[SI-LIST] Re: switching LVDS from loose to tight coupling

This was discussed here before. The problem may arise if you have common
mode /differential noise: the noise signal may get some reflections at the
transition point. As long as it is small - it is OK to match tight to loose.

-----Original Message-----
From: Robert Sefton [mailto:rsefton@xxxxxxxxxxxxx]
Sent: Friday, June 21, 2002 1:39 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] switching LVDS from loose to tight coupling


I'm running a ~400MHz LVDS bus (source synchronous, data on both edges of
clock) from a main board through a connector to a daughter card. For various
reasons I'm using loose coupling on the main board, routing each line as
single-ended 50 Ohm. The bus is terminated into 100 Ohms at the load on the
daughter card. I am not designing the daughter card so it's likely that the
diff pairs will be tightly coupled there. My question is whether switching
from loose coupling to tight coupling across the connector is ok. My
instincts say it's fine as long as lengths within each pair are closely
matched to my side of the connector, but I don't have the tools to simulate
it.

Any potential for problems here?

Thanks,

Robert

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