On Tue, 2 Dec 2003 10:25:31 EST, K5RC@aol.com wrote:
>This may
>be an RFI issue coming through the CAT 5 network cables. I am replacing them
>with shielded cable and just ordered a load of ferrite chokes.
I would avoid the shielded CAT5 cable. A mechanism called shield
current induced noise will convert shield current to voltage on the
"balanced" pair. On the other hand, a better grade of CAT cable like
Belden Mediatwist may help. It uses a closer tolerance construction
that can result in fewer errors and fewer data re-transmissions. That
has been the effect in my office.
The ferrite chokes ARE a good idea, but not necessarily on the network
cables. For HF, you need a lot of turns around one of the bigger
toroids commonly used for baluns (#43 material). The clamp-on chokes
won't do much below VHF.
Another common source of RF coupling into equipment is a design error
that we in the audio industry call "the pin 1 problem," because pin 1
is the shield terminal of the XLR connectors used for balanced audio.
The only "right" connection for the shield of a cable is to the
shielding enclosure of equipment. A very common wrong connection is for
the shield to go to circuit common, wander around the circuit board for
a while, then to the shielding enclosure at some point. This wandering
path is a common impedance that can have considerable inductive
reactance at RF, and the drop across it from current flowing on the
shield will be added to the signal!
One common way this happens is for the shells of connectors that are
the shield contact to be insulated from the enclosure and be connected
only to the circuit board. This is an inexpensive way to build gear,
but is an invitation to RFI.
The pin 1 problem is quite common in computer sound cards, small
mixers, and a lot of both pro and semi-pro audio gear. Although I
haven't had time to verify it, the "help files" and applications notes
for several pieces of equipment in my ham shack suggest that they too
have a pin 1 problem.
There are three easy fixes for a pin 1 problem.
1. Modify the gear to take the shield contact of the cable connector
straight to the enclosure at the point of entry.
2. Remove the cable shield from the improperly wired shield contact and
take it straight to the enclosure at the point of entry.
3. Eliminate or reduce the shield current. The toroidal chokes at the
input of the defective equipment can do this. Remember that nearly all
detection of rf is square law, so a 10 dB reduction in current will
result in 20 dB less detected RF.
Jim Brown K9YC
http://audiosystemsgroup.com
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