RE: [SI-LIST] : connector model for differential signal

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From: Daniel, Erik S. (Daniel.Erik@mayo.edu)
Date: Thu Oct 26 2000 - 05:29:58 PDT


John-
 
I agree that the terminology is confusing for impedances of two coupled
lines. The answer is that both sets of equations are "right" -- it is a
matter of semantics and definitions.
 
The terminology I prefer defines the odd mode impedance Zodd and the even
mode impedance Zeven of two completely uncoupled 50 ohm lines as Zodd=50
ohms, Zeven=50 ohms. Then, if these two lines are brought closer together
(without other changes in geometry), Zodd will drop below 50 with Zeven
rising above 50.
 
Some people define Zdifferential=2*Zodd because a resistor of this value
places between the two traces provides reflectionless termination of odd
mode signals, but this is equivalent to placing termination resistors of
Zodd from each trace to a "virtual" ground node between the traces, so the
termination resistance allocated to each trace is really Zodd. In fact, in
practice, one sometimes wants to terminate a differential line with
resistors of value Zodd to "true" ground to provide some termination of even
mode signals as well as the odd mode signals.
 
Similarly, some define Zcommon=Zeven/2 because resistor of this value
connected from ground to both traces (shorted together) will provide
reflectionless termination of even mode signals, but again, I view this as
equivalent to placing resistors of value Zeven from each trace to ground
separately -- there is a virtual "short" between the two traces for true
even mode signals.
 
To further confuse things, some define Zodd or Zeven not as I did above, but
with these extra factors of 2.
 
I prefer the terminology without the additional factors of 2 for a number of
reasons, but primarily because it then can be generalized to systems of more
than 2 coupled conductors, or to two conductor systems that are not exactly
symmetric (in which case "Zeven" and "Zodd" are ill-defined quantities, and
the system has to be described by two "eigenmodes" that are neither quite
even or quite odd).
 
In the end, you can use whatever system of definitions you like as long as
you, your simulation tools, and your board vendor use the same terminology
(or at least know how to convert from one to another) so that embarrassing
factor of 2 discrepancies in impedances can be avoided.
 
Regarding your question of whether one cares about anything besides Zodd (or
Zdiff), I personally think it is useful to be aware of both Zodd and Zeven
when designing differential line structures as many systems issues (EMC,
noise immunity, common mode rejection, propagation across split planes, ...)
involve considerations of both quantities.
 
                    - Erik

==================================================================
Erik Daniel, Ph.D. Mayo Foundation
Voice: (507) 284-1634 Guggenheim 1011B
Fax: (507) 284-9171 200 First Street SW
E-mail: daniel.erik@mayo.edu Rochester, MN 55905
==================================================================

-----Original Message-----
From: John@quantatw.com [mailto:John@quantatw.com]
Sent: Wednesday, October 25, 2000 8:43 PM
To: Dima Smolyansky
Cc: eric@bogent.com; si-list@silab.eng.sun.com
Subject: RE: [SI-LIST] : connector model for differential signal

Dear Dima,
 
Thank you for your response. After downloading the application note for the
web site, I find
it is exactly what I am looking for.
 
However, I got a little confusion about common mode impedance in the paper
"Characterization of Differential Interconnects from TDR Measurement"
,DIFF-1199.pdf.
 
On the page two, equation 3, the common mode impedance is Z_common=Zeven /
2.
 
Checking the article "Differential impedance ... finally made simple"
(CAPGLTIF.pdf) by Eric Bogatin, from www.BogatinEnterprises.com,
<http://www.BogatinEnterprises.com> Slide 26, I find the
Z_common=Zeven=Z11+Z12.
 
Checking the HP54754A TDR user's manual, I find a description with
measurment waveform shown
"The common mode impedance of two 50 ohm uncoupled lines is 25 ohms". It
match what you described Zcommon.
 
Shall the Z_common be Zeven/2 or Zeven?
 
To meet Zdiff is always mentioned when doing PCB impedance control. Usually,
I totally forget Zcommon
when try to meet Zdiff. What circumstance shall I maintain Zcomm , instead
of Zdiff , or both?
 
 
Thank you for your inputs.
 
Best Regards,

John Lin
SI Engineer, ARD4
Quanta Computer Inc.,Taiwan, R.O.C.
Email: John@quantatw.com <mailto:John@quantatw.com>
Tel: 886+3+3979000 ext. 5183

 

-----Original Message-----
From: Dima Smolyansky [mailto:dima@tdasystems.com]
Sent: Wednesday, October 25, 2000 9:10 AM
To: si-list@silab.eng.sun.com <mailto:si-list@silab.eng.sun.com>
Subject: Re: [SI-LIST] : connector model for differential signal

John:
 
Even and odd mode TDR analysis will give you a pretty good model for your
connector.
 
You may want to check our application note on SCSI connector and cable
characterization and the more generic appnote on differential line
characterization, which can be found on www.tdasystems.com/support.htm
<http://www.tdasystems.com/support.htm> . The appnotes discuss in detail
characterization of differential lines in SCSI connector/cable assembly.
 
Thanks,
 
-Dima
 

----- Original Message -----
From: John Lin <mailto:John@quantatw.com> (L·)
To: si-list@silab.eng.sun.com <mailto:si-list@silab.eng.sun.com>
Sent: Tuesday, October 24, 2000 12:03 AM
Subject: [SI-LIST] : connector model for differential signal

Dear SI gurus,

How to do connector modeling for differential signals, such as SCSI on a
backplane?

Usually, I am used to use IPA510 modeling software with TDR to build a
connector pin model.
The IPA510 will generate a Z-profile for the connector pin, create a RLC
model, and then run spice simulation to verify model's accuracy. All
mentioned above are for single connector pin. But, how about differential
signals on the connector pins? What does the model look like?

Thanks for your comments in advance.

John Lin
SI Engineer, ARD4
Quanta Computer Inc.,Taiwan, R.O.C.
Email: John@quantatw.com
Tel: 886+3+3979000 ext. 5183

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