Newsgroups: sci.electronics.misc
Subject: Re: due to some kind of interference, monitor display is "shaking"
References: 
"John H."  writes:

> hi, I have two monitors connected to my ati card.
> 
> I have
> coax on wall - > cable modem surge protector (very small 
> -> 2- way splitter -> cable modem 
> 		    -> 	tv card on pc
> 
> there is a line off the surge protector which goes into screw 
> in order to ground it.
> 
> 
> 
> OK, so there are several weird situations from this.  If I have 
> coax from splitter to TV card, and I do not ground it to wall 
> outlet, both monitor displays shake REALLY heavily.  

Sounds like a large current starts to flow through the cables 
from electrical power ground to the TV cable ground. 
This current can cause effect inside the devices/cables and also 
in some cases the magnetic fields from the cables 
can carry noise. 

> If I ground 
> it to wall outlet, large 19 inch CRT shakes some, but not nearly 
> as much(where I am at now).

Still there is considerable current floating and causign problems. 
No grounding is ideal. 

> Now, if I move computer and two monitors to other side of room 
> with no peripherals attached, no shaking.  

As such the computer system is working well. 

> If I simply plug the 
> surge protector into the coax on this side of the room, and ground 
> is connected, even though NOTHING Is connected to surge protector, 
> on the clear other side of the room, both monitors begin shaking!

My quess:
Your connection of the calbe to electrical ground causes 
lots of current to flow in the TV cable shield and the 
electrical power ground wires. This current creates considerable 
magnetic field in the room and causes problems to your monitor. 

> I need a way for everything to live in harmony:(  The shaking, 
> even though it is minor right now, is mild, but still too much:(
> I know a lot about computers, but not too much about electronics.  
> How can I get rid of ALL shaking?

In most cases the problem like you can be solved with antenna 
cable signal isolator and/or better grounding of the TV cable 
when it enters the house (better grounding together with 
mains panel ground reduces the potential differences between 
your electrical outlets and cable TV outlets if your elctrical 
wiring is done right).

Usually the easiest is to install an antenna cable signal 
isolator between the antenna outlet and the component that 
causes problem. In this case all problems related to 
the shaking most propably would be solved by installing 
an auitable isolator between your antenna outlet 
and your surge suppressor. This gets rid of the ground loop. 

One potential problem with this setup: 
The cable modem system might not work well with all 
of the isolators in the market. Not all antenna signal 
isolators pass properly the cable modem return channel 
signals (lower frequency and much higher amplitude than 
normal cable TV signals). 

Here are some links for more information:
http://www.epanorama.net/documents/groundloop/index.html
http://www.epanorama.net/documents/groundloop/home_solving.html
http://www.epanorama.net/documents/groundloop/antenna_isolator.html
http://www.epanorama.net/documents/groundloop/antenna_isolator_building.html

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/