[SI-LIST] Re: Buried Microstrips

Ty,
You might look at "Networks and Devices Using Planar Transmission Lines",
DiPaolo.  The equations yield excellent insight into non-coupled and coupled
line theory.  You can get a good match for the effective relative
permittivity by adjusting the value of "h", where "h" is the height of the
conductive enclosure boundary above the trace (similar to the solver
approach).  The equation set should yield solutions as good as or better
than the authors you have mentioned for the non-coupled case.  You are
talking exclusively about the non-coupled case, right?  However, be aware
that closed-form solutions break down when you use them for coupled lines.
Error can be as high as 30%.  A simple 2-D solver will yield far more
accurate results when you need to know Zodd, Zeven, and crosstalk.

Ken

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of steve weir
Sent: Thursday, July 16, 2009 8:45 AM
To: Ty Beeman
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Buried Microstrips


3D field solvers like Simberian, Ansoft, CST, when properly set-up yield
very accurate results.  If the object is to develop CF equations that
are more accurate than existing over some data range, I think you need
to do the leg work of obtaining / generating the data set, and then
performing the fit.  To put it another way:  If there was a straight
forward way to yield accurate results from CF equations over a wide data
range, there wouldn't be much demand for 3D solvers in this space.

Ty Beeman wrote:
> Hello,
> As part of a class project, we have been given the assignment of using
> closed loop equations to "accurately" predict the characteristic
impedance,
> etc. of microstrips.
>
> To solve for surface microstrips, we have used the equations from: E.
> Hammerstad and O. Jensen, "Accurate models for microstrip computer-aided
> design." These equations give us results that are very close to field
> solvers.
>
> However we're kind of stuck in getting accurate results for a buried
> microstrip. We are using the equations found in: Brian C. Wadell,
> "Transmission Line Design Handbook." When we use the surface microstrip
> equations above to solve for Z-buried we can get close, end up with some
> errors as much as 4% and off by as much as 3 ohms or more.. If we use the
> Wadell equation for surface microstrip to solve for Z-buried, our error is
> around 5% when comparing to field solvers.
>
> Looking at it a little closer, we find that our calculated effective er is
> different than that given by the field solvers.
>
> All we can seem to find in the reference materials is the same (or
similar)
> calculation that is in Wadell's book or old IPC equations.
>
> So our questions are:
>
> 1. Are there any other equations for buried microstrips that others have
> found to be more accurate, especially for effective er and impedance?
>
> 2. Can anyone recommend any other reference material on buried
microstrips?
> Are there any good IEEE papers on the subject?
>
> We know we may have to settle for what we have now, given the reference
> material, but we're hoping there's something a little more accurate
> available.
>
> Thank you,
> Ty Beeman
> Logan Talada
> Students
> Tompkins-Cortland Community College
>
>
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