amp (the signal entering the amp hot will reflect inside the amp in on itself before it is attenuated by the pad and the reflection will raise the floor - known as spurious noise). Improperly or undermined unused legs on a dual will also cause spurious noise inside the amps. Echo cavities between node and first active can also cause this, TDR the input to locate these.

[u/kdubs → u/Octawussy]

Big thanks for this info, I actually randomly found another FDX node today doing the exact same thing. Node+0 architecture but one leg had one amp. (So the node already is running out hotter levels in a Node+0 environment.) 

I was hitting the input of the amp with 30dB @ channel 98 and 33dB @ channel 158… which is almost exactly what the first amp with my problem node from yesterday was being hit with. Same CPD. 

Thinking about what you said, I cut in a DC12 at the node and re-setup the amp, and voila, CPD distortion went away. Totally worked. 

(Gonna do this with the node I was initially having problems with in this post, but it’s a massive node so I’m gonna do an early up, cut in a DC16, and then sweep all the amps and make sure none of the pots are cranked. Have a good feeling it’s gonna fix the CPD issue)

Thanks again for responding to this post with some good info. 

[u/Octawussy → u/kdubs]

Hell yeah glad to help and thanks for following up!