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In article <g6f5mj$15n$1@xxxxxxxxxxxxxxxxx>, PaulP@xxxxxxxxxx says...
> A twisted pair of cables will not prevent induction! You should be using
> a SHIELDED cable. I run my PC cables everywhere near to mains
> cables and it doesn't make any difference.
You are correct in what you say BUT the induced signal is common-mode.
That is, the signal that is induced is of the same phase and magnitude
in each wire. Thus, a balanced receiver will not see this noise.
Practially, a twisted pair performs the same as a shielded single-wire
cable at the order of a foot, depending on the frequency of the
interfering signal.
If you are using non-twisted cable, or one that is loosely twisted, e.g
cat1 or phone cable (2.5 twists/ft), then it will present induced noise
with a normal node component, This is seen as noise by the receiver.
If you run a twisted pair alongside a mains cable carrying home power
without electronic switching (or home ethernet adaptors...), then any
interference is unlikly to be a problem for a good balanced receiver.
However, most receivers have a limit to the maximum amount of common
mode noise they can tolorate with respect to ground, since they don't
use high isolation voltage transformers to couple the signal into the
detection circuit.
Ethernet cat5e cables (non shielded) have each twist at a different
pitch. This is to ensure there is little chance of each pair running
with individual wires adjacent to a wire in another twisted pair, over a
substantial length. If it did this, then the high level TX signal in one
pair would interfere with the low level Rx signal on the adjacent pair
producing a normal node coupling. Once you have added even the cable
shieth into cat5e. there is little problem with cables outside the
shieth. Indeed, in every small home and most business installations
I've done, cat5e is fine running 100Mb Ethernet. There is no need to go
to cat6, with all the hassles of maintaining the screen over patch panel
cords, This needs special connectors.
How are you terminating the shields on your cables? If they are
unconnected, they they are performing little function other than
providing armouring. If the shield isn't foil, then it stops working at
a relativly low frequency - just ask a TV aerial installer... What
connectors are you using on the end? I've not seen one on a home
computer that connect the shield to ground.
--
John W
To mail me replace the obvious with co.uk twice
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