On Fri, 15 Oct 2004 23:06:38 +0000 (UTC), "Reg Edwards"
wrote:

Sure did, Steve! I ran it through a spice program and you're right in
every detail.
=======================

What makes you think Spice is correct? Its only a buggy computer program.
Rubbish in - rubbish out!

Buggy?? I'm using Multisim (formerly EWB) which has been around longer
than most spices and therefore has probably less bugs than the rest
put together!

--

Fat, sugar, salt, beer: the four essentials for a healthy diet.

On Fri, 15 Oct 2004 14:23:24 -0500, "Steve Nosko"
wrote:

While not necessary for understanding this circuit, I'll fill-in this bit
here. A "clamper" is a diode and cap circuit which will clamp a particular
point on a waveform to a specific voltage (kinda like being clamped in a
vice on the bench), but NOT change the wave's wiggling *shape*. Sometimes
this is needed. In the coupling circuit in question, it is the POSITIVE
PEAK of the signal that gets clamped to +0.7 volts. The wave's SHAPE is
un-changed, but the whole thing is shifted in its DC component.

What you are thinking of is called a "CLIPPER" because it CLIPS *off*
part of the waveform like barber's scissors.

Gotcha, Steve. I won't forget that distinction in a hurrq! It\s
amazing after nearly 2 years of studying this subject that I didn't
tumble the true meahing of clamping. Duh!

you state (and the spice progs agree) that what *actually* happens in
this case is that the whole AC waveform gets shifted south into
negative territory. It's still a full wave, but it's way down into
the negative and only the highest peaks just creep above zero volts.
Is this effect *solely* attributable to the steady build-up of
negative charge on the cap's RHS?



YUP !




I think what's really freaking me
out here is the fact that the signal source is grounded on the neg.
side and yet we have that same signal that after going through a cap
can end up going fully negative *below* ground. It just seems like any
such voltage beneath zero/ground potential is breaking the laws of
physics. Ground should be the 'absolute zero' of the potentials in any
circuit and here it is being violated. I need some help to get my
thick head around the concept! :-(



Oooooo. BEWARE! GROUND IS NOT ABSOLUTE ! ! ! Nope, nope, nope. This is
going to take some time to explain and more experience/study will be needed
for you to really _get it_.

The bad news for you may be that ground, I must sadly inform you, is
relative. There is no one, solid, never varying, absolute thing which is
ground, except in our imaginations. Many hams believe there is, but there
isn't.



That being said, let's start out simply and build.



Here is a very applicable analogy:

Voltage, also called "potential difference", is a lot like altitude --
height. We can talk about the height above the street level. We might
consider the street level to be "ground". In Physics, moving some object to
a higher level gives it the "Potential" to do damage if it falls on your
head, so the "potential energy" of it is greater. Holding it three feet
above your head gives it a certain potential, right? Voltage is just like
this.

HOWEVER, what about standing on the roof of a building THEN moving the same
object three feet above your head. You must agree that it has the SAME
potential to do damage TO YOUR HEAD that it did in the first example, right?
In this case, the roof of the building is our ground. So ground is relative
and *we* get to pick it. It is usually a known point in our circuit and we
use special symbols to show it. Note that this is why voltage is also is
called potential *DIFFERENCE*.

Unfortunately, this analogy will fall apart when trying to use it for
negative voltage, if we put this negative voltage "Below" our ground, but
for the "relative" concept, I hope it worked.



Now I'll try *negative*.


Okay, Steve, I've snipped your explanation about negative voltages as
that wasn't quite what I was getting at. I'm familiar with the
analogos you used (which I'm sure will help ohters in future via
Google). I've done quite a lot of experimentng with opamps using split
supplies, hence I don't have a problem envisaging below ground
voltages where the supply is say +15V - 0 -15V. The problem I was
having was with below ground voltages in a circuit with only one
ground and one V+ supply! I'm gojng to have a good think about this
before posting back for further clarification. I'm not completly sure
that its all down to the cap alone. Mebe the diode has an effect on t
producing his sub-ground signal as well?

Steve


--

Fat, sugar, salt, beer: the four essentials for a healthy diet.

Sure did, Steve! I ran it through a spice program and you're right in
every detail.
=======================

What makes you think Spice is correct? Its only a buggy computer
program.
Rubbish in - rubbish out!

Buggy?? I'm using Multisim (formerly EWB) which has been around longer
than most spices and therefore has probably less bugs than the rest
put together!


=========================

There's far too much blind faith placed in computer programs.

Programs can be no better than their authors who are only fallible human
beings.

Good programmers may be able to write practically bug-free programs. But
their technical knowledge of the subject matter might be no better than the
old-wives who write magazine articles and often contribute to these threads.

Without any intended offence, your Multisim program is worthless to me for
the purpose of checking or confirming anything. Why? Because I've never
even heard of it! Therefore it carries no weight in attempting to convince
me of anything.

Your use of the word "probably" is significant. In the absence of knowledge
of the probabilities involved I think it inadvertently displays a measure of
lack of confidence in the program.

The only way of accumulating confidence in a computer program is to use it
and compare results with what you are already aware of as being true. But if
what you are already aware of is untrue and so also is the computer (because
you both make the same easily-made mistakes) then your own confidence will
be improved but the confidence of others (who may think they know better) in
what you say will be undermined.

So, on balance, quoting (or misquoting) computers, measuring instruments,
magazine and other articles and contributors to this newsgroup is just a lot
of hot air and nobody gets anywhere. Reliabilty depends solely on the
confidence which can be placed in the writer.

In extreme cases some authors are worshipped as being infallible such as in
ARRL and RSGB handbooks, Terman and Kraus (who I have heard of). Name
dropping is better not practiced by name-droppers as a means of supporting
and reinforcing their technical arguments.

In the end, statements made by newsgroup contributors are made on their own
responsibility without the assistance of free adverts of type numbers of
particular measuring instruments, names of computer programs which the
great majority of readers have never heard of, the 3 gentlemen who
pronounced that 120 radials was a magic number but who forgot to measure
ground conductivity, and various worshipped authors whose printing errors
and misquoted sermons occasionally disagree with each other, etc.

I'll allow mention of Clerk Maxwell but only by people who have read and
understood him. And there's very few of them around. ;o)

Well, I've wandered around and probably said too much. I'm unable to swig
wine of any sort tonight because I'm on a 7-day course of anti-biotics and
it says on the associated leaflet, in capital letters, alcohol is barred.
----
Reg.

Reg Edwards wrote:
. . .
The only way of accumulating confidence in a computer program is to use it
and compare results with what you are already aware of as being true. . .
. . .

SPICE has been used for decades in the design of countless products that
you undoubtedly use daily. It's an extremely useful and valuable tool,
without which many modern designs simply wouldn't be possible.

Roy Lewallen, W7EL

On Sat, 16 Oct 2004 16:35:00 -0700, Roy Lewallen
wrote:

Reg Edwards wrote:
. . .
The only way of accumulating confidence in a computer program is to use it
and compare results with what you are already aware of as being true. . .
. . .

SPICE has been used for decades in the design of countless products that
you undoubtedly use daily. It's an extremely useful and valuable tool,
without which many modern designs simply wouldn't be possible.

Here again, Roy I"m confused. You say its indespensible; Reg Edwards
says its' unreliable. Who am I to beleive? When the experts disagree,
its imposible to form a reliable conculsion.
--

Fat, sugar, salt, beer: the four essentials for a healthy diet.

On Sat, 16 Oct 2004 22:43:50 +0000 (UTC), "Reg Edwards"
wrote:

Without any intended offence, your Multisim program is worthless to me for
the purpose of checking or confirming anything. Why? Because I've never
even heard of it! Therefore it carries no weight in attempting to convince
me of anything.

I can't believe you haven't heard of Multisim, Reg! It used to be
Electronics Work Bench and is with little doubt the worst heap of sh*t
simulator on the market. One can but hope the OP's got the demo
version for nowt or he's spunked a load of cash on SFA. They wanted 4
grand for this hopless crock of sh*t last time I looked!! :-D
--

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

It's only impossible to form a reliable conclusion when "experts"
disagree if your sole source of knowledge and information is from those
"experts". That's a bad spot to be in. There are many sources of
information available to help you learn about the topic and come to a
more informed opinion. That's the solution to your dilemma -- develop a
wider range of sources of information and decrease your dependence on
the "experts".

Originally developed at Berkeley, SPICE has been commercialized by a
number of companies, one of the most popular being PSpice by MicroSim.
It's in very wide use, and has been for decades. Do you think that a
couple of generations of engineers would have paid several thousand
dollars each for software with a reputation of unreliability?

My own experience includes about 30 years designing a variety of test
and measurement equipment for several companies, followed by several
years doing electronics design as a consultant. In that time, I and my
colleages at Tektronix and other companies used SPICE very often. (In
fact, recognizing it value, Tek spent a large amount of money and
devoted resources to development of its own internal version of SPICE,
which included schematic entry and other features before they were
available in outside commercial versions.) It's virtually impossible to
design an analog or mixed analog-digital integrated circuit without it,
and I and my colleagues found it indispensible for many other projects.

You might compare this with Reg's experience with SPICE, if he'll tell
you what it is, and see what brought him to make the unequivocal
statement about it which he did. I don't personally think he really
believes that it's unreliable, though, but was just making one of his
characteristic trolls in order to relieve boredom.

Roy Lewallen, W7EL

Steve Evans wrote:

On Sat, 16 Oct 2004 16:35:00 -0700, Roy Lewallen
wrote:


Reg Edwards wrote:

. . .
The only way of accumulating confidence in a computer program is to use it
and compare results with what you are already aware of as being true. . .
. . .

SPICE has been used for decades in the design of countless products that
you undoubtedly use daily. It's an extremely useful and valuable tool,
without which many modern designs simply wouldn't be possible.


Here again, Roy I"m confused. You say its indespensible; Reg Edwards
says its' unreliable. Who am I to beleive? When the experts disagree,
its imposible to form a reliable conculsion.

Some years back, after retirement, I bought out of curiosity a copy of
Electronics Work Bench. It was and still is the only such program I have
ever had my hands on. I think it arrived on a collection of floppies.

After a few days curiosity was satisfied. Then I junked it.
---
Reg

All programs have bugs. I only asked why people think it IS reliable.

I am aware of its existence only from the frequent mentions made on
newsgroups.

Why do the arguments continue after Spice has arbitrated?

I have no reason to think Roy's opinion of Spice is anything other than
true.

But whatever it is, it is not a device intended to be used as a means of
instructing learners on the theory of electrical circuits. All programs
have many limitations which eventually always become serious and which are
UNKNOWN to the user. Very often they are unknown even to the programmer.
Limitations should not be allowed to cross over the borders of knowledge.
Programs should not be worshipped for always telling the gospel truth. They
don't.

For example, a sensible circuit designer invariably checks the output of a
program by making a hardware prototype - or several. Why? Because he
trusts neither himself nor the program!
----
Reg

"Steve Evans" wrote in message
...
On Sat, 16 Oct 2004 16:35:00 -0700, Roy Lewallen
wrote:

Reg Edwards wrote:
. . .
The only way of accumulating confidence in a computer program is to use
it
and compare results with what you are already aware of as being true. .
..
. . .

SPICE has been used for decades in the design of countless products that
you undoubtedly use daily. It's an extremely useful and valuable tool,
without which many modern designs simply wouldn't be possible.

Here again, Roy I"m confused. You say its indespensible; Reg Edwards
says its' unreliable. Who am I to beleive? When the experts disagree,
its imposible to form a reliable conculsion.
--

Fat, sugar, salt, beer: the four essentials for a healthy diet.

"Reg Edwards" wrote:

Some years back, after retirement, I bought out of curiosity a copy of
Electronics Work Bench. It was and still is the only such program I
have ever had my hands on. I think it arrived on a collection of
floppies.

After a few days curiosity was satisfied. Then I junked it.
---
Reg

Just an explanation to the person who asked:

Reg is one of the few people in the world who have no use for a spice
simulator. He writes lots of very good calculation programs and knows
electronics like he had a built-in electronics simulator in his brain. He
has studied electronics during a long life and knows what he talks about.

For other people though, a spice simulator can be very useful.

Multisim has a very bad reputation because is has a lot of bugs.
The best version of EWB is 5.c
After that version multisim replaced the central parts of the program and
the new program had a lot of problems.

Nobody uses later versions of multisim, so we do not know if they have
solved the bugs yet.

EWB 5 is used by beginners, because it has a very good user interface.
But even EWB 5 is not regarded as a good spice simulator.

Professionals often like the freeware spice simulator Switchercad3 from
Linear Technology. http://www.linear.com/company/software.jsp

I use EWB 5.c myself, because it is good enough for my purposes, and it
is a lot easier to work with than Switchercad3. But I use Switchercad3
sometimes too, because people in newsgroups often give a circuit in
switchercad's text file format.

Another advantage for EWB is that it has been around a while, so there
are many add-ons to the program, like a converter from EWB circuit to an
Eagle layout (Eagle is a freeware layout program for circuit boards).
Translation programs from ewb to standard netlist is also available.

After multisim changed the program there was a big wave of protests among
the EWB users and buyers. New buyers demanded to get a copy of the old
working program when they bought the new version, and Multisim accepted
that and delivered a EWB version 5c for free, on demand, to all buyers of
the new program.

I don't know if they still do.


--
Roger J.

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

Is it based upon successive delta-time increments, and if so, what
is the increment? What prompted the last question is an attempt
I made to create a sine-wave generator using the identity that
sin dTheta = dTheta, but I had to go for an _extremely_ small
value of dTheta (ISTR 10^ -18) before getting anything like a
decent sine wave, and even that degenerated after a few cycles.

So, these circuit simulators - what is their underlying technique
for circuit simulation?

"Roger Johansson" wrote in message
...
"Reg Edwards" wrote:
Some years back, after retirement, I bought out of curiosity a copy of
Electronics Work Bench. It was and still is the only such program I
have ever had my hands on. I think it arrived on a collection of
floppies.
Multisim has a very bad reputation because is has a lot of bugs.
The best version of EWB is 5.c

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.

On Sun, 17 Oct 2004 05:59:02 +0000 (UTC), "Reg Edwards"
wrote:

Some years back, after retirement, I bought out of curiosity a copy of
Electronics Work Bench. It was and still is the only such program I have
ever had my hands on. I think it arrived on a collection of floppies.

After a few days curiosity was satisfied. Then I junked it.
---
Reg

In 1970 I bought the latest issue RSGB Handbook, obviously a decade
before the transistors were being discovered in England, and several
decades before the spectrum analyser were applied over there. So after
a week I managed to find another person to keep the book, not sure if
he paid for the rubbish

73, Jan-Martin
---
J. M. Noeding, LA8AK, N-4623 Kristiansand
http://home.online.no/~la8ak/c.htm

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.

It's been the case in Britland for many years now, that if you
want info to build modern gear, then buy the ARRL handbooks.
OTOH, if you want a mediocre book that is many
years out of date and seems to owe more to self-congratulation than
it does to technical excellence, then go for the RSCB offering. Odd, really,
when you consider that the RSCB is a publishing corporation.

"J M Noeding" wrote in message
...
In 1970 I bought the latest issue RSGB Handbook, obviously a decade
before the transistors were being discovered in England, and several
decades before the spectrum analyser were applied over there. So after
a week I managed to find another person to keep the book, not sure if
he paid for the rubbish

On Sun, 17 Oct 2004 15:00:49 +0100, "Airy R. Bean"
wrote:

It's been the case in Britland for many years now, that if you
want info to build modern gear, then buy the ARRL handbooks.
OTOH, if you want a mediocre book that is many
years out of date and seems to owe more to self-congratulation than
it does to technical excellence, then go for the RSCB offering. Odd, really,
when you consider that the RSCB is a publishing corporation.

I do not agree, and soon the RSGB is the only IARU organization I
support, have been a member from january 74. They now contribute with
a lot of useful material, but it was different earlier, as one had the
feeling that high-ranked persons or duke and knights with outdated
experience was telling you what to do.

But for Radcom, I must admit that I mainly read G3VA's "Technical
topics"

My radio club was Worcester &DARC, suppose it is not so much activity
there now....

---
J. M. Noeding, LA8AK, N-4623 Kristiansand
http://home.online.no/~la8ak/c.htm

Roy Lewallen wrote:
(In
fact, recognizing it value, Tek spent a large amount of money and
devoted resources to development of its own internal version of SPICE,
which included schematic entry and other features before they were
available in outside commercial versions.)

You owe me a keyboard to replace the one that just died under three
quarts of drool.

--
Most dying mothers say, "I love you, son," or "Take care of your sister."
Why were the last words of Kerry's mother a lecture on integrity?

On Sun, 17 Oct 2004 16:52:38 +0200, J M Noeding
wrote:

On Sun, 17 Oct 2004 15:00:49 +0100, "Airy R. Bean"
wrote:

It's been the case in Britland for many years now, that if you
want info to build modern gear, then buy the ARRL handbooks.
OTOH, if you want a mediocre book that is many
years out of date and seems to owe more to self-congratulation than
it does to technical excellence, then go for the RSCB offering. Odd, really,
when you consider that the RSCB is a publishing corporation.

I do not agree, and soon the RSGB is the only IARU organization I
support, have been a member from january 74. They now contribute with
a lot of useful material, but it was different earlier, as one had the
feeling that high-ranked persons or duke and knights with outdated
experience was telling you what to do.

But for Radcom, I must admit that I mainly read G3VA's "Technical
topics"

My radio club was Worcester &DARC, suppose it is not so much activity
there now...

In my limited experience over the lats 22 months Iv'e read both (well
looked through both!) and they aer both highly infromative books. Id
just give the edge to the ARRL version, it's more readable and has
more coentent, but hte RAdcom version is still well worlth having on
the shelf. I do'nt see any problem with the Brits anachronistic
attachment to valve gear.
--

Fat, sugar, salt, beer: the four essentials for a healthy diet.

On Sun, 17 Oct 2004 23:07:14 GMT, Steve Evans
wrote:



In my limited experience over the lats 22 months Iv'e read both (well
looked through both!) and they aer both highly infromative books. Id
just give the edge to the ARRL version, it's more readable and has
more coentent, but hte RAdcom version is still well worlth having on
the shelf. I do'nt see any problem with the Brits anachronistic
attachment to valve gear.

I is really a myth, TV sets were fully transistorized in Europe
compared to USA by several years


---
J. M. Noeding, LA8AK, N-4623 Kristiansand
http://home.online.no/~la8ak/c.htm

"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.
. . .

"Airy R. Bean" wrote in message
...
Question - what is the internal modelling technique used
by these various programs, and can we produce our own package?

Is it based upon successive delta-time increments, and if so, what
is the increment? What prompted the last question is an attempt
I made to create a sine-wave generator using the identity that
sin dTheta = dTheta, but I had to go for an _extremely_ small
value of dTheta (ISTR 10^ -18) before getting anything like a
decent sine wave, and even that degenerated after a few cycles.

So, these circuit simulators - what is their underlying technique
for circuit simulation?
[clip]

The simulators basically offer 2 types of direct analysis ...
An "AC" analysis and a "Transient" analysis. Their answers come via
different maths methods.
Basically the much less useful 'AC' analysis examines all the wire
connection points 'nodes' in the circuit diagram and enters the found
components in 2 matrices. Each (square) matrix is sized to hold node^2
elements. One is for 'real' components, the other for quadrature components.
All non linear components in the circuit must first be simplified/replaced
by linear equivalants (messy). Matrices are filled in a manner similar to
kirchoffs loops anaysis. Reactive components entering the imaginary matrix.
Ie lots of real/imag simultaneous equations need solving which is of course
why the computer is handy.
After they are filled the matrices are mathematically inverted to give a
complete set of solved phase and ac voltage data for all the node points in
the circuit. Nice for filters useless for oscillators!.

The more useful transient analysis is similar to as you mention (Babbage's
difference engine?) but based simply on the differentials V=L i/t and V=i/C
and uses near complete maths models for the semicons or other non linear
elements. Nodes examined in turn and time steps selected purely on the
basis of how fast the results are changing. time steps can be a problem as
too long and the final results get 'smeared out' hence phase lag artifacts
cause overall loop stability problems.
The TA is conceptually very simple and surprisingly easy and fun to
programme for set piece or well observed circuits but gets *really* spagetti
code messy if it is to work smoothly with any input circuit. Big problems
can turn up getting the results to converge or balance and much progging
effort is needed in this direction.
regards
john

"J M Noeding" wrote in message
...
On Sun, 17 Oct 2004 15:00:49 +0100, "Airy R. Bean"
wrote:
[clip]
But for Radcom, I must admit that I mainly read G3VA's "Technical
topics"

My radio club was Worcester &DARC, suppose it is not so much activity
there now....

---
J. M. Noeding, LA8AK, N-4623 Kristiansand
http://home.online.no/~la8ak/c.htm


To me, Pat Hawker is the defining spirit of UK amateur radio.
I've also picked up much fascinating stuff from his technical-topics and the
mentions of his SOE work in WW2.
regards
john