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Las Vegas: Signal level falling
I periodically check the signal level on my cable modem page. Earlier this year, it was usually around -7 to -9 dBmV. But then in the spring, it dropped to the -12 to -14 dBmV range. Shortly after that drop, my cable modem lost the signal altogether and I had to have a technician come out to fix it. The technician replaced the TV filter at the curb access, changed the port my house was connected to at the curb access, replaced the connector at the curb access, removed an unused 3.5dB splitter where the cable enters my home, and replaced most of the connectors in my home. Then the signal came back, and it was in the -9 to -10 dBmV range.
The technician considered that a borderline signal level (he said that the preferred range was -10 to +10 dBmV, and mine was bumping against the -10 dBmV lower limit). So he replaced the balanced 3-way splitter that I had in my home (to feed the cable modem, TV, and tuning adapter) to an unbalanced one, with the cable modem getting the -3.5 dB port. Since the balanced splitter had about -5.5dB insertion loss, that gained us about 2dB of signal. And indeed, the cable modem reported about -8 dBmV for a few days after that.
However, shortly after that, the signal dropped, and for most of July the signal has been hovering in the -13 to -15 dBmV range, occasionally dropping to -16 dBmV (which is outside the DOCSIS spec of +15 to -15 dBmV).
So, looking at the signal coming into my house before any splitters, that was a 5dB drop in the spring, another 5dB drop when my signal was lost completely and then repaired (since the technician removed a 3.5dB splitter and changed the balanced three way to an unbalanced, that is 3.5 + 2 = 5.5dB of improvement in the house, so the incoming signal to the house must have dropped about 5dB), and then another 7dB drop after the technician left. That is a total of 17dB decrease in signal from Cox that is feeding into my house, over a period of several months.
Now, my upstream power level has held steady at about 35 to 38 dBmV since the technician fixed the lost signal, even though the downstream signal has dropped 7dB during the same period. So I think that the signal level is dropping at the cable coming into my house, and the signal level dropping is NOT a problem from inside my house (another reason I think it is not inside my house is that the technician replaced most of the connectors inside, and I have not so much as touched them since).
Why is the signal Cox is feeding my house always dropping to just barely acceptable, or even slightly below acceptable? Is there some sort of automatic gain control on Cox amplifiers that is not functioning properly?
Here's an example of recent signal levels at the cable modem:
http://i.imgur.com/p1HCrER.png
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