If you are seeing the garbage in the return bandwidth ONLY at the return optical receiver and NOT at the node, it sounds like you have a return laser clipping problem.
This can be best identified by measuring the amount of "trash" on the spectrum amalyzer in the HE. Have someone go to the node and insert a 3dB pad (or increase the existing pad by 3dB) that feeds the optical laser input. If the "trash" decreases by 3dB the problem MAY be in the node or the fiber link. If it drops by more than 3dB (6, 10 or whatever) and you see an improvement in overall noise floor performance, you have a "clipping" problem. This is mostly caused by excessive noise and ingress overdriving the return laser.
One would expect that putting 3dB pad in the return (at the laser input) would cause the modems to transmit 3dB higher. This is NOT true if the laser is in clip. If the 3dB (half the power BTW) causes the laser to reduce the power and eliminate the clipping, this could drop the "trash" level by 10-20dB and the modems now will transmit lower than without the 3dB pad. Every scenerio is different.
Remember, if the "trash" drops on a 1:1 ratio, (3dB pad = 3dB drop) this would suggest that the return laser is not clipping. If the ratio is significantly different, I would look at clip. You may need to, depending on how sever your problem is, use more than a 3dB pad to prove this. I would use a 6dB pad if the 3dB pad has no effect. The idea is to keep "dividing the power by 50%" with each 3dB increment to the laser input.
The way ANY optical laser is rated is through POWER input drive level. Anything over 100% drive will produce distortions (clip) and show up only at the optical receiver. Noise and ingress are POWER, unwanted power, but power nonetheless. The laser has no way of knowing what input is good or bad, it just sees it as power input. The BIGGEST unwanted "garbage" in the return is usually the junk below 20MHz that increases in level the closer to 5MHz you get.
Sorry for the soapbox!
Good Luck,
Greg Tobin
Interactive Broadband Technical Services, LLC
Re: Need help with random Motorla/Arris SG4000 iss
Member #: 7190
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Name:
Greg Tobin
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Greg Tobin
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