Re: gEDA-user: Spice model coding

[al davis <ad151@xxxxxxxxxxxxxxxx>]

On Thursday 04 September 2008, Gary L. Roach wrote:
> I have just read through Stewart Brorsan's Circuit simulation
> using gEDA and SPICE -- HOWTO. I am really confused about how
> one is supposed to add parameters to components. Strings of
> hieroglyphics after the model parameter doesn't help. Is
> there any concise treatise that explains all of the spice
> parameters and their uses.

I thnk what you are asking about is how to specify model 
parameters and the like.

In my experience, usually I need to manually edit the generated 
netlist.  Also, I never put simulation commands in the 
schematic.  Instead I run the simulator interactively or make a 
separate script.

In the schematic, you need a "value" attribute, which is passed 
to the simulator as is, without checking.

As to figuring out the "strings of hieroglyphics", you need to 
understand the parameters of the individual models you are 
using.  There are hundreds of these models, all with different 
parameters.  These models come from a variety of sources, each 
separately documented.  Some of these documents are hundreds of 
pages long.

By "models" here, I refer to the code describing the equations 
and their parameters, not the ".model" (dot-model) statements 
used by spice and its derivatives.

Mostly, the parameters are supplied by the fab.

If you are doing simple circuits, it is best to use the simple 
models, and supply just a few parameters that you can 
determine, such as beta of transistors.  Then leave the rest to 
their default values.

Unfortunately, teachers often don't teach it this way.  They 
tell you to just use overly complex models, which don't 
necessarily match to anything.  Some commercial demo simulators 
come with parameters for common parts, often bogus or not 
representative of the spread of real devices.  The teachers 
tell you to use them without any understanding of what they are 
about, or what to do when what is there doesn't match what you 
need.  They are doing a lot of damage.  Sometimes I think they 
are as clueless as you are.

I know I didn't answer your question.  There is too much there.  
Hopefully, I gave you some background that may help understand 
why there is a problem.  If you can be more specific, perhaps I 
can really answer it.


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