[SI-LIST] Re: Ethernet emissions.
- From: Todd Hubing <HUBING@xxxxxxxxxxx>
- To: "<si-list@xxxxxxxxxxxxx>" <si-list@xxxxxxxxxxxxx>
- Date: Sat, 6 Apr 2013 17:54:38 +0000
Bryan,
Please don't cut up your ground. I wouldn't want anyone on this list to get the
idea that isolating grounds is going to solve a radiated emissions problem
above 100 MHz. From the symptoms and the measurements you describe, it appears
that you are driving the cable relative to your circuit board ground with a CM
voltage produced directly by the PHY and insufficiently attenuated by the
connector magnetics. If you've maintained balance in your layout between the
PHY and the connector, you may need to consider using a different connector
with better CM rejection.
At these frequencies, nothing you do with your ground is going to improve upon
the solid plane that you currently have. Trying to establish separate grounds
that are at different potentials is likely to make things worse.
Todd
-----
Todd Hubing
LearnEMC<http://www.learnemc.com/>
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Bryan Ackerly
Sent: Saturday, April 06, 2013 5:12 AM
To: <verma.nitya@xxxxxxxxx>; olaney@xxxxxxxxx
Cc: <epd2001usa@xxxxxxxxxxx>; <si-list@xxxxxxxxxxxxx>
Subject: [SI-LIST] Re: Ethernet emissions.
Sorry, i should have indicated there are TWO ethernet connectors. The loopback
used at the test house is between the two different ports using a standard
CAT5e cable.
The connector with integral magnetics is supposed to include common mode
rejection. Though a quick test with a network analyzer didnt show anything
sectacular. CM Attenuation at 100MHz was around 40dB but at 500MHz it was more
like 10dB.
The board has no ground plane splits anywhere. It has no supply plane splits
that active signal traces run over.
The highest near field probed levels at 375/500/625MHz were detected directly
above the PHY, not on the rx/tx lines coming from it (though it is definately
evident there at higher levels than anywhere else on the board.)
My reasoning as to why the signals are common mode is that i can also detect
them with an inductive probe easily off the ethernet cable itself, but not off
the power or serial cables from the board.
Interestingly another product (using a different PHY) done some time ago showed
some emissions at 250MHz (the power plane had an impedance hole near this
frequency, which probably didn't help, but it passed) and a lot of spurs at
100MHz (which were very marginal pass with UTP cable. And that looks similar
with this implementation. I have yet to deal with that). But the other product
had everything else passing with considerable margin.
Ken, Id certainly be interested in looking at the "image plane" idea. If i
understand it right it involves having a separate "quiet" ground for all the IO
connectors. That would be rather tricky in this case as one of them is an
antenna connector that carries several watts at UHF. Splitting its ground is
likely to create a number of design challenges, a significant one being trying
to convince my peers to implement it!
Regards,
Bryan Ackerly
On 06/04/2013, at 7:19 PM,
<verma.nitya@xxxxxxxxx<mailto:verma.nitya@xxxxxxxxx>> wrote:
> Hi Bryan,
>
> You mentioned that the test was done as loop back on the same Ethernet port
> which means there is no cable attached to the port but only the loopback
> connector.
> Could it be possible that source of these emissions might be from the
> Ethernet lines but they are coming out of the product via another medium?
>
> Is it that any other small cable wire or mesh is nearby which is picking up
> from and radiating. May be if it is there you might want to isolate them?
>
> This is just a thought.
>
> Thanks and Regards
> Nitya
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx<mailto:si-list-bounce@xxxxxxxxxxxxx>
> [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Bryan Ackerly
> Sent: Saturday, April 06, 2013 8:39 AM
> To: keithK EPD; si-list@xxxxxxxxxxxxx<mailto:si-list@xxxxxxxxxxxxx>
> Subject: [SI-LIST] Re: Ethernet emissions.
>
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