FreeLists / si-list / [SI-LIST] Re: Understanding S-Parameters

["Eric Bogatin" <eric@xxxxxxxxxxxxxxx>]

Ralph-
 

You may gain a little insight in single ended and differential via design by
looking at the talk I am giving at the Agilent ADS roadshow, on "Designing
transparent vias". I cover both single ended and differential via design.

 

The schedule for the tour is posted at:
http://www.home.agilent.com/agilent/eventDetail.jspx?cc=US
<http://www.home.agilent.com/agilent/eventDetail.jspx?cc=US&lc=eng&ckey=8737
87&nid=-536900532.536905430.08&id=873787>
&lc=eng&ckey=873787&nid=-536900532.536905430.08&id=873787

 

If this link doesn't go through, there is another link posted on my web
site, www.bethesignal.com <http://www.bethesignal.com/> , under the "where's
eric" tab at the top of the splash page.

 

I also posted a copy of the handouts for this talk on my web site under the
features articles tab.

 

The bottom line is that to make a transparent differential via, you want to
make the return loss, SDD11, as large a negative dB value as you can, and
make the insertion loss, SDD21 as close to 0 dB as you can. 

 

As a quick estimate, and to provide guidance on which knobs to tweak, and in
which direction, you can fit the via to an ideal differential pair model,
with a diff stub attached, as needed, to extract the differential impedance
and the degree of coupling.

 

In looking at a number of stack up configurations, I typically find that if
nothing special is done in the design, the coupling, Z21, is small and the
diagonal element, Z11, is very low. To match the via to a 100 ohm diff
impedance usually requires increasing the single ended impedance by such
things as removing the NFPs and increasing the antipads clearance holes. As
I show in my talk, since there still is considerable coupling, you can't
just take two  50 Ohms single ended vias and have them be optimized for 100
Ohms differential vias. You have to include the impact for the coupling.

 

Feel free to drop me a note off line if you have any questions about the
method I used. I hope to have this talk posted as an online lecture in the
next month.

 

--eric

 

 

**************************************

Dr. Eric Bogatin, President

Bogatin Enterprises, LLC

Setting the Standard for Signal Integrity Training

26235 w 110th terr

Olathe, KS 66061

v: 913-393-1305

f: 913-393-0929

c:913-424-4333

e:eric@xxxxxxxxxxxxxxx

www.BeTheSignal.com 

**************************************** 

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Ralph A Wilson III
Sent: Wednesday, May 02, 2007 8:02 AM
To: si-list
Subject: [SI-LIST] Understanding S-Parameters

 

The question was asked, "which of these differential

vias is better?"  Ignoring all the philosophical issues

of the meaning "better", I put together a simple 3D-model

of each of the vias in Microwave Studio and took a

look at S11 and S21 (among other things).  I have

several questions regarding the setup of the simulation

and the interpretation of the results:

 

1) Is a differential via (pair) a two-port or four-port

   model?  The model I put together has two vias, with

   pads/connections (a "P" and an "N" pair) on the top layer

   and on an inner layer.  The model includes a number of

   reference planes tied together with additional "ground

   vias".  In modeling this, do I attach a single port

   between the "P" and "N" leads at the micro-strip, and

   a second port between the "P" and "N" striplines (hence

   a two-port model)?  Or, should I attach a port from

   a reference plane to each "P", and from a reference

   plane to each "N", hence having a four-port model?

   Exciting the two-port model in MWS is straight

   forward - I haven't figured out how to stimulate the

   four port simulation to give me "differential"

   operation - any hints?

 

2) Assuming I get #1 figured out, do I want to normalize

   the resulting S-parameter to 50 Ohms?  If a 4-port model

   is the right one, I'm assuming normalizing to 50 Ohms is

   the "right thing to do".  However, if the answer to #1 is

   a 2-port model, shouldn't the normalization be done to

   100 Ohms?

 

3) Finally, regarding S11 - I'm assuming I can use this to

   determine how "transparent" my vias are - that is, how

   close each via is to matching the characteristic

   impedance of the rest of my transmission line

   (normalized to 50 or 100 Ohms).  True or not true?

   Can I use S11 to calculate the "Z0" of the via?

   I'm intuitively drawn to wanting to equate S11 to the

   reflection coefficient, and therefore use that to calculate

   the Z0 of the via (given that S11 is normalized to 50 or

   100 Ohms).  However, since S11 is always positive,

   this implies I could never have a via whose impedance is

   less than that of the transmission line - and I don't

   believe that.  Or, do I need to look at the phase information

   as well as S11 to determine positive vs. negative reflection?

 

Thanks in advance for the help!

Ralph Wilson

Alcatel-Lucent

 

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