[SI-LIST] Re: How accurate is HSPICE's field solver?

Hassan,

We've done some characterization work with it and found the solver to be 
quite sensitive to the mesh size that is used.  This is controlled by 
the Accuracy and Gridfactor statement in the field solver call. (Yes, 
and do not believe the comment in the pdf documentation of the 
Gridfactor, it does in fact work.) The field solver uses a filament 
method, similar to the old Pacific Numerics field solver and equivalent 
to the FastHenry solver for resistance and inductance.  This type of 
solver is sensitive to the number of filaments used in the extraction, 
which can also greatly increase the execution time.  Just like these 
solvers, there is no adaptation of the mesh for the problem that you are 
solving, so you are left to your own devices to determine if the result 
is accurate enough, unlike a finite element field solver that is slow, 
but uses a minimum energy approach to determine the optimal mesh. 
 Unfortunately, the Hspice solver does not give you any visability into 
the way in which filaments are divided, how many their are ... etc, so 
all you can do is to "play" with the settings and compare the results to 
another "golden" field solver.  We've done this and compared it to 
Ansoft Maxwell 2D.

We found that for simple models of single or differential transmission 
lines, the solver gives reasonable accuracy with 2% using the standard 
settings.  (I am talking about versions 2001.4 or later.  Earlier 
versions had some significant flaws.)  But for large coupled models with 
6 or more coupled elements, the accuracy falls off with the standard 
settings.  Increasing the gridfactor will increase the resolution and 
give convergence to Ansoft Maxwell 2D results, but at the cost of 
increased solve time.

Ultimately, for frequency dependence the solver only outputs an 
equivalent Rs or Gd, which is perfectly acceptable for lower frequency 
work (below 1 GHz), but is quite lacking at higher frequencies where 
losses are dominant.  At that point we switch to w-element table models, 
with a table of frequency dependent RLGC tables.  THe Hspice field 
solver will not produce this sort of model.  What we do is to use Ansoft 
Maxwell 2D, create a frequency dependent parameter table to sweep 
against the trace geometry, and then extract the matrices from the 
Ansoft Admittance and Impedance results for each frequency, from 1 Hz to 
100 GHz.  These are then placed into a w-element table model format and 
simulated to a high degree of resolution.


regards,

scott


Hassan O. Ali wrote:

>Hi All,
>
>Has anybody checked the accuracy of differential RLGC parameters computed by 
>HSPICE's 
>internal field solver against those computed by other full-wave field solvers 
>such as 
>Ansoft Maxwell 2D Extractor, HFSS, etc, over a broad frequency band? 
>
>Thanks.
>
>Hassan.
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-- 
Scott McMorrow
Teraspeed Consulting Group LLC
2926 SE Yamhill St.
Portland, OR 97214
(503) 239-5536
http://www.teraspeed.com




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