[SI-LIST] : Gigabit eithernet board.

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From: Nick Dietz ([email protected])
Date: Fri Feb 04 2000 - 09:47:55 PST

Hi. I've been reading this list for several months now, but I still can't
figure out what to do now. I'm a new engineer working on a new board on
which I am trying to minimize noise. Cost is not a driving issue.

The board is probably routable on two layers. It has a 3.3V-IO / 2.5V-CORE
FPGA communicating with two 5v transceiver chips. I am trying to figure out
how to do the stackup for low noise up to the 1500 Mbaud-serial /
100?MHz-parallel speeds that the board will be running at.

I'm thinking option 1:

Top (routing, and ground copper under the transceivers)
 GND (full plane)
 POWER (+5v under transceivers/2.5v under FPGA)
 POWER (3.3v full plane)
 GND (full plane)
Bottom (routing and capacitors)

This is my take on option 1: All of the signals will route with minimal
vias on the outer layers, which might be bad for emissions. The board is
only 3 inches by 4 inches, with the fastest signals moving about an inch or
so, so this should not be terribly bad.

All the signals will be directly adjacent to a solid ground plane, thus
eliminating traces passing from the 5v area to the 2.5v/3.3v area. This
should allow a good return path for all of the signals on top and bottom.

Is this a reasonable stackup?

Would it be significantly better or worse to put in a bunch more vias and
do something like this (option 2):

Top (minimal routing, ground copper under the transceivers)
 GND (full plane)
 GND (full plane)
 POWER (5v under transceivers/2.5v under FPGA)
Bottom (minimal routing)

This puts a 3.3v plane underneath all the components, even the 5v parts. Is
that a problem? Would it be better to put the split 5v/2.5v plane as the
second layer? Would it be better to switch the power layers in some other
order? (e.g. GND on layer 2 and 3.3v on layer 3, or 5v/2.5v split on layer
2? and 3.3 on layer 7)

Would the capacitance between the top layer grounded copper and the 3.3v
plane in option 2 be a bad thing since the transceivers are 5v parts?

We are not as worried about emissions as we are about having a board that
works reliably (=quietly). It will run inside a PC, so noise coming from
off the board may be a consideration.

Any more light you can shed on this issue will help a lot.

Nicholas Dietz

Nicholas Dietz
Applied Telecom, Inc.
3060 Ogden Avenue, Suite 300
Lisle, IL 60532-1690
(630) 357-9290 x 219

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