Help wanted determining decibel levels

Help wanted determining decibel levels

Post by SL » Thu, 22 Jan 2004 08:06:46

Hello all,

I have been helping my boss get his powerlink up and running. The tech who
was there with me at the time was having a little trouble getting the modem
to sync. He mentioned the house was messed up with many outlets going
through several low grade splitters that were not two-way. That solved many
of his "outages" problems with his multi digital box outfitted house. The
modem does work, and so do the digital boxes. But one thing remains. One
outlet in a bedroom has no box, just straight to the t.v. It is sharing a
2-way splitter with the cable modem. It was the simplest to get things
going. Problem is, that occasionally the cable modem when the pc is turned
on/off will not get a signal from the cable.

The tech had his decibel meter and said splitting it would be a problem. A
direct run will solve the problem. I am at this point trying to avoid a new
run. I can hook a barrel connector, effectively removing the splitter and
getting the one shared tv out of the situation. It works fine. If I hook
the tv and the modem back up, It still is fine.

He measured and said with them both sharing the line, I was at about a -9
decibel reading. The modem by itself is a better -4.5 decibel reading. I
guess I will eventually run a dedicated run to the outside box. However, I
was wondering if there is a way to measure the results as I go along. I
dont want to buy the expensive meter he had. But I was wondering if the is
more "laymans" way of determining the db levels.

Is this possible?

Thanks all,

Help wanted determining decibel levels

Post by $Bil » Thu, 22 Jan 2004 10:26:03

Have you tried logging into the modem at
Most of them will have a power levels page.


Help wanted determining decibel levels

Post by SL » Thu, 22 Jan 2004 10:42:15

Not yet $Bill. Will that give me a "snapshot" type of reading? Or will it
be a continual one? I ask now before I have a chance to test on his pc in
the next 24-48 hours.


a -9
However, I

Help wanted determining decibel levels

Post by Robert Hon » Thu, 22 Jan 2004 12:38:41

If a complaint modem, it will allow you to set up the polling time (in sec).
It gives upstream and downstream info. I'm hoping the negative numbers you
were reporting wasn't in dBmV. That's really low. You should be up to 10+
dBmV with a SNR over 30 dB. Look up Shannon's law to relate that to


Robert T. Honea, AKA;


Help wanted determining decibel levels

Post by SL » Fri, 23 Jan 2004 01:16:31

I cannot recall his modem model. Its not the typical SB5100 that myself and
others have.

I also cannot recall if the reading was a dbmV reading. I only know we were
referring to decibel readings. When things were bad, we were getting -16db
readings. As it improved, it was basically dropping -4db at a time until
we reached the desired 4-5 db readings that the techs reported at my house.
In both situations, they reported that as the ideal reading for the modems.

I hope i gave your more info.



Help wanted determining decibel levels

Post by Ed Nielse » Fri, 23 Jan 2004 04:24:14

Snapshot. DOCSIS calls for the input level to be within the -15 -
+15dBmV range. As long as it's within that window, it'll work just
fine. You do want to be at least a few dB inside each end, though.
Keep in mind, also, that that is the QAM level, which is typically ~10dB
below analog.

If you want to get a rough estimate of levels at an outlet, start with
12dBmV at the groundblock. Most 2-way splitters lose ~4dB @750MHz, and
RG 6 loses ~5.6. Do the math from there. Just remember that's analog.
Subtract 10dB for QAM.


Ed N.