I would do more simulation if there were more and better models for the
chips I want to use.  Also, if the simulator software was easier to use (not
talking about just Protel here).

A lot of the designs I work on are mixed-signal, not strictly analog or
digital.  It's not easy to simulate these types of systems.  Of the analog
components, very few have good models.  For example, I could use a generic
diode statement and tweak the parameters, but I DON'T WANT TO.  I should be
able to get accurate SPICE model tailored for that particular diode from the
manufacturer.  But they don't offer them, because they think it is a
"trivial exercise left to the reader".  Of the digital components, there are
IBIS models for some fancy chips, but not for commodity standard logic.

To gather and generate all the models I need would take a lot of time.  I
can't justify it for the return on time investment.  I use a simple method
of manual simulation:  derive the equations for various parts of the
circuit, and apply worst case values to them.  Find out what is out of
bounds, and tweak the design until every component is happy and all the
specs are met.  All I need for this is the fundamental circuit formulas
(Ohm's law, energy, power, inductance, capacitance, RC, RL, RLC formulas,
etc.) and my trusy HP-28C calculator.  Why, just a couple of months ago I
designed a custom power supply for a customer.  The supply had some truly
bizarre requirements, such as:  constant current into load (voltage varies
over a 2:1 ratio), able to run on half-wave AC, 90%+ power factor, up and
running within 40 ms, etc.  Did I run SPICE simulations - heck, NO!  Guess
what:  it worked on the first try (no smoke)!  Lucky?  I'd like to think
it's because I know what I'm doing.  So, good designers (like me, I hope)
don't necessarily use simulation.  But if we don't, it's not because we
don't believe in the value of it, but because it's too much trouble.

BTW, in engineering school I knew some straight-A students that did nothing
but simulation.  When they actually built the circuits, none of them ever
worked as desired.  It seems that there is more than "book larnin' "
required ;-)  I didn't make A's in all my courses, but all my circuits
worked.  I guess playing around with electronic junk since the age of 8 had
some advantages.  There were 2 other guys that were electronics whizzes.
Some of the A students came to us for help.  One had an interesting design
methodology:  he would plug resistor values into the SPICE circuit and keep
simulating it until it worked.  Then he built it on a solderless breadboard.
Guess what - it didn't work.  At his request, I looked over his circuit.  He
had connected the transistors wrong.  I pointed this out, and his response
was "shouldn't those transistors work any way you connect them?".

Best regards,
Ivan Baggett
Bagotronix Inc.
website:  www.bagotronix.com


----- Original Message -----
From: "Gary Packman" <[EMAIL PROTECTED]>
To: "Protel EDA Forum" <[EMAIL PROTECTED]>
Sent: Tuesday, July 02, 2002 5:54 AM
Subject: Re: [PEDA] SPICE sim question


> Mira,
>
> The few good designers I have know in my 25 years of design *always*
> simulate their designs.  Engineers who cannot design a functioning
> product without a simulator shouldn't be engineers in the first place.
> However, every engineer that takes simulation seriously can *improve*
> any design with simulation. I don't consider simulation a debug tool,
> rather an optimizing tool.   Have you ever purchased a bad piece of
> electronic equipment. If yes, this underscores the importance of
> simulation.  If no, you have had the good fortune of purchasing gear
> that probably received a thorough regimen of simulation before going
> into production.  Do you think that camcorder you take so much for
> granted just fell out of somebody's schematic generating package onto
> the production line?  People who argue that simulation takes too much
> time to be worthwhile are, in my experience, not proficient at
> simulation.  It takes me less time to simulate a circuit than it takes
> to fill out the purchase orders for the parts that go into the
> prototypes.  It does takes time to become proficient and you need tools
> that are easy to use.  Pspice is easy, Protel is not (probably because I
> haven't taken the time to learn properly, I already know PSPICE so why
> bother with Protel). Also, with more experience you learn to recognize
> which portions of a circuit actually merit simulation.  Voltage
> dividers, not necessary. High-gain multi-stage process control
> amplifiers that must operate in a plus or minus 100 degree temperature
> environment, a must.
>
> Gary Packman



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