[SI-LIST] 1000Base-T Transmitter Distortion Testing
- From: Matthew Fornero <mfornero@xxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Fri, 3 Aug 2012 11:18:06 -0400
Hello All,
I am working on IEEE 802.3 testing for a system with two Intel
gigabit Ethernet controllers (82577 and 82574), and am currently looking
into transmitter distortion testing (40.6.1.2.4). One of the controllers
(82577) narrowly (but consistently) passes this test (< 10 mV distortion).
While the other (82574) consistently fails (~16 mV distortion). My power
supplies and clocks appear to be good, and the clearance from the MDI lines
to any aggressors appears good as well.
Looking at the sample waveforms output from the two controllers in test
mode 4, there are some slight differences (maybe some extra attenuation of
the high frequency components for the 82574), but given the way that
gigabit Ethernet transmits, it's hard to determine what the "ideal"
waveform is.
My initial idea is that it may be something in the MDI traces, though I'm
not sure exactly what. The traces are fairly short (30 - 50 mm; Intel's
design guides recommend < 100 mm) and are matched to within 0.25 mm (10
mils).
Both controllers use internal termination, and both are routed entirely as
microstrips (82674 on the bottom of the board, 82577 on the top) so there
are no/minimal stubs.
My initial thoughts:
1) The MDI traces are routed as 95 Ohm differential pairs (55 ohm single
ended), with +/- 10% controlled impedance fabrication. This deviation from
100/50 was due to space constraints, but should be acceptable based on
Intel's design guide. The guide calls out 100 Ohm +/- 15%, but specifically
allows 95 Ohm +/- 10%. One concern I have is that the Intel design guide
recommends using the 100kHz - 100 MHz relative permittivity (Er) value for
the dielectric rather than the traditional 1 Ghz value for calculating a
stackup-- which I believe my fab house failed to do. They do, however,
measure the actual impedance via TDR, so maybe this is not an issue?
2) I am using a TE (Tyco) MagJack for my connector/magnetics with through
hole leads. For the controller on the bottom (82577-- which passes
narrowly), there should be no/minimal stub. For the controller on the top
(82574), there will be a ~3.5 mm stub just before the magnetics. Maybe this
is contributing to the issue?
Any thoughts on this, or suggestions of an area I should be investigating?
Best Regards,
Matthew Fornero
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