[SI-LIST] Re: Via stub math help needed....
- From: Lee Ritchey <leeritchey@xxxxxxxxxxxxx>
- To: Todd Westerhoff <twesterh@xxxxxxxxxx>, si-list@xxxxxxxxxxxxx
- Date: Sat, 14 Jan 2012 16:40:06 -0800 (GMT-08:00)
Try placing a 0.3 pF capacitor in your model where the via would be. Bet you
get the same effect.
-----Original Message-----
>From: Todd Westerhoff <twesterh@xxxxxxxxxx>
>Sent: Jan 13, 2012 9:00 PM
>To: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: Via stub math help needed....
>
>Interesting thread. I'd like to offer this up for your consideration:
>
>http://www.eeweb.com/blog/michael_steinberger/the-long-and-the-short-of-vi
>as
>
>Todd.
>
>--
>
>Todd Westerhoff
>VP, Software Products
>SiSoft
>6 Clock Tower Place, Suite 250
>Maynard, MA 01754
>(978) 461-0449 x24
>twesterh@xxxxxxxxxx
>www.sisoft.com
>
>
>"It doesn't matter what you've heard
> Impossible is not a word
> It's just a reason
> For someone not to try"
> -Kutless
>
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
>On Behalf Of Eric Bogatin
>Sent: Friday, January 13, 2012 7:14 AM
>To: si-list@xxxxxxxxxxxxx
>Cc: eric@xxxxxxxxxxxxxxx
>Subject: [SI-LIST] Via stub math help needed....
>
>Scott-
>
>
>I thought I would chime in on the question of the higher Dk for vias. I
>learned a lot about vias working on a project with Bert a few years back
>and continue to learn more every time I look at them, so I always
>appreciate your comments and others on this topic.
>
>
>
>I think the first question is, is there a higher propagation delay through
>a via, if you take as its start and finish the top and bottom pad region
>where the transmission line feed hits the annulus clearance hole?
>
>
>
>It's really hard to measure directly, but easy to simulate in many 3D
>tools. I find that when you keep the stack height fixed, by just changing
>the clearance holes, NFPs and other physical features inside the via pad
>stack, you can get a variety of propagation delays, as extracted from the
>phase of S21.
>
>
>
>I have to say that I did not use generalized modal S-parameters, however,
>this higher delay is seen even when the return loss is less than -25 dB,
>where you would not expect a phase distortion from reflections.
>
>
>
>I interpret this higher delay as a higher "effective Dk". I don't think
>the intrinsic material properties have changed, nor is there an issue of
>the anisotropic effects of the Dk from glass weave. I think this has been
>shown to be only on the order of ~ 20% at most.
>
>
>
>As an aid in describing the via propagation delay, I interpret the extra
>delay as a higher Dk. In some multi layer via structures, I extract a
>Dk_eff as high as 16, if there are NFPs.
>You see this clearly in the lower stub resonance frequencies.
>
>
>
>Why does the prop delay increase thru a via? I think it is due to the non
>TEM fields. In a TEM propagation, I think we all agree that the delay is
>due to the physical length and the bulk Dk.
>
>
>
>In a non TEM propagation, I think all bets are off. The speed of the
>signal will depend a lot on the shape of the fringe fields.
>The more non-TEM, as you get with all those fringe fields from the barrel
>and pads to the planes, the more different the speed is from the bulk Dk.
>
>
>
>I think the confusion is over the use of the term Dk. It may be less
>confusing thinking of this as an effective Dk, useful to predict the prop
>delay based on the physical length.
>
>
>
>I can model many complex via structures as uniform transmission lines up
>to the 10 GHz range, using a Z0, the physical length and the Dk_eff. This
>helps in estimating the stub resonances.
>
>
>
>What do you all think?
>
>
>
>--eric
>
>
>
>
>
>
>
>*******************************************************
>Dr. Eric Bogatin, Signal Integrity Evangelist Bogatin Enterprises Setting
>the Standard for Signal Integrity Training web site: www.beTheSignal.com
><http://www.bethesignal.com/>
>
>beTheSignal Blog: www.beTheSignal.com/blog
>26235 W 110th Terr
>Olathe, KS 66061
>e: eric@xxxxxxxxxxxxxxx
>
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>***********************************************
>
>
>
>Msg: #1 in digest
>
>Date: Thu, 12 Jan 2012 16:41:44 -0500
>
>Subject: [SI-LIST] Re: Via stub math help needed....
>
>From: Scott McMorrow <scott@xxxxxxxxxxxxx>
>
>
>
>Bert and Ralph
>
>There are layered-anisotropic variations in Er for many materials,
>
>especially those that include fiberglass weave. However, Er does not
>
>change with non-TEM modes or different TEM modes (stripline, via-coaxial,
>
>circular cavity ... etc). Different propagation modes merely concentrate
>
>the field in different directions and select a different set of localized
>
>material characteristics.
>
>
>
>I've read the papers and seen the claim that the dielectric constant of
>
>layered fiberglass material is higher for propagation through a via, due
>to
>
>the direction of the field, however, I've not seen a systematic study of
>
>this. (Adjustment of material Er(effective) to obtain a match to modeling
>
>does not constitute proof.) My experience for launch vias with coaxial
>
>ground rings has been that the resonance computed by full wave solvers
>
>matches measurements quite well in a multitude of materials, if the
>
>dielectric has been characterized correctly. I find that most of the
>
>mis-correlations that I've seen are due to improper material
>
>characterization. I do not discount the possibility of a higher localized
>
>Er region around a via in some measurements, its just that I find little
>
>evidence for fiberglass being the sole culprit. In many cases I've found
>
>that mismatch in stub resonance could be easily accounted for by adding
>the
>
>correct amount of soldermask to the bottom pad in modeling. In other
>cases
>
>I've found that material variations between layers were not correctly
>
>modeled.
>
>
>
>As a thought experiment, take a section of a PCB with a via along the
>
>z-axis that is fully surrounded by a coaxial metal wall.
>Calculate the
>
>average Er from top to bottom, and then calculate it radially out. The
>
>volume of material is the same. The composition of the material is the
>
>same. Thus the average Er is the same. The only way to come up with a
>
>higher Er for the radial direction is to conclude that somehow the
>
>drilling process selectively removes more epoxy than fiberglass from the
>
>mixture. There are layered variations as we travel down the via passing
>
>through fiberglass rich, and then epoxy rich layers. But I see no reason
>
>why they would not average out. I can make a case that individual pairs
>of
>
>signal and ground vias can have Er variation, just as I can for traces,
>but
>
>I cannot come up with any reason why the Er would not average out in the
>
>limit.
>
>
>
>There is one other potential reason why a via could have a higher
>localized
>
>average Er. But it has nothing to do with the fiberglass itself.
>I will
>
>probably use it as a topic for next year's DesignCon paper, as a follow up
>
>to the paper I'm involved with this year.
>
>
>
>
>
>regards,
>
>
>
>Scott
>
>
>
>
>
>
>
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