[SI-LIST] Re: Via stub math help needed....
- From: steve weir <weirsi@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Sun, 15 Jan 2012 11:31:07 -0800
Bert, your paper does a nice job of reinforcing the point of using
models appropriate to the situation. At sufficiently high rates we
would need to break the Tx line down further into segments that
represent each significant change in Tx line parameters that extend for
a substantial fraction of incident Tr/Tf. I believe we are going to see
some of this in the Oracle paper on vias in two weeks.
Best Regards,
Steve.
On 1/15/2012 11:10 AM, Lambert Simonovich wrote:
> Lee,
>
> It depends. You probably will get the same effect up to a certain risetime
> and bandwidth. If you are concerned about stubs and the resulting resonant
> frequency, then using a capacitor will not give you the right information.
>
> Here is an analysis I did on my blog on the perils of lumped element via
> modeling.
>
> http://bit.ly/jqnEGm
>
> -Bert
>
> =================================
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> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
> Behalf Of Lee Ritchey
> Sent: January-14-12 7:40 PM
> To: Todd Westerhoff; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Via stub math help needed....
>
> 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
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>> Olathe, KS 66061
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>>
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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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