[SI-LIST] Re: Quasi Static Assumptions
- From: Geoff Stokes <gstokes@xxxxxxxxx>
- To: "'si-list@xxxxxxxxxxxxx'" <si-list@xxxxxxxxxxxxx>
- Date: Fri, 1 Aug 2003 09:40:59 +0100
Hi
I am grateful for Hassan's description, in particular the comment about a
quasi-static model where the fields are strictly TEM in the y-z plane (no
longitudinal x-component). In microstrip, unfortunately the wave is
quasi-TEM and other modes are generated, so on a PCB this can lead to
problems in sensitive applications - it's better to avoid long tracks, or
sandwich them between power/ground planes. However maybe Hassan is right
that, for the low GHz range (including significant harmonics) in a lot of
cases a quasi-static analysis/simulation is enough.
Hassan is also right about the comment only being a guide. I have found
that sometimes a specific problem may require unusual accuracy for the
frequency range. For example, high isolation may be required. Then the
low-level propagation of higher wave modes requires a better model. That
model is extracted from full-wave analysis or a series of difficult
measurements.
Another reason why the quasi-static approach breaks down is that the
waveguide has discontinuities, such as various transitions - between chip
and PCB, between layers of the PCB, connectors, etc. At these
discontinuities, new wave modes are generated and may cause unexpected
reflections and couplings.
Another point is that because of the presence of several wave modes, the
fullwave analysis may include coupling paths via significantly retarded
(radiated) fields, which need evaluation relative to the required accuracy.
The question of how to deal with the fullwave simulator output remains a
difficulty depending on the application. So far I have had moderate success
with trials involving EM analysis especially CST Microwave Studio.
Geoff
> -----Original Message-----
> From: Hassan O. Ali [mailto:hassan@xxxxxxxx]
> Sent: 31 July 2003 17:53
> To: Si-List (E-mail)
> Subject: [SI-LIST] Re: Quasi Static Assumptions
>
>
> Merrick,
>
> I believe Young gives just a guideline. For you to know
> exactly when a quasi-static tool
> (that assumes negligible longitudinal EM field components) is
> not good enough, you need
> to look at the error in the "tangible" parameters of your interest.
>
> For example, if you use a quasi-static tool to extract a
> model of a realistic
> interconnect (microstrip line, stripline, via, coaxial line,
> etc.), you may wish to look
> at how the characteristic impedance and attenuation provided
> by the quasi-static model
> compare with those parameters provided by a fullwave model,
> for the frequencies and
> interconnect lengths of your interest. You may be surprised
> to see how the two models
> compare.
>
> From my experience, I can tell you (also in a general sense)
> fullwave interconnect
> models are indispensable for frequencies above 5GHz,
> especially on lossy substrates, and
> long transmission lines. Below 5GHz, the error is within the
> general error level of
> everything else in typical printed circuit board systems.
>
> Regards.
>
> Hassan.
>
> On Jul 31, "Moeller, Merrick" <mmoeller@xxxxxxxxxxxxxxx> wrote:
> >
> > When does a Quasi Static 3D simulation tool become
> obsolete? B. Young's
> > book describle the limitation by maximum circuit dimension
> d < wavelength/10 or d <
> wavelength/30.
> > With wavelength of interest being calulated from the
> standard digital frequency of
> > interst equation f = .35/Tr, and the maximum circuit
> dimension or effective electrical
> length
> > to be, SQRT(Er)*Mechanical Length, Is this a correct
> approximation? Why the large
> range of wavelength ratios?
> > Are there tell tale signs that the Quasi Static assumption
> has broken down?
> > Thanks,
> > Merrick
> >
>
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