[SI-LIST] Re: Diff line
- From: Scott McMorrow <scott@xxxxxxxxxxxxxxxx>
- To: zanella_fabrizio@xxxxxxx
- Date: Thu, 14 Feb 2002 19:16:01 -0800
All,
Both loosely coupled and tightly coupled differential pair designs work
as all authors have previously stated. The following is a list of some
of the general differences between the two types of edge-coupled
differential pairs:
Loosely coupled
better impedance control
less sensitivity to manufacturing variations
lower track density
lower even mode return loss
lower insertion loss
less impedance variation at 2mm connector transitions
less impedance variation at BGA escapes transitions
less impedance variation at via transitions
Tightly coupled
worse impedance control
greater sensitivity to manufacturing variations
higher track density
higher even mode return loss
higher insertion loss
greater impedance variation at 2mm connector transitions
greater impedance variation at BGA escape transitions
greater impedance variation at via transitions
As with all things in interconnect engineering, there are tradeoffs. At
3.125 Gbps and above, it's the impedance mismatches that get you into
trouble, especially near connectors, vias and device escapes. It is
extremely easy to create unwanted resonances which will cause unexpected
losses at the strangest frequences, some easily within the frequency
spectrum of data streams.
For high speed circuits where trace density is not an issue, I always
choose to use loosely coupled differential pair conductors, where
impedance control is much more precise, and resonances caused by
impedance mismatch can more easily be reduced.
regards,
scott
--
Scott McMorrow
Principal Engineer
SiQual Interconnect Engineering
18735 SW Boones Ferry Road
Tualatin, OR 97062-3090
(503) 885-1231
http://www.siqual.com
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