[SI-LIST] Re: Tightly coupled VS loosely coupled diff pairs

  • From: MikonCons@xxxxxxx
  • To: hali@xxxxxxxxxxxxxx, si-list@xxxxxxxxxxxxx
  • Date: Wed, 12 Sep 2001 16:37:41 EDT

In a message dated 9/10/01 4:04:36 PM Pacific Daylight Time, 
hali@xxxxxxxxxxxxxx writes:

> Some folks claim that loose coupling is better because it results in less
> reflection due to intra-pair trace spacing changes (hence impedance
> mismatches) in connector pin fields, in device break-out area and associated
> vias. You didn't say much about that. Just wondering, have you ever
> quantified that particular reflection effect?
> Around the same impedance matching issue, is it not the case that in some
> applications there is a need to tightly match both single-ended and
> differential impedances? If that is the case, is it then always right that
> tight coupling is better than loose coupling?


First item:  Where the trace pairs split upon entering the connector pin 
fields, there are two options; viz, the traces can be widened to maintain the 
differential impedance or the trace width can be maintained and balanced 
against the parasitic capacitance of the pin through-hole to achieve Zo 
continuity. For critical (>2 GBPS) lines, I combine both techniques in the 
pin field, depending on how far the penetration into the field. This takes a 
reasonable modeling effort to accomplish, but I like to make SI a "non-issue" 
for cutting-edge designs.

Second item: Unfortunately, one invariably gets into trouble by assuming 
anything is "always right." I have found that maintaining the accuracy of the 
differential impedance is most beneficial for CML systems. For complimentary, 
single-ended logic systems, the clocks are generally (inherently) slower 
because the unbalanced technology is more susceptible to parasitics, unequal 
transition slew rates, induced supply ripple, etcetera. As a consequence, the 
tight matching of even and odd impedances you question above is of less 
importance. If you are pushing to the higher speeds, I recommend you look to 
balanced CML technology which better tolerates fluctions in even-mode signals 
and responds more accurately to the differential signal.


Michael L. Conn
Owner/Principal Consultant
Mikon Consulting

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