[SI-LIST] Re: spice math question
- From: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
- To: <jellis@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Wed, 30 Oct 2002 09:16:40 -0800
My experience with issues like this have almost always resulted in my =
finding out about some wierd option setting, rather than exotic =
physics/mathematics. A while ago, someone found ideal transmission =
lines affecting rise-times, which they shouldn't do. The problem was in =
Hspice and was solved by setting the Transient sweep spec. properly for =
the T-lines of interest:
.TRAN 50p 4n $ doesn't work - gives rise-time degradation
.TRAN .5p 4n $ works correctly
Perhaps you have a similar problem?
Jeff Loyer
-----Original Message-----
From: John Ellis [mailto:jellis@xxxxxxxxx]
Sent: Wednesday, October 30, 2002 9:08 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: spice math question
Thanks Fred,
At least with the modal and tline models, it seems to be time step =
related.
I'm going to have to look closer at the ladder models. When I ran that =
I
got the same results for a single 40 inch model and 10 4 inch models,
however it was a shorter delay than predicted.
John
-----Original Message-----
From: fred@xxxxxxxxxxxxxxxxxxxxxx [mailto:fred@xxxxxxxxxxxxxxxxxxxxxx]On
Behalf Of Fred Balistreri
Sent: Wednesday, October 30, 2002 11:43 AM
To: jellis@xxxxxxxxx
Subject: Re: [SI-LIST] spice math question
John, there are a variety of SPICE engines. They do not all treat
transmission
lines with the same algorithms. I am surprised that the ladder network
also
exhibited the same problem. By ladder network I presume that you had 750
L, C and perhaps R's cascaded to approximate the transmission line. In
lieu
that you broke up the transmission lines into 4 inch pieces and you got
the
correct response I would have said: Force a smaller time step on the one
40 inch line simulation. The problem could be due to the time stepping
routine.
If the algorithm involves FFT and convolution then one has to look into
the
sampling point routine. My SPICE does not suffer from this problem. So I
do
not think it is generic of SPICE.
Best Regards,
John Ellis wrote:
>
> I've got a question about how spice mathematically handles long
transmission
> lines. I was recently comparing the output of different spice =
structures
> for a 40" long transmission.
>
> For modal structures, I found that a single modal element produced a =
delay
> that was a little fast for the structure. The signal arrived about 300 =
ps.
> before it should have. At 158 ps/inch I was expecting a delay of 6.32 =
ns.
> The result was 6.02ns. When I cascaded 10 4" lengths the delay was =
more
> accurate, giving me a believable 6.38ns.
>
> I saw this same effect when I modeled with W-elements as well. I =
vaguely
> remember an app. note on W-elements describing something akin to this.
>
> I then looked at ladder models for the same structure and found the =
same
> slightly short delay. For this structure I cascaded >750 ladder =
elements
to
> build the model.
>
> There seems to be some numerical inaccuracy native to spice. These
results
> were seen in Hspice and Berkeley spice as well.
>
> Is it rounding error or something else?
>
> Thanks,
>
> John Ellis
>
> Vice President
> Interconnect Technology
>
> TriCN, Inc.
>
> 3605 Vartan Way
> Suite 301
> Harrisburg, PA 17110
>
> Phone:717-657-1002
> Fax: 717-221-1185
> email:jellis@xxxxxxxxx
>
> www.tricn.com
>
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--
Fred Balistreri
fred@xxxxxxxxxxxxx
http://www.apsimtech.com
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