On Sun, 17 Oct 2004 13:52:17 +0100, "Airy R. Bean"
wrote:

Question - what is the internal modelling technique used
by these various programs, and can we produce our own package?

Most of them (exceptions being the harmonic balance types for RF) use
the old Berkeley Spice engine developed by the good folks at the
eponymous university. The simulation package authors just adapt the
engine with their own preferences WRT to features, GUI, gimmicks etc.
So yeah, you can certainly come up with your own flavor of Spice just
by adapting the basic Berkeley engine to your tastes. It's highly
unlikely to be worth the effort, though. There's already a spice out
there for everyone - if you can find the right one for you.
--

"What is now proved was once only imagin'd." - William Blake, 1793.

I'm not interested in someone else's engine - that's not the
way of the _REAL_ Radio Ham

It is _ALWAYS_ worth the effort to do things for yourself - that
is the essence of _REAL_ Ham Radio - it is the CBer and the
CBer-Masquerading-As-A-Radio-Ham who buy things
off-the -shelf!

"Paul Burridge" wrote in message
...
On Sun, 17 Oct 2004 13:52:17 +0100, "Airy R. Bean"
wrote:

Question - what is the internal modelling technique used
by these various programs, and can we produce our own package?

Most of them (exceptions being the harmonic balance types for RF) use
the old Berkeley Spice engine developed by the good folks at the
eponymous university. The simulation package authors just adapt the
engine with their own preferences WRT to features, GUI, gimmicks etc.
So yeah, you can certainly come up with your own flavor of Spice just
by adapting the basic Berkeley engine to your tastes. It's highly
unlikely to be worth the effort, though. There's already a spice out
there for everyone - if you can find the right one for you.
--

"What is now proved was once only imagin'd." - William Blake, 1793.

"Airy R. Bean" wrote in message
...
I'm not interested in someone else's engine - that's not the
way of the _REAL_ Radio Ham

It is _ALWAYS_ worth the effort to do things for yourself - that
is the essence of _REAL_ Ham Radio - it is the CBer and the
CBer-Masquerading-As-A-Radio-Ham who buy things
off-the -shelf!

"Paul Burridge" wrote in message
...
On Sun, 17 Oct 2004 13:52:17 +0100, "Airy R. Bean"
wrote:

Question - what is the internal modelling technique used
by these various programs, and can we produce our own package?

Steve (Evans),
Correct me if I am wrong (like I need to say this here, eh?)
I believe the underlying basis is the collection of loop / node equations
used (by Engineers) to model circuits. We know the behavior of resistors,
inductors and capacitors and have mathematical models for them. To this we
add the active devices, etc. and develop an "engine" which does all the
calculations for us. [[we used to do them by hand/slide rule -- yes, I am
included in this we]]. These loop and node equations provide us with a
mathematical model of the behavior of electronic circuits. If done
carefully, this is a general purpose model which applies to all the
situations for which our component models are valid.
Some time later there were bare engines into which we had to type the part
values and node numbers (the sane things you can see in printouts from
Spice). As computers got more powerful, schematic entry was developed. I
believe these programs to be very useful, but as with any model or
simulation, it is best to understand the limitations.
Thre is an alternate method. It is also possible to derive equations for
each type of situation and use these calculations each time you need to
solve that type of problem. I am sure you are familiar with the equations
for things such as parallel capacitors and resonance and so forth. These
are specific solutions of the properties of components in those specific
circuits.
From some postings here I get the idea that Reg is providing various
"calculators" in the form of computer programs for hams to use to
solve/design various circuits. Not one thing wrong with either this or the
general type of software...Except that the limitations argument applies to
all calculations and it is our responsibility to determine whether or not
our situation is adequately covered by a particular math model.
I am also not familiar with the programs mentioned here (except to have
heard the names), except for OrCad's PSpice ver 9, which is relatively easy
to use (for me) and provides results adequate for my purposes--not to
mention the fact that I was given a CD with the student sample version on
it). I was introduced to is by the department chair at the county college
where I was asked to teach some classes and like it. I just draw a circuit
and can then do various forms of analysis. I modeled a recent project and
all worked the first time when I assembled the one and only unit. It was a
simple RS-232 to Kenwood TH-F6A handheld interface.

I agree 100% with Reg in that a circuit simulation program is not intended
to *teach* circuit theory, That needs to come first, then the simulation
tool can help us gain a better understanding by letting us try out the
things we learn and "see" them happen with out having to collect all the
parts and wire it up. I find it much faster to "assemble" a PSpice circuit
and test my design ideas than go into my basement and collecting all the
parts.

BTW it *IS* the cap AND diode which cause the negative voltage in the
coupling circuit described so long ago...

Airy,
While I applaud your desire to understand how these "engines" work and
perhaps build your own, I suggest that it is a most formidable task by any
measure. If you understand the concept of loop and node equations then you
know the math. Now figure out how to write software to handle any circuit
and you have it...then there is the user interface...(what I believe is the
most important [and most difficult to do well] part of any program)

73,
--
Steve N, K,9;d, c. i My email has no u's.

When solving node/loop equations manually, it's generally necessary to
resort to phasor analysis with its underlying assumptions, or Laplace
transforms. The latter does have the capability of producing a time
response. But the solution requires finding the inverse transform, a
process similar to integration in that there's no single direct rule,
and often it's impossible to find a solution except for simple cases.

Computers can be programmed to solve complex problems numerically, using
fundamental time-domain current/voltage relationships (such as the
relationship V(t) = L di/dt for an inductor, or even more generally,
V(t) = L di/dt + I dl/dt for a time-varying inductance). This is
basically what SPICE does, and it's able to easily solve problems which
are simply not possible to do manually, either because of the enormous
time that would be required, or the impossibility of finding a reverse
Laplace transform -- or its equivalent, the solution to a high-order
differential equation if Laplace transforms aren't used. A google search
on 'SPICE "time step" equations' brought a number of hits. I'm sure you
can find an adequate explanation of the inner workings of SPICE among them.

Roy Lewallen, W7EL

Steve Nosko wrote:
. . .
Correct me if I am wrong (like I need to say this here, eh?)
I believe the underlying basis is the collection of loop / node equations
used (by Engineers) to model circuits. We know the behavior of resistors,
inductors and capacitors and have mathematical models for them. To this we
add the active devices, etc. and develop an "engine" which does all the
calculations for us. [[we used to do them by hand/slide rule -- yes, I am
included in this we]]. These loop and node equations provide us with a
mathematical model of the behavior of electronic circuits. If done
carefully, this is a general purpose model which applies to all the
situations for which our component models are valid.
Some time later there were bare engines into which we had to type the part
values and node numbers (the sane things you can see in printouts from
Spice). As computers got more powerful, schematic entry was developed. I
believe these programs to be very useful, but as with any model or
simulation, it is best to understand the limitations.
Thre is an alternate method. It is also possible to derive equations for
each type of situation and use these calculations each time you need to
solve that type of problem. I am sure you are familiar with the equations
for things such as parallel capacitors and resonance and so forth. These
are specific solutions of the properties of components in those specific
circuits.
. . .