[SI-LIST] Re: Via modeling & de-embedding
- From: David Siadat <dsiadat@xxxxxxxxx>
- To: gstokes@xxxxxxxxx
- Date: Mon, 19 Jul 2004 08:22:48 -0700
Hi Geoff,
I agree with your comments.
I should also point out that it is quite acceptable to replace multiple
inductance value of a distributed model with a single inductance value.
This does not mean that we are modeling the object with a lumped elements.
3D field solvers use this to calculate characteristic impedance of the object.
Regards,
David
At 09:21 AM 7/19/2004 +0100, Geoff Stokes wrote:
>Hi David
>
>In your point no. 1 you mention higher order modes. This was described very
>well by Hassan O. Ali. Your point no. 2 mentions "characteristics of the
>via". The RF current is confined to the surface because of the skin effect.
>At the via this surface current changes direction abruptly from horizontal
>to vertical. The discussion seems to indicate to me that there is no
>purpose in considering characteristics of the plated via hole alone, because
>in the vicinity of the via there are various wave modes. Therefore
>conventional de-embedding becomes impossible, because it uses TEM wave
>analysis. If the via is sufficiently far from other wave discontinuities in
>the signal path, there will be some value in a TEM model of a via together
>with its connecting traces, and this can be de-embedded to a reasonable
>distance, but not right up to the edge of the anti-pad where the higher
>order modes are quite strong. The varying concentrations of field and
>surface current can be studied in various 3D tools such as Ansoft HFSS, CST
>Microwave Studio or EM3DS. Generally, the TEM mode settles down at a
>horizontal distance many times larger than the dielectric height. In some
>cases it might be best not to de-embed closer than say 3h, and other cases
>could benefit from a yet larger de-embedding distance. The higher order
>modes provide interactions that are not represented on a two-terminal
>schematic - at higher frequencies there is no such thing as a simple
>inductance: that's an approximation suitable only for lower frequencies. In
>the case of a straight wire or plated via, if the impedance is significant,
>it is because the length is a significant part of a wavelength, even if the
>significant part is only a tiny fraction. The lumped inductor model is
>better replaced by a transmission line model, or better still, by a
>multi-element distributed physical analysis as provided in CST Microwave
>Studio or the like. In practice, for good accuracy, such analysis demands
>single-mode stimulus and termination.
>
>So finding the "inductance of a via" or the characteristic of the via alone
>is to chase a rainbow.
>
>Cheers
>Geoff
>
> > -----Original Message-----
> > From: David Siadat [mailto:dsiadat@xxxxxxxxx]
> > Sent: 16 July 2004 18:04
> > 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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