Re: [SI-LIST] : Accuracy of HSPICE W-element transmission lines

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From: Chris.H.Simon@gd-is.com
Date: Mon Nov 27 2000 - 05:06:27 PST


A few years back I made some measurement vs simulation comparisons of the w-element model
to TDT measurements. I used a Tektronix TDR (definitely less than 100 ps risetime) to
drive a ~25 cm line and compared the measurement at the end of the line to what HSPICE
predicted. I was using a 1997 version of HSPICE (I don't recall the exact release). At
that time I had to be careful to set an option called "risetime" to the proper value - if
I didn't then I got a glitch in the simulated rising edge. I have heard that setting this
option is not necessary in newer versions of the simulator.

I made comparisons for FR-4 and Nelco N4000-13 materials, both microstrip and stripline.
Line widths were 3 to 9 mils, depending on PWB layer, with impedances around 50 to 60
Ohms. I only looked at single ended lines. I found that the simulations matched the
measurements amazingly well. Of course this was completely dependent on the care that I
took to properly enter the geometry and material properties into the w-element model.

You state that "The W-element model appears to give lower loss than the other techniques."
Do you know (through measurement) that the other techniques are giving the right answer?
If so then I would take a closer look at the model information that was entered into the
w-element.

<>-<>--<>----<>------<>--------<>--------<>--------<>------<>----<>--<>-<>
  Chris Simon chris.h.simon@gd-is.com
  General Dynamics Information Systems

  To an optimist, it's half full.
  To a pessimist, it's half empty.
  To an engineer, it's twice as large as necessary.
<>-<>--<>----<>------<>--------<>--------<>--------<>------<>----<>--<>-<>

Hello SI-People:

We have now several times encountered differences in results from other modelling packages
and results from HSPICE w-element models for applications involving fairly long striplines
> 30cm on FR4 and GETEK with 2.5Gbps signals with ~100ps edge rates. The W-element model
appears to give lower loss than the other techniques. Typical track widths are 6-8 mils
with 100Ohm loosely coupled differential pairs.

Avant! claims the W-element method is accurate without providing a paper showing how it
was validated.

How does the rest of the SI community feel about the accuracy of the W-element model?

Thanks in advance,

Kai Keskinen

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