**From:** Daniel, Erik S. (*Daniel.Erik@mayo.edu*)

**Date:** Thu Oct 26 2000 - 05:29:58 PDT

**Next message:**Silvano Bettinzana: "[SI-LIST] : Search for ADSP-21160 IBIS model"**Previous message:**Dima Smolyansky: "Re: [SI-LIST] : connector model for differential signal"**Maybe in reply to:**ªL´Â·×: "[SI-LIST] : connector model for differential signal"**Next in thread:**Daniel, Erik S.: "RE: [SI-LIST] : connector model for differential signal"

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> (ªL´Â·×)

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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**Next message:**Silvano Bettinzana: "[SI-LIST] : Search for ADSP-21160 IBIS model"**Previous message:**Dima Smolyansky: "Re: [SI-LIST] : connector model for differential signal"**Maybe in reply to:**ªL´Â·×: "[SI-LIST] : connector model for differential signal"**Next in thread:**Daniel, Erik S.: "RE: [SI-LIST] : connector model for differential signal"

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