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RE: XAUI AC coupling


It seems to me that AC coupling deserves more to be "required" than
"not required".

PHY module manufacturers will gain maximum economies of scale by
being able to sell the same PHY module to multiple switch
manufacturers, a scenario made possible by XAUI. But each party will
control only one end of the link. Without AC coupling, that may
become a risky scenario.

Take PECL as an example. PECL gate inputs need resistors to set up
DC bias and to match impedance. Some ICs implement this
resistor/bias network internally, some don't. PECL outputs (the
"hanging" emitters) also need a similar resistor network to set up
DC bias and to match impedance. Often, a termination is a single
resistor; often it is two resistors - one pulled high and one pulled
low, to achieve the same (small signal) impedance but a different
level of bias current.

Now consider the COMBINATIONS of inputs and outputs, if DC coupled.
If the transmit side uses a different VDD than that assumed by the
receive side (or vice versa), the bias points at both ends of the
link will change. When the AC signal is added, it will span a
voltage range it wasn't optimally designed to span. Even with the
same VDD at both ends, the different ways of implementing resistor
networks would change the assumed bias points. If we make AC
coupling optional, and some use it, the bias point assumptions
change again. It all gets more chaotic if you add LVDS option to
this scenario.

The safest way to eliminate DC bias uncertainties is to ensure AC
coupling. I would find it risky to blindly design a DC coupled
driver/receiver where one end of the link is under my control and
the other end is not. In a market where you will see more than 15
switch manufacturers and more than 20 PHY module manufacturers
announcing products within a 12-month span, it is not realistic to
assume that they will work it out easily. Therefore, I think we
should specify AC coupling.