FreeLists / si-list / [SI-LIST] Re: Tightly coupled VS loosely coupled diff pairs

[MikonCons@xxxxxxx]

Hi, Lee. I have added this response to the SI list to perhaps hear of others 
experiences on the topic.
****
You asked, "Do you have some analytical evidence that tight coupling is 
better?  All of the analysis I have done shows that tight coupling actually 
reduces signal quality, specifically edge rate and jitter.

This becomes prominent at 2.5 GB/S.

I'd be interested in some analysis that shows what the benefir of tight 
coupling is."
****

I have been on the road for the last month, and will continue to be so for at 
least four more weeks; therefore, my comments are brief. The abrupt answer to 
your inquiry is "Yes, absolutely." A longer answer follows.

For the last three years, I have redesigned non-functional, loosely-coupled 
designs, and designed multiple original, tightly-coupled designs (both 
backplanes and line cards) for new, upcoming 3G systems for multiple 
companies, including Nokia and CacheFlow. I am dealing with 50 ps rise times 
and 2.5 to 5 GBPS systems.

As I'm sure you are aware, edge rate degradation for any line configuration 
is partially dependent on the losses of the harmonics of the fundamental. 
That degradation is universal and must be dealt with in all designs. However, 
a potentially greater impact on edge rate occurs because of different delays 
of the signal propagation on each of the two traces of a differential pair. 
The edge rate degradation is 1/2 the difference in propagation delays 
(originating from whatever source). Closer routing (and therefore tighter 
coupling) of traces minimizes this delta in two ways; viz, minimization of 
physical length differences and different effective dielectric values caused 
by localized glass-resin variations in the core and pre-preg layups. More 
tightly coupled traces minimize both effects. Note that the localized 
variations in effective dielectric constant are near impossible to model, 
even for one board vendor with a known material source and a given stackup. 
These variations will vary from board to board even within one lot.

Another major contributor in edge rate degradation is non-symmetrical 
coupling to adajent signal traces and pins in connector pin fields. Contrary 
to recommendations made by another well-known consultant for AMP HS-3 
connectors, I have conclusively demonstrated superior eye diagram performance 
of pairs that bracket the signal pin rows, as opposed to the ground pin rows. 
This comment is particularly applicable to CML designs (like for Vitesse 
parts) as opposed to complimentary, but single-ended, drivers (like the IBM 
DASL line). This degradation effect can also be encountered when dual 
stripline construction includes short parallel segments near the trace pair, 
but on the adjacent layer.

Still two other (perhaps obvious to you, but not to many SI analysists) 
degradation mechanisms are encountered because of assymmetrical rise and fall 
times of complimentary outputs, and different clock-to-output delays (skew) 
of the complimentary driver outputs.

And, yes, I have modeled and analytically correlated ALL of the above effects 
with real hardware designs. Most of these analyses were performed and are 
parts of proprietary, cutting-edge reports to my clients; therefore, I have 
not publicly published my findings for consumption by the SI community. As I 
TRY to back off from a busy consulting schedule, I plan to update and expand 
my own earlier seminar and publish two or three papers on this subject.

Respectfully,

Mike

Michael L. Conn
Owner/Principal Consultant
Mikon Consulting
(408)727-5697

           *** Serving Your Needs with technical Excellence ***

------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List archives are viewable at:     
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages 
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu