SB6121 Digital Modem, Comcast disconnect

Recently, we started having problems with our internet disconnecting.  After some searching, I found a few helpful links that shed light on the issue.

A really good Amazon review.

Acceptable upstream and downstream power levels here, hereand here.

For the SB6121 digital modem, the "surfboard" interface can be found by steering your web browser to  You can then get upstream and downstream signal strength information by clicking on the "signal" link at the top of the page.

In summary, you want:

  1. Downstream strengths between -15 dBmV to +15 dBmV.
  2. Upstream strengths between 37 dBmV to 55 dBmV.
  3. SNR (signal to noise ratio) greater than 30 dB.

I couldn't find a definition online, but it seems obvious to me that the downstream power is the power seen by the cable modem, from the cable company; and the upstream power is the power that the cable modem is required to send to make the connection.  Thus, you don't want too strong of a signal coming in, and you don't want the modem to have to try too hard to get a signal out.  For the RF inclined, I imagine that at too high of a power level, the gain of the digital modem starts to become non-linear, causing distortion problems.

When the internet started disconnecting, I went to the surfboard page and found that the upstream power was 57 dBmV at night, and 54 dBmV during the day (apparently it can change as the outdoor weather conditions change).  The connection problem was much worse at night, consistent with the higher upstream power at night.

At this point, I went and looked at my setup.  The Comcast coaxial line came down to a three-port splitter/attenuator: the input, a -2.1 dB output (which went to the TV), and a -9 dB output (which went to the digital modem).  Eureka!  a 9 dB diminished signal mean that the digital modem had to push that much harder to get its signal out.  So I did an experiment, and it all came out as expected.

  1. I removed this splitter/attenuator from the connection, so that the Comcast coax went directly to the digital modem.  The upstream signal decreased to about 43 dBmV, which is roughly a 9 dB drop!
  2. Since I wanted the TV to have Comcast cable too, and I had a splitter lying around, I put it into the circuit.  The splitter has no added attenuation, other than that is splits the signal in half, and has a little path loss of its own.  Thus each output port puts out -3.5 dB.  A rule of thumb: -3 dB = half, -3.5 dB is a little more than half.  When I substituted this into the circuit, I got about 46 dBmV strength, which is roughly 3.5 dB more than a straight through connection.

In sum, after putting in the splitter, my downstream was at 46 dBmV, and the upstream was at 5-6 dBmV, with an SNR of 38 dB.  Everything is now where it is supposed to be!  I will try and update this post if the problem ends up not being fixed...  So far so good.

Comments !