[SI-LIST] Re: How accurate is HSPICE's field solver?
- From: Pat Diao <Pat_Diao@xxxxxxxx>
- To: "'scott@xxxxxxxxxxxxx'" <scott@xxxxxxxxxxxxx>, mherndon@xxxxxxxxx
- Date: Thu, 6 Feb 2003 15:00:19 -0800
Scott,
Did you mean 1 aggressor and 4 victims below? It won't change the coupling
coefficient either way, but physically in most simulators the aggressor is
the one that carries the active voltage.
Just want to clarify...
Pat
Pat Diao
ASAT Inc.
Fremont, CA
phone: (510) 249-1227
-----Original Message-----
From: Scott McMorrow [mailto:scott@xxxxxxxxxxxxx]
Sent: Thursday, February 06, 2003 12:48 PM
To: mherndon@xxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: How accurate is HSPICE's field solver?
Matt,
For single ended simulations I most always use 5 coupled lines, 1 victim
and 4 aggressors. And for differential simulations I use 6 or 10
coupled lines (3 or 5 differential pairs) There are several reasons why
I do this:
1) By extending the simulation trace geometry out to 2 aggressors on
either side, I guarantee that the adjacent aggressors are operating in
their normal electromagnetic field configuration, with their impedance
altered correctly by the adjacent aggressors traces. This reduces some
errors in the overall crosstalk and eye pattern simulations. Although
neighbors that are 2 conductors away from the victim have very little
direct influence upon the victim, they do have an indirect influence due
to their coupling to the nearest crosstalk neighbor, having a tendency
to alter the dynamic impedance of the nearest neighbors, and therefore
the amount of energy available for crosstalk on the victim.
2) Signals always travel through packages and often travel through
connectors. In both cases, coupling is almost always much stronger than
it is on the PCB. (Unless the PCB stackup is poorly designed.) It is
often necessary to simulate many neighbors in packages and connectors.
In order to keep the simulations symmetric and not induce artifacts due
to different driven phases, I find it useful to extract as many
conductors in the PCB as will be simulated through the package and
connectors. For packages that are designed with poor power/ground
structures, I find that there are additional modes of propagation
between the signal conductors and package power conductors that cannot
be accounted for if all of the signal conductors are not driven. This
will sometimes require a large number of conductors to be extracted from
the PCB, in order to include all of the system effects. Oftentimes I
find that for non perfectly terminated busses the crosstalk will
saturate in the package before ever reaching the PCB.
3) At high frequencies fine pitch BGA via breakouts can introduce a
large amount of crosstalk, similar to that caused by connectors. Since
these patterns are square arrays, I will use a 3x3 array of vias,
extract them using a full wave field solver, convert the s-parameters
into a spice circuit with BroadBand Spice, and then assign the signals
and grounds based upon the ball-out pattern of the BGA. This will often
cause the simulation to need quite a few parallel lines to be extracted
from the PCB.
I hope this answers your question, Matt.
Best regards,
scott
--
Scott McMorrow
Teraspeed Consulting Group LLC
2926 SE Yamhill St.
Portland, OR 97214
(503) 239-5536
http://www.teraspeed.com
Matthew Herndon wrote:
>I did some simple comparisions among 2D field solvers as part of a
>paper at DesignCon in January. If anyone wants details, email me
>offline.
>
>Briefly, I looked at 7 field solvers: Ansoft, HSPICE (2001.4), IFS,
>Quad, Greenfield, SigXp, and ApsimRLGC. Presence on the list is not an
>endorsement, nor is absence from the list a criticism; these were
>simply the 7 I had easy access to. I looked at two SIMPLE
>configurations: a 1-conductor microstrip case and a 2-conductor
>stripline case (nothing as complicated as what Scott describes below).
>
>The results were that all the solvers were consistent within 2.5%, and
>most results were within 1%. My conclusion was that accuracy is not a
>concern with these simple structures; rather, when choosing a 2-D field
>solver, one should look at ease of use and fit into existing design
>flows, and price.
>
>One member of the audience at my presentation commented that he saw
>good correlation between 2-D solvers and measurements of boards, but
>that the measured characteristic impedance within a package was often
>off by 15% from the 2-D solver (I forget if he said higher or lower).
>Does anyone have related experience?
>
>One question for you, Scott: I work mostly at the board level, and
>usually don't see much crosstalk except from the traces nearest the
>victim (and so wouldn't need structures with more than 3 coupled
>elements). Do you see significant crosstalk from farther away? Am I
>missing something important?
>
>-matt
>Matt Herndon
>ECAD Group
>Apple Computer
>
>
>On Friday, January 31, 2003, at 10:27 AM, Scott McMorrow wrote:
>
>
>
>>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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