[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?
>
Ali:
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.
Mike
Michael L. Conn
Owner/Principal Consultant
Mikon Consulting
(408)727-5697
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