Re: gEDA-user: lost newbie

[Dan McMahill <dan@xxxxxxxxxxxx>]

Marc wrote:
> Dan you forgot to mention its cheaper to run a sim before you actually 
> build anything im not currently in college as everyone suggests here but 
> i might go do Physics next year im currently tired of doing college 
> courses i just finished Cisco.
>
> Marc :)

For IC design I agree completely and in that area I simulate things 
almost every day.  For many analog board level designs, I don't agree. 
There are many circuits for which a hand analysis is completely adequate 
and a spice simulation is the wrong tool for the job.  If it's a 
commercial environment, you can quite easily spend more paying for 
someone to try and come up with a simulation which is even close to 
reality than having them just build the thing.

What you very quickly run into unless it is something like a filter or a 
transistor amplifier is that the models either don't exist or aren't 
good enough to be worth anything.  Take a switching power supply.  You 
can simulate the power stage (with some modeling effort), but the 
controller chips are complex and you can spend more time trying to come 
up with a behavioural model which is even 0th order correct than 
actually building something.  A lot of the transistor models which are 
available for discretes aren't very good either.  Sometimes it's due to 
a large process variation, sometimes because of poor extraction of the 
model, sometimes because the model used isn't capable of predicting the 
sort of behaviour of interest.  I came across an example of this 
recently where someone had some discrete FET models which are not 
capable of subthreshold modeling but the designer was designing a 
subthreshold circuit.

I've designed a fairly large number of analog board level circuits and 
essentially none of the problems I've run into when testing them in the 
lab would have been predicted by a simulation.  About the only board 
level simulation I've ever found to be useful is for some filters where 
I wanted to look at some statistical analysis and in cases where I 
wanted to model some finite Q's and board parasitics on some RF filters.
Certainly other cases can be found, but by in large I have not felt that 
board level analog simulators have been important. I'm sure others will 
disagree with me on this.

-Dan