via clearance question

John Lipsius (johnl@agathon)
Fri, 7 Mar 1997 16:25:43 -0800

Hello All,

This concerns how much trace to put between a via on it
and the terminal optical transceiver component pin.

I have a 1+GHz trace that must use a via to cross another;
both are a differential pair. It's needed to match up crossed
pcb routing to a component pinout that doesn't match.

Presently, both traces are on the top layer to avoid via usage
(since smt passives are present on the nets in question).
All stackup and impedance aspects are covered. And the
via in question is less than 150 mil from the terminating
component, a transceiver. The pair is transmit data.

The question is, should the via be backed away at least 500
mil or more from the pin on the transceiver that the trace
terminates on? If not, then distortions could show up as
phase jitter, no?

Reference PCB Design magazine, Aug. 96, "The Effects of Vias
on PCB Traces", where it's shown that the initial via can have
an effect over 250 mil in both directions.

John Lipsius
Sun Microsystems
Internet and Network Products Group
tel 415-688-9442
fax 415-688-9112