[SI-LIST] Re: Via modeling & de-embedding
- From: "Craig Clewell" <cclewell@xxxxxxxxx>
- To: <dsiadat@xxxxxxxxx>, <hassan@xxxxxxxx>
- Date: Fri, 16 Jul 2004 10:32:55 -0800
Here is a good tongue twister.....could you not simulate your line
without the via and then with the via to get the value of the via?
Craig
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of David Siadat
Sent: Friday, July 16, 2004 9:04 AM
To: hassan@xxxxxxxx
Cc: Ivan Ndip; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Via modeling & de-embedding
Hello Hassan,
There are two related issues that should be discussed separately.
1. The required length of the transmission line before the higher order
modes from
the excitation wave port settle down as Giancarlo indicated in his
email.
2. How much of the transmission line can be de-embedded to investigate
the
characteristics
of the via its feeding.
So in a simulation model, you must have long enough transmission line to
satisfy the first issue
and you should de-embedded to the edge of your anti-pad radius (assuming
you have a reference
ground plane below the signal layer).
The impedance of the via consists of the inductance of the transmission
stub hanging off the via and
the via length as well as capacitance of the via which is function of
via
diameter, pad size (and number of them
in the stack up. you could minimize that by removing pads on some
layers),
anti-pad size ( you could choose
different size at different layers to optimize your impedance).
Lets not forget the via stub length (non terminated transmission line)
that
can oscillate at its resonant frequency.
I should also point out the importance of return path for the SINGLE via
model which impacts the inductance value
at lower frequencies.
Regards,
David
At 12:17 PM 7/16/2004 -0400, Hassan O. Ali wrote:
>Ivan,
>
>To know how long is long enough, you need first to understand what a
via
>model should
>accomplish.
>
>Assuming a stripline on either side of the via, a TEM wave gets into
the
>via and comes
>out as a TEM wave. At the via itself, though, the incident signal in
TEM
>mode gets
>converted into several other (higher-order) modes and then back to TEM
>mode as it exists
>the via. The types of higher order modes depend on the environment
around
>the via. Some
>of the modes are evanescent (non-propagating) and some are propagating
>modes. Some of
>the higher-order propagating modes have much higher attenuation
constants
>(i.e. they die
>off short distance away). Ultimately, one cares only about the TEM mode
>that propagates
>far into the striplines on either side of the via.
>
>So, in this case, the via model should essentially capture the mode
>conversion at the
>via. For that reason, you need to place your reference planes at points
>where only the
>TEM mode exists (substantially). Unfortunately, the determination of
such
>points is
>problem-dependent. Certainly you shouldn't just use the attenuation
>constant of the
>first propagating stripline higher order mode to determine the location
of
>such points.
>That's because you also need to be away from the effects of evanescent
>(non-propagating)
>modes and other modes that propagate through the via in a manner not
>supported by
>stripline.
>
>For each via configuration, I would suggest you start with very short
line
>segments
>(e.g. of length equal to the pad radius) and increase the length in
>several steps to
>determine S21 (insertion loss) convergence for the frequency band of
>interest. You
>should have to fix your own convergence criterion. I would be
comfortable
>with 0.5% S21
>covergence.
>
>Regards.
>
>Hassan.
>
>
>On Jul 16, Ivan Ndip <ndip@xxxxxxxxxxxxxxxxx> wrote:
> >
> > Hi Hassan,
> > How "long enough" must the trace segments be? Do you have any
experience
> > in determining the exact length?
> >
> > Best regards,
> > Ivan Ndip
> >
> > Hassan O. Ali wrote:
> >
> > >It seems that "de-embedding" in this thread has been used to
suggest
> that one can
>create
> > >an "isolated" via model that can be used in various other
environments
> such as with
> > >different trace widths and thicknesses; different signal rates,
etc.
> If that is the
> > >case, then I think that suggestion is not accurate enough.
> > >
> > >I tend to believe that via characteristics (hence its model) cannot
be
> isolated from
>the
> > >surrounding environments - especially for higher frequencies. The
> trace/via-pad
> > >transition itself presents a discontinuity that contributes to the
> parasitics of the
> > >via. The pads have their own parasitics. Neighboring ground/power
vias
> also can
> > >influence the signal via performance. All of those along with the
via
> barrel
>parasitics
> > >contribute to the overall via characteristics and have to be
> appropriately considered
>in
> > >model creation.
> > >
> > >As to de-embedding, I think you won't get accurate results if your
> reference (de-
> > >embedding) plane is right at the transition. In other words, an
> accurate via model
> > >should include trace segments long enough to ensure that the field
at
> the segment
>ends
> > >are mainly of the dominant mode of field propagation. In fact most
(if
> not all) de-
> > >embedding techniques make that assumption.
> > >
> > >Regards.
> > >
> > >Hassan.
> > >
> > >
> > >On Jul 15, David Siadat <dsiadat@xxxxxxxxx> wrote:
> > >
> > >
> > >>Hello Ivan,
> > >>
> > >>De-embedding the transmission line feeding the via is limited to
the
> > >>edge of the anti-pad region that the transmission line is
referenced to.
> > >>That could mean an additional 17.5 mil transmission line for 35mil
> diameter
> > >>anti-pad. The inductance value of this transmission line can be
> significant
> > >>compare with the inductance of the via itself.
> > >>
> > >>Regards,
> > >>David
> > >>
>
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