[SI-LIST] Re: Ethernet emissions.
- From: <verma.nitya@xxxxxxxxx>
- To: <bryan@xxxxxxxxxxxxxx>, <epd2001usa@xxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Sat, 6 Apr 2013 08:19:37 +0000
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] On
Behalf Of Bryan Ackerly
Sent: Saturday, April 06, 2013 8:39 AM
To: keithK EPD; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Ethernet emissions.
Keith. Thanks for the advice.
Not sure what more i can do with the layout. The traces to the connector are
rather short (<10mm) and run as equal length differential pairs on an outer
layer to preserve trace to ground impedance. No other digital signal tracks for
the PHY run on the outer layers. The board is 10 layers with paired supply and
ground planes to maximize plane capacitance. The PHY runs off these planes and
the supplies and grounding are well stitched to them.
Close Inductive probing using a spectrum analyzer shows these parts are the
"hottest" on the board at these frequencies. Use of a shielded ethernet cable
drops most of the emission levels by about 10-15dB. But this is not a practical
solution as we have no control over what the customer may use as a cable. This
also applies to ferrite clamps, which so far i have not tried.
This seems to suggest that the signals are common mode out of the PHY. I
thought this would largely be removed by the magnetics in the connector. The
connector is well bonded to the board lower layer which is a continuous ground.
The case does NOT form a shield as this is not practical. Though i would have
thought if connector flange grounding was important, the shielded cable would
not have improved things so much.
The test was done as a loop back on the same product so i am measuring only its
emissions, not those coming from any connected equipment.
Regards,
Bryan Ackerly
On 06/04/2013, at 11:48 AM, keithK EPD <epd2001usa@xxxxxxxxxxx> wrote:
> Layout - I've done this for 15 years..
>
> keith
>
>
> > Subject: [SI-LIST] Ethernet emissions.
> > From: bryan@xxxxxxxxxxxxxx
> > Date: Sat, 6 Apr 2013 10:39:45 +1100
> > To: si-list@xxxxxxxxxxxxx
> >
> > I have a question regarding a (10/100) ethernet implementation that failed
> > radiated EMC. The phy is an SMSC LAN-8720. Due to space requirements a
> > connector with integral magnetics is used.
> >
> > The main problem seems to be emissions at 375, 500 and 625MHz being
> > radiated from the UTP cable.
> >
> > For an expeiment i added an LC lowpass filter on each line from the PHY
> > cutting off from 125MHz which seems to improve things a bit, especially at
> > the higher frequencies.
> >
> > But i dont know what impact this will have on the ethernet signalling
> > itself.
> >
> > Has anyone else done this to pass emissions? Is there a better way of doing
> > it?
> >
> > Regards,
> > Bryan Ackerly
> >
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