In article , amdx wrote:

He said he was looking for noise. The cable guy didn't have a high
understanding, he just knew if it had a signal over a certain amount
that was to much noise. The test showed a peak at 300MHz of -40 unknown
label. But he said, nothing to worry about.
I'm not sure what this test was, unless is is just a spectrum analyzer
looking at noise on a coax going into the house.

If that is so, where would a 300MHz signal originate in the house?


The coax was connected to the Modem/router on one end and the spectrum
analyzer? on the other end. Could the Modem/router generate a 300MHz signal?

Sure... if one of its onboard clock/oscillator chips is running at
that frequency (or a subharmonic of it like 100 or 150 MHz).

Some years ago I helped chase down an "over the air" interference
problem, where a strange signal was getting into the inputs of a bunch
of repeaters in the 2-meter band (144-148 MHz). We tracked down the
source to an apartment building in San Jose. One apartment dweller
had an Ethernet switch which was radiating a harmonic of its
oscillator frequency, whenever it was passing Ethernet traffic, and
this signal wandered around between 144 and 145 MHz. The signal was
apparently leaking out via both the Ethernet cables and through the
router's power wiring and wall-wart... strong enough to be picked
up miles away!

I traded the owner one of my spare Ethernet switches for his noisy
one, and the problem went away. Never did learn what was actually wrong
with the noisy one (it was a well-known brand with a good reputation for
quality).

Would overloading the input of a modem cause it be slow?

It's not impossible. If a signal is too strong it can push the RF
circuitry into saturation, which causes several sorts of Badness...
distortion of the strong signals, suppression of weaker ones, and
the creation of intermodulation "images" between strong signals.

You could try adding a 75-ohm coaxial "pad" (a bidirectional attenuator
of the correct impedance) to see if it helps... try a 6 dB or 12 dB pad.

Another thing to check for, is to make sure that the home's coaxial wiring
is a "straight run" from the service entrance to wherever you have the
modem... no dividers or side branches. If you do have a signal splitter
(so e.g. you can run both the modem and a TV or DVR) make sure that all
unused ports on the splitter have 75-ohm terminators screwed into them.
If there are any branches coming off of the splitter that go to other
rooms, make sure they're either in use (with e.g. a TV) or have a terminator.

Unterminated branches on a coax network can cause signal reflections, and
admit outside noise... both can hurt reception.

Cable guy did suggest he try a new modem.

Well worth trying. Maybe see if you can get a loaner from someone to
try, before spending $$.