I think you meant to say that the SNR should be >35 (greater than 35).


"Ed Nielsen" <egnlsn@comcast.net> wrote in message
news:eN-dnTixSIfg-9DfRVn-hg@comcast.com...

Those numbers are well within the operating range of a cable modem. Cable
modems are designed to wrok with an inout window of -15 - +15dBmV, and an
output of 8 - 55dBmV. SNR is excellent -- supposed to be <35dB.

hardware giving attenuation to the cable line once it comes in: a surge
protector, a splitter, and a male/male coupler.

The cable plant losses increase as the ambient temperature rises. Cable
plants work best cold. The active devices in the cable company's plant
have
correction circuitry to compensate for this. ALC (automatic level control)
holds the output level of an active device constant and ASC (automatic
slope control) holds the pre-emphasis level constant. Cable plant active
devices pre-emphasize the higher frequencies of the baseband because of
the
higher loss of the coaxial hardline as frequency increases. This "slope"
as
it is called allows the downstream active device to see a flat (or nearly
so) frequency response on the baseband. Since the loss of hardline changes
with increasing temperature, the ALC and ASC circuitry automatically
increases the gain and slope as appropriate to maintain a constant level
to
the next downstream active device. This is adjustable, and may be
operating
incorrectly in your case.

This is pretty low, though legal. I'd expect to see +15 dBm in a properly
set up and designed system at your demarcation point.

Good so far.

Describe the coupler? What kind of device is this?


Dump the surge supressor. Get yourself a Regal gas-discharge ground block,
and place this at the demarcation point.

As to cable plant signal level variations, expect some change, but the
changes you are reporting would indicate that your plant is incorrectly
set
up or has incorrectly operating ASC/ALC circuitry.

Call your cable company and have them send out a line tech to take a look
at
their plant.

Rick Kunath

I have about 30 feet of coax cable
running from the outside box to the surge protector inside. Thanks for
the
tip on the ground block. I'll look into doing this and not running
through the surge protector.

What surges are you protecting your equipment from by running the coax
through the surge protector? Lightning? A little surge protector isn't
going
to help. Even if the surge causes it to disconnect, a lightning hit is
going
to be able to jump that tiny, tiny little gap. And the chance of some
other
kind of surge is very low. And even if you beat the odds, and have a power
surge from one of these rare causes, what would you loose if it made it to
the modem? A $50 modem?

There really isn't enough of a risk involved to bother with a surge
protector on the coax cable line. Compare that to the problems it causes
by
adding extra exposed, unshielded connectors, and choking-off the signal.
If
conditions are well enough that adding these adverse conditions doesn't
make
a difference, that's one thing. But if it's having a negative impact, and
doesn't have a positive impact to off-set that negative, the choice is
easy:
Stop running the coax through the surge protector.