It is my understanding there is a spec relating to both the voltage handling
capability and the impedance between ethernet coax and earth ground.
At low frequency (must be more than a certain level) it is quite high in
order to prevent potentially damaging ground loops from forming. At high
frequency (must be less than a certain level) to effectively reference the
shield to chassis potential and make certain that the coax doesn't radiate.
There are manufacturers that sell coax panel connectors with the "proper"
built in capacitor. I recall $10 each price tag.
I further recall that we used to use 0.001uF 2kV caps. WELL DRESSED AND
MOUNTED EXTREMELY PROPERLY.
Vaguely remember that the impedance was to be more than 1Meg at 60Hz and
less than 50 at 3MHz, but you should check the ethernet spec.
You can tell a lot about the cap's mounting (and quality) by looking at the
spectrum of the radiated emissions. For example, internal clock and the cap
is referenced to a noisy spot. Or, spectrum related to the ethernet traffic
and a loop exists around the coax terminations and bypass cap.
- Robert -
Robert A. Macy, PE [email protected]
408 286 3985 fx 408 297 9121
AJM International Electronics Consultants
101 E San Fernando, Suite 402
San Jose, CA 95112
From: "Ken Javor" <[email protected]>
To: <[email protected]>
Sent: Wednesday, March 05, 2003 12:37 PM
Subject: Ethernet coax connection
>
> Question for list members:
>
> Background: I am troubleshooting a complex integration of military
hardware
> and COTS. One COTS piece of equipment has an RG-58 coaxial connection,
but
> the coax connector is an isolated feedthrough bnc. From a radiated
> emissions point-of-view, that is hurting us. One of the engineers here
said
> that is part of the spec - Ethernet shields are not supposed to be chassis
> grounded.
>
> Question: Can someone please explain the reason for that, and how this is
> usually handled to minimize radiated emissions?
>
> Thank you.
>
> Ken Javor
> EMC Compliance
> Huntsville, Alabama
> 256/650-5261
>
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