Derek,
If the wall-wart is insulated, the cabling to AE provides the earth
reference. For UTP Ethernet the RS-232 gnd is the earth potential of
the system. For STP Ethernet it is a large ground loop with an RF
generator in the middle.
.
Honeywell
David W. Sterner
Quality Assurance Engineering
Security & Custom Electronics
165 Eileen Way
P.O. Box 9035
Syosset, NY 11791
Phone: (516) 921-6704 x6970
Fax # (516) 364-6953
[email protected]
<http://www.honeywell.com>
[email protected]
<http://www.IEEE.LI>
Electromagnetic Compatibility
IEEE 05888136
From: [email protected] [mailto:[email protected]]On Behalf Of Derek
Walton
Sent: Thursday, October 05, 2006 8:56 AM
To: [email protected]
Cc: [email protected]; IEEE EMC Discussion Group
Subject: Re: Ethernet emissions
HI All,
thanks for the great suggestions. I didn't get chance to work on this
yesterday too much, but hope to this evening ( USA time ).
Just to clarify, this is a small plastic box about the size of a
matchbox with 3 connections. Ethernet, RS 232, and low voltage dc from a
wall wart.
I did observe that putting a ferrite bead on each cable made little (
maybe 1 dB ) difference. Nor did wrapping the box in AL foil and bonding
the foil to the shield. The emission is a single spike @ 125 MHz about
20 to 25 dB higher than surrounding noise.
The cables are brand new, and have foil and braid.
I hope to share the findings tomorrow,
Sincerely,
Derek Walton
[email protected] wrote on 10/5/2006, 7:39 AM:
> Hi Derek,
>
> I've seen a couple of posts with good advice. I'm wondering if
you've
> already taken care of bonding the shield; and you're still having
issues.
>
> Forgive me for repeating your question...but I'm thinking out loud
(which
> is difficult for email :-)...for some reason when you just have the
> Ethernet cable plugged into the transmit end (The DUT) in the
chamber...
> the Ethernet signal conductors don't change potential at 125Mhz with
> enough magnitude to couple to the measurement antenna...but when you
> terminate the receive end, outside the chamber..they do.
>
> That could be due to the symbol rate of the actual ethernet
> traffic...which would probably increase when the receive end is
plugged
> in. Could it also be a coincidence...how good does the receive end
> filter
> its Ethernet cable? Could the receive end have another, unrelated
clock
> at 12.5 or 25Mhz that's catching a ride out?
>
> Now...somehow, the emissions get higher with a shielded cable...is
this
> with or without the receive end plugged in?
>
> For the shield to work best...it would need to be bonded at both ends
to
> the same, infinite capacity, zero resistance ground at Earth
> potential...which would also, hopefully, be the same ground as the
shield
> of the coax for the measurement antenna...this would minimize
measured
> emissions.
>
> Any break in that chain...and you have an issue. I saw "ground
currents"
> mentioned. This would be due to some potential difference between
the
> shield's end at the DUT, the shield's end at the receiver and or the
> ground of the measurement antenna system.
>
> If you've already messed with shield bonding...Have you also tried
the
> following experiments?
>
> -Are you sure of the shield integrity within the cable? Is it old?
> Has it
> been bent a few times?
> -What if you try a different device at the receive end?
> -Have you played with the grounding of the chassis of the device at
the
> receive end? (grounded, ungrounded...grounded back to chamber ground)
> -What if you leave the cable hooked up at the receive end, leave it
> running into the chamber and disconnect it from the DUT? What
happens?
> -Are there any power filters that could possibly be breaking the
Earth
> path between the DUT and the device at the receive end of the
Ethernet
> cable?
>
> ---What if you just turn it all off, go to the local pub and catch a
ball
> game over some food and spirits...maybe the emissions will still be
> there;
> but you won't care :-)
>
> I don't know what's at the receive end; or what your DUT is; but many
> Ethernet devices are powered by Wall Warts and carry no real chassis
> ground. Many "shielded" ethernet cables have wimpy shielding and
poor
> termination to the connector...to the point where the termination to
the
> connector could be ruined after some hard bending cycles. Running
this
> type of shielded cable between two devices with unknown ground
> characteristics is always "fun". I don't know if I've ever gotten a
> device through a suite of EMC testing unscathed if it had an Ethernet
> cable...seems as though we always had to fiddle with it; either at
our
> end, the peripheral end, shielded, unshielded... very similar to the
> issues that you're having now...I just don't remember the differences
> being so pronounced as what you're seeing.
>
> The other problem with Ethernet is that it's a high frequency signal,
so
> the ports are usually poorly filtered (for EMC anyway), which means
that:
> if somebody didn't take care of business with other clocks in the
system,
> they can easily catch a ride out on the Ethernet cable. That's why I
was
> wondering if the 125Mhz measurement is only coincidentally related to
an
> Ethernet frequency.
>
> Hope this helps and good luck.
>
> Chris Maxwell
> Electrical Engineer
> Critical Imaging LLC
>
>
>
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