[sdiy] Spice....

[Magnus Danielson]

From: "Paul Perry" <pfperry at melbpc.org.au>
Subject: Re: [sdiy] Spice....
Date: Sun, 9 Jan 2005 16:44:05 +1100
Message-ID: <001801c4f60e$3c5699e0$0a01a8c0 at frostwave>

> 
> The UK mag "Electronics World" recently did a survey of 
> various Spice versions & the Linear Technologies one came top..
> although lacking a lot of the things people on this list would like.
> 
> For me, Spice (in all the forms I've seen) is still a tool
> that looks too difficult for the casual user. At least this one..

The original Berkley Spice is certainly not an easy tool to master just given
it out of the box (which is source BTW) and with no or low experience. Given
some shorter introduction you can however learn how to do things with it.

For ease of use you need a schematics side and a waveform-viewer/postprocessing
tool. The Berkley Spice only has a waveform-viewer/postprocessing tool for X,
which most Windows and Mac uses classically have not been able to handle very
well.

But then, there are many hidden trappdoors in Spice for the unexperienced,
which neither of the graphical frontends does nothing to solve. Some of them is
related to issues which far overshoots the knowledge of the beginner, even if a
rought feeling of what needs to be done can be learned by the trial-and-horror
method.

Lately there have been a couple of interesting developments in the field,
besides a number of specific Spice extentions used to aid certain development
fields. NGspice is one of these. It is attempting a re-write of the original
Berkley Spice 3f5 (last official version as I recall it) but under GPL instead
of the original BSD licence (which has a specific clause which was later
removed but Spice and other codes have not been released with the new version
so it can't be used freely according to the current free/open software
interpretation).

Another interesting project which is more forward going is the GNUCAP simulator
(previously known as ACS - Al's Circuit Simulator). GNUCAP builds on a fresh
ground in which the internal methods of Spice have been changed. It is however
fairly compatible with Spice, but it lacks some of the Spice features but have
other features which is really nice. When talking to Al, you realize that he
has set up the vision beyond being "just another spice" but rather a complete
analog/digital simulation tool. I think GNUCAP could be a very valuable tool.
For those into chip/RF-simulation would start ask questions about having the
additional MOSFET models (which was additional packages to the original spice
but not released with it even if many commercial Spice included them), and to
the best of my knowledge GNUCAP has those models added.

The main problem for non-commercial Spice tools is the lack of Spice models of
commercial components. You can usually download them for free from manufacters
(like Philips, Texas Instruments, Motorola) but their licenses are usually such
that the free software community can't redistribute them.

In my mind, Spice is a very valuable tool in the hands of those who know how to
use it, just like any other simulation tool. You can model things and try it
out. Some stuff you may not be able to measure because it is just too hard or
you don't have or can't afford the necessary tools. However, the real danger of
any simulation tool is that you beleive that this is how the real world circuit
is behaving. This is a danger since both models and simulators are limited
models of the real world. Also, another danger is that even if the simulation
is correct, you don't actually test the circuit with suitable signals
representative to the real world. Thus, it is another swamp form of swamp, but
it may be better than the real world lab swamp for certain things. I think this
is the point that Bob Pease and other known Spice-haters maybe not fully
articulate, but their point is that you can't trust simulations and that is
true, all simulations need to be evaluated for their accuracy to the real world
and the real world situation. I see their disgust for Spice being more related
to the fact that many engineers incorrectly uses Spice in total blindness and
assumes that this is how things work, together with having fallen into a bunch
of trapdoors when using spice themselfs.

As for myself, I haven't used Spice in ages and actually think I should be
using it more.

Cheers,
Magnus