[SI-LIST] 1000BaseT Common-mode output voltage test
- From: Matthew Fornero <mfornero@xxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Sat, 6 Aug 2011 11:34:37 -0400
Hello All,
I'm performing system evaluation on an embedded SBC using a couple of Intel
ethernet controllers/PHYs 82577 and 82574). I am able to pass parts of the
1000BaseT compliance test, with the exception of common-mode output voltage.
Here, the spec requires < 50 mV pk-pk, while I am measuring ~70-90 mV pk-pk.
I've run the same tests on reference platforms for the two controllers, and
they are able to pass without difficulty (< 25 mV pk-pk). Comparing my
schematics and layout to the reference platforms, I can find no obvious
differences other than the following:
1) Different grounding of the ethernet connector shield: The reference
platforms tie the shield to digital ground (commercial boards), while mine
is tied to frame ground/earth. Shorting the connector ground to digital
ground does not seem to make any difference
2) Different connectors/magnetics: one reference platform uses an ethernet
connector (Pulse I think) with built-in magnetics and USB ports, the other
platform uses discrete magnetics. I am using a Tyco connector with built-in
magnetics. All are rated for gigabit ethernet.
I've looked over my circuit in general, and can't find anything obvious--
the power delivery is good, and well within spec for the controllers, the
MDI traces follow the Intel recommendations for isolation from aggressor
signals, and are all length matched to within 10 mils.
Any ideas what would be causing this level of common-mode output voltage?
For reference, the common-mode output voltage test is described here:
ftp://ftp.iol.unh.edu/pub/ethernet/test_suites/CL40_PMA/PMA_Test_Suite_v2.6.pdf
Test 40.1.8 (IEEE Std 802.3-2008, clause 40.8.3.3)
Best Regards,
Matthew Fornero
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