[SI-LIST] Re: HSPICE S-element with a large number of ports

Hi All,

Thanks to all who have provided me with various suggestions on trying to solve 
the 
HSPICE transient convergence problem. The suggestion I received from Jason 
Pritchard 
<pritchard_jason@xxxxxxx> (thanks again Jason) turned out to be the best in my 
case. 

He suggested that I reduce the port numbers of my 36-port S-parameter file to 
remain 
with only the ports I was interested in driving or probing. Please find below 
his 
suggestion (which I think should be captured in the SI-list FAQ archive):

On Mar 30, "pritchard, jason" <pritchard_jason@xxxxxxx> wrote:
> 
> Hi Hassan,
> 
> If you only want to look at a diff pair then convert the 36 port to a 4
> port. Tie off all the other nodes with 50 ohms to ground.
> 
> p1,p2,p3,p4 are the nodes of interest in the s-parameter model you want to
> reduce. I use p1 and p2 as my input nodes and p3,p4 as output. 
> 
> This creates a new 4 port touchstone file......
> 
> .ac LIN 1601 0 10G
> 
> .LIN sparcalc=1 modelname="NEW model name" filename="NEW file name"
> + format=touchstone noisecalc=1 gdcalc=1
> 
> P1 p1 0   dc 0 ac 1 port=1  z0=50 
> P3 p3 0   dc 0 ac 1 port=3  z0=50 
> 
> P2 p2 0   dc 0 ac 1 port=2  z0=50 
> P4 p4 0   dc 0 ac 1 port=4  z0=50 
> 
> Overlay the s-parameters to make sure you set it up correctly and that
> hspice converged on all nodes. This approach almost always works for me. 
> 
> NOTE: When you start cascading s-parameters together you will run into
> more problems. Use this approach to combine s-parameter blocks together which
> will help with convergence in HSPICE too...
>  
> -Jason


The 1601 number appearing above in Jason's message is the number of frequency 
points in 
the new S-parameter file - it can be different or same as in the original 
s-parameter 
file. In the example above the frequency range is DC (0) to 10GHz - it should 
preferably 
be same as in the original S-parameter file.

Another thing to consider while making S-parameter port number reduction: If 
the 
actual loads tied to the ports you wish to eliminate are other than 50-ohm,
and you would like to include the effects of those loads in the new s-parameter
file, then you should not terminate those ports with 50-ohm loads while making 
port 
number reduction. You should instead terminate them with the actual loads. The 
new S-
parameters of the remaining ports will be different from the original ones. 

Hassan.


>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx 
>[mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Hassan O. Ali
>Sent: Tuesday, March 29, 2005 3:05 PM
>To: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] HSPICE S-element with a large number of ports
>
>
>I'm trying to perform a very simple (or so I thought) HSPICE simulation 
>with a 36-port S- element for 18 coupled package lines. Only one port 
>is driven with a pulse, the rest are
>terminated with RC loads. 
>
>The simulation fails with the dreaded "internal time step too small in 
>transient analysis" error message.
>
>I don't use any fancy options in my S-element model line. Just:
>
>.model s-para-line18 s tstonefile='../s-parameter/line18.s36p'
>
>Could any one share tricks for better transient convergence with large 
>S-elements?
>
>Thanks.
>
>Hassan.
>


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