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RE: XAUI AC coupling

Title: RE: XAUI AC coupling

From a user:

This looks like a replay of the Transceiver/SERDES mess on 100 Mb/s SFF and 1G 1 x 9 systems.

What happened there was that nobody agreed on the input common-mode DC level of either direction. Users were lectured to about "classic" PECL coupling networks by companies whose own wares did not follow it. Bipolar chips wanted a different DC input level from CMOS chips, which was again different from GaAs designs.

Worse, in many cases the transceiver inputs relied on the Thevenin coupling network to set the (correct) DC bias point.

The net result was that many transceiver/SERDES combinations did not operate together at all. In the end, vendors ended up having to build two versions of everything-- AC coupled and DC coupled. And there were still incompatible combinations.

We ended up insisting on AC coupled designs for everything just to get interchangeability of parts between vendors, time, temperature, phase of the moon, etc. And we also ended up insisting that the transceivers have the caps IN the units, where they belong from an EMI standpoint.

I suspect that our company will again insist on capability for AC coupling. That way each vendor gets to control the DC characteristics on "his" side of the coupling capacitor. The corollary here is that each vendor is responsible for providing an input circuit bias means that sets the DC level to the "sweet spot" for that chip.

Larry Miller
Nortel Networks