[SI-LIST] Re: Via Impedance!
- From: "Ken Willis" <kwillis@xxxxxxxxxxx>
- To: <gedlund@xxxxxxxxxx>, <srini@xxxxxxxxxxxx>
- Date: Fri, 16 Jul 2004 04:44:42 -0700
Hi Greg,
You are right; SQ has traditionally used closed-form lumped
approximations
to estimate via parasitics, which did a decent job up to 1GHz. You are
also
right in that there is new capability available. With the 15.2 release
we put
out last month, there are new via modeling algorithms (and a new Via
Model
Generator GUI) in there that work well up into the GHz realm. Output
formats
for the new via models include S parameter, wideband circuit, and
narrowband
circuit.
For G. Srinivasan; typical usage we have seen during the Beta cycle
regarding
your question on via impedance is that people plot the S parameters for
the
specific via they are looking at, and study the insertion and return
loss. Then,
they may go in and adjust some of the padstack parameters, re-generate S
params,
and overlay the before-and-after for comparision.
Please contact your local AEs off-line if you have questions or want to
find
out more about this.
Ken Willis
Cadence Design Systems
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Gregory R Edlund
Sent: Thursday, July 15, 2004 4:18 PM
To: srini@xxxxxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Via Impedance!
Dear G. Srinivasan,
The last time I checked, SpecctraQuest was using a closed-form equation
to calculate via capacitance. Perhaps there is a new version available.
Off the top of my head, I would estimate that if you're below 0.062 in.
card thickness and 1 GHz bandwidth, this simple lumped model is
sufficient. My colleagues and I did a paper for DesignCon 2004 about
full-wave electromagnetic modeling of vias at frequencies between 1 and
10 GHz. Let me know if you'd like a copy. Somewhere between the lumped
capacitance model and the full-wave model there is probably a place for
quasi-static modeling, e.g. if you're looking for a more accurate
prediction of capacitance. However, if magnetic effects are significant
I don't think you're going to get the right answer using a quasi-static
solver because the path the current takes is very much a transient
phenomenon.
I read a good paper on via modeling a while back:
"Designing Controlled-Impedance Vias," Thomas Neu, EDN, October 2, 2003
If you're looking for a simple formula to calculate via lumped
capacitance, try this:
"High-Speed Digital Design," Howard Johnson & Martin Graham, page 257
Greg Edlund
Senior Engineer
Signal Integrity
IBM Engineering and Technology Services
3605 Hwy. 52 N, Dept. HDC
Rochester, MN 55901
gedlund@xxxxxxxxxx
Msg: #4 in digest
Date: Wed, 14 Jul 2004 12:17:52 -0000
Subject: [SI-LIST] Via Impedance!
From: "G.Srinivasan" <srini@xxxxxxxxxxxx>
Hi,
I have a querry regarding how to ensure that the impedance of the via
is=20
the same as that of the traces. For example, if I have a trace of 50
ohms running between two layers through a via, how can I calculate its
impedance?=20
In Specctraquest, I saw that there is a field solver that generates a
model for the via based on its geometry and uses it for simulation. Does
it also=20
gives the impedance of the via and provides help to achieve the required
via impedance?=20
Regards,
G.Srinivasan.
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=20
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