Blind simulation without understanding

[Pimpom]

I've been spending some time in a web-based electronics forum
that seems to attract a lot of engineering students, hobbyists
and beginners while the more experienced members seem to be very
knowledgeable. What surprises me is that many of the questions
are about some circuit that the OP blindly plopped into a
simulator without any clear idea of how it's supposed to work. Is
this the approach students and newbies are being encouraged to
take these days?

 

[Joerg]

I've been spending some time in a web-based electronics forum
that seems to attract a lot of engineering students, hobbyists
and beginners while the more experienced members seem to be very
knowledgeable. What surprises me is that many of the questions
are about some circuit that the OP blindly plopped into a
simulator without any clear idea of how it's supposed to work. Is
this the approach students and newbies are being encouraged to
take these days?

That sounds more like laziness to me. Maybe they do not want to put in
the time to learn and truly understand a circuit.

But there were some disturbing trends even all the way back at my
university in the early 80's. A professor said, literally "Well you have
to learn all this analog circuit level stuff for the exam but in a few
years you'll never need it again". What a fool that was. But he was
professor so most students believed that nonsense. I didn't

 

[mike]

I've been spending some time in a web-based electronics forum
that seems to attract a lot of engineering students, hobbyists
and beginners while the more experienced members seem to be very
knowledgeable. What surprises me is that many of the questions
are about some circuit that the OP blindly plopped into a
simulator without any clear idea of how it's supposed to work. Is
this the approach students and newbies are being encouraged to
take these days?

Back in the late 60's, I had ONE, count 'em, ONE prof who I thought
knew what he was doing. He was an engineer for TI. Teaching was
some kind of outreach program to teach/acquire new engineers for TI.
He taught a practical approach to engineering with the math reserved
for fine tuning what you had designed based on your UNDERSTANDING
of the problem and the devices used in the solution.

I had a thermodynamics prof who's approach to teaching was to discuss
a concept at the first part of the class then spend the bulk of the time
painstakingly working thru solving the differential equations with us.
That was the ONLY class where I thought I understood how to do the math
involved in a practical context.

All the rest of the profs had a teaching style that could be described
as RTFM.

A couple of years later when I was hiring new grads, I experienced the
same results. High GPA students who could solve equations, but had no
idea how to decide which equations to write.

I was lucky enough to land a job with a mentor. I learned more from him
in a few months than I did in 5 years of school.

Mentors are rare. If you don't get one, you're not likely to be able to
mentor your charges.
Don't get me started on management and the Peter Principle...

So, the more computing power became available to engineering students,
the stupider they got.
I remember standing in line to access one of the few calculators the
university had for our use. Good times.....

Simulation is a great thing for prototyping your design.
It's a lousy substitute for a synthesis process.

 

[Joerg]

And you learn a great deal less. Building stuff and doing your own math
is the way to wizardhood. Poking simulations will _not_ get anyone there.

Absolutamente.


I use Spice a fair amount these days--a lot more than I used to--but
that's primarily to give clients a warm fuzzy feeling that the final
circuit will do what I say it will. It's also very useful as a sanity
check and for optimization, once a circuit topology and most of the
component choices have been made on other grounds.

I (and a client) had our SPICE comeuppance almost right after I started
self-employment for the first time. Ultrasound front end boards were
deteriorating. Some channels would become "weaker" and it kind of
depended on how many hours each machine had been running and at which
pulse power level. Since this was, of course, not a tube design but all
semiconductors they could not explain it. They were at wits end.

Probing around the unit I couldn't see anything suspicious. Then I put
part for the front end on PSPICE. That was always an adventure back in
the days of DOS. Lo and behold this showed a spike of such short
duration that even really expensive scopes from that era could not
capture it. From there it was only minutes until I had figured out that
there was some race conditon in their logic circuitry. The guys were
highly skeptical about my findings. All this voodoo and witchcraft and
whatnot, and then blaming it on the digital part. The chief digital dude
was a guy similar to Hulk Hogan so I had to be careful ... But, they
decided to fix this race condition. The number of front end boards that
degraded in the reworked machines dropped to ... zero.

Real circuit design is a mental exercise, and mostly doesn't require
computers.

But it does sometimes require good beer

 

[Tim Williams]

I use Spice a fair amount these days--a lot more than I used to--but
that's primarily to give clients a warm fuzzy feeling that the final
circuit will do what I say it will. It's also very useful as a sanity
check and for optimization, once a circuit topology and most of the
component choices have been made on other grounds.

Real circuit design is a mental exercise, and mostly doesn't require
computers.

I find it's often easier to brainstorm a circuit, hash it out on paper to
get the implementation correct, then do the layout, and finally, tweak the
simulation until it produces realistic results.

Simulation is only a design tool in as much as verifying circuit
performance, based on educated guesses of real world parameters; it will
never assist you with circuit synthesis. Especially to the untrained
student, it can be downright dangerous because real transistors explode,
most often from circuit elements you didn't realize are there.

Tim

 

[Martin Riddle]

I've been spending some time in a web-based electronics forum that
seems to attract a lot of engineering students, hobbyists and
beginners while the more experienced members seem to be very
knowledgeable. What surprises me is that many of the questions are
about some circuit that the OP blindly plopped into a simulator
without any clear idea of how it's supposed to work. Is this the
approach students and newbies are being encouraged to take these days?

It's the new trend of Social Engineering.
Take someone else's work and expect it to work for you....
Then take credit.

I read some where it is common for those labeled 'Generation X'


Cheers

 

[Tom Del Rosso]

Yes :-(

Wanna hear something worse? Ok. At the Vaughan College of Aeronautics near
LaGuardia airport, they used to have labs with engines and stuff. Now they
have computers in every room to practice aircraft maintenance in simulation.

 

[legg]

I've been spending some time in a web-based electronics forum
that seems to attract a lot of engineering students, hobbyists
and beginners while the more experienced members seem to be very
knowledgeable. What surprises me is that many of the questions
are about some circuit that the OP blindly plopped into a
simulator without any clear idea of how it's supposed to work. Is
this the approach students and newbies are being encouraged to
take these days?

I see more questions that involve a misunderstanding of the
simulator's function, the graphical user interface, and the models
'provided'.

Lack of understanding of circuit function is universal to SW and
hardware implimentations - that's why the 'student' going through
either exercise.

A dumb question is usually just a dumb question, but not always.
That's why its important to ask them.

RL

 

[Joerg]

If you're merely claiming that computers can be useful, I agree. If you're
claiming that spending more time on computers and less time doing algebra and
bench work produces wizardly engineers faster, better, cheaper, or whatever, I
disagree 100%.

Full confession: That is my MO since years. I have done bench testing on
maybe 30% of projects last year that involved the design of new or
revised circuitry (not EMC works and such). The other 70% were born
totally in cyberspace, meaning on this here computation machine. And
they work, they are in production, they make my clients money.

I bet Jim's percentage of total cyberspace designs is 100% like it is
with almost any IC designer, or close to that. With the exception of
honey-do projects, of course

Naturally, this does not mean one can approach the computer without a
solid plan of actions or without a solid circuit design know-how.

[...]

 

[Joerg]

If you're merely claiming that computers can be useful, I agree. If you're
claiming that spending more time on computers and less time doing algebra and
bench work produces wizardly engineers faster, better, cheaper, or whatever, I
disagree 100%.

Full confession: That is my MO since years. I have done bench testing on
maybe 30% of projects last year that involved the design of new or
revised circuitry (not EMC works and such). The other 70% were born
totally in cyberspace, meaning on this here computation machine. And
they work, they are in production, they make my clients money.

I bet Jim's percentage of total cyberspace designs is 100% like it is
with almost any IC designer, or close to that. With the exception of
honey-do projects, of course

Naturally, this does not mean one can approach the computer without a
solid plan of actions or without a solid circuit design know-how.

[...]

P.S.: When young, engineers must experiment and solder, not matter what.

 

[Tim Williams]

Sometimes the errors can be rather subtle. If you simulate the
Baxandall class-D oscillator with bipolar transistors and use an extra-
large inductor to feed the centre-tap of the tank circuit inductor/
transformer to minimise the harmonic content of the output, the
circuit eventually settles down to produce a constant output amplitude
sine-wave signal.

In real life, such a circuit "squegs" and never settles down into a
stable state. Apparently this doesn't happen if you build the circuit
with MOSFET switches.

It's my experience, both in real life and simulation, that slightly too
much feedback, combined with unfortunate oscillator response (too many
poles in the amplitude response), causes it to overdrive itself, forcing
bias down, cutting it off, causing it to recover, restarting the cycle.

I once made an amplitude regulated VFO with a peak detector and error
amplifier (to give better regulation than a light bulb, at lower power
levels as well, though the implementation as-is contains a Vbe tempco).

The oscillator, in the linear (non-saturated) range, is roughly an
integrator; ideally, it's an exponential integrator, because as gain is
varied (by bias, in this case), the oscillation should grow or decay
exponentially with time, not linearly. The toe of that exponent can be
approximated as linear, so it therefore acts as an integrator to a
reasonable degree.

Now, the detector and error amplifier have a nice long time constant, to
keep ripple out of the loop and ensure low distortion. This absolutely
trashes the phase margin, because it looks like an integrator at some
upper frequency as well.

I simply put a pole-zero compensator in the loop and it went from
unconditional squegging (even under variation in coupling capacitance,
detector capacitance, etc.) to rock steady action. It doesn't even have a
startup transient, it just goes... bloop and stays there, with 10ms of
applying power.

Tim

 

[Joerg]

Bill Sloman wrote:
[snip]
Real circuit design is a mental exercise, and mostly doesn't require
computers.
But computers can do the arithmetic a lot faster than our mental
machinery. We are much better at pattern recognition than computers,
and consequently much faster at coming up with configurations of parts
that may do what we want, but computers are a handy tool for exploring
what a particular configuration of parts might do in reality - if
mathematical models of the parts do a good job in the region we are
trying to explore.

--
Bill Sloman, Nijmegen
If you're merely claiming that computers can be useful, I agree. If you're
claiming that spending more time on computers and less time doing algebra and
bench work produces wizardly engineers faster, better, cheaper, or whatever, I
disagree 100%.

Full confession: That is my MO since years. I have done bench testing on
maybe 30% of projects last year that involved the design of new or
revised circuitry (not EMC works and such). The other 70% were born
totally in cyberspace, meaning on this here computation machine. And
they work, they are in production, they make my clients money.

I bet Jim's percentage of total cyberspace designs is 100% like it is
with almost any IC designer, or close to that.

Total cyberspace in that "testing" is done only on the simulator... no
more breadboarding. BUT most initial work-up is by pencil and paper
and Algebra.

Yeah, I had to buy Mathcad for a client's project last year and thought
about maybe using that for the math stuff at the beginning of a design.
But ... ain't happening, pencil and green engineer's pad are faster.

Pretty much the same way... except I, personally, am the "process
house" ;-)

Sometimes vendors are and then it gets ugly. Like my favorite job this
afternoon: Did the regular cleaning of the kitchen sink waste pipes and
sure enough now that all the gunk is out there's a tiny leak. Because
this stuff is all cheap plastic these days. Hurumph.

[...]

 

[Joerg]

Bill Sloman wrote:

On Jan 8, 12:56 am, Phil Hobbs
BillSlomanwrote:
I've been spending some time in a web-based electronics forum
that seems to attract a lot of engineering students, hobbyists
and beginners while the more experienced members seem to be very
knowledgeable. What surprises me is that many of the questions
are about some circuit that the OP blindly plopped into a
simulator without any clear idea of how it's supposed to work. Is
this the approach students and newbies are being encouraged to
take these days?
Simulation is a great deal cheaper than building bread-boards. Lab
space is expensive, and has to be supervised. Students learning to
solder burn themselves from time to time, which leads to acrimonious
complaints from parents and guardians.
Virtual breadboards are much less trouble.
And you learn a great deal less.
Obviously. Mathematical models are always over-simplifications of
reality, and you have to test them against reality from time to time.

Building stuff and doing your own math
is the way to wizardhood. Poking simulations will _not_ get anyone there.
They certainly won't get you all the way there, but the wizard is just
somebody who has decent mathematical models inside their own head, and
simulations can help to build these models.

I use Spice a fair amount these days--a lot more than I used to--but
that's primarily to give clients a warm fuzzy feeling that the final
circuit will do what I say it will. It's also very useful as a sanity
check and for optimization, once a circuit topology and most of the
component choices have been made on other grounds.

Real circuit design is a mental exercise, and mostly doesn't require
computers.
But computers can do the arithmetic a lot faster than our mental
machinery. We are much better at pattern recognition than computers,
and consequently much faster at coming up with configurations of parts
that may do what we want, but computers are a handy tool for exploring
what a particular configuration of parts might do in reality - if
mathematical models of the parts do a good job in the region we are
trying to explore.

--
Bill Sloman, Nijmegen
If you're merely claiming that computers can be useful, I agree. If you're
claiming that spending more time on computers and less time doing algebra and
bench work produces wizardly engineers faster, better, cheaper, or whatever, I
disagree 100%.

Full confession: That is my MO since years. I have done bench testing on
maybe 30% of projects last year that involved the design of new or
revised circuitry (not EMC works and such). The other 70% were born
totally in cyberspace, meaning on this here computation machine. And
they work, they are in production, they make my clients money.

I bet Jim's percentage of total cyberspace designs is 100% like it is
with almost any IC designer, or close to that. With the exception of
honey-do projects, of course

Naturally, this does not mean one can approach the computer without a
solid plan of actions or without a solid circuit design know-how.

[...]

I still do about as much breadboarding as simulation, but a lot of
designs get neither.

Lots of parts, especially fast ones, don't have decent Spice models.
By the time I evolve a model and verify it against a real part, I may
as well just breadboard the real circuit.

Recall my recent experience with the AD8014 bootstrap ramp generator?
The simulation, using ADI's model, was useless.

Yes, I remember. However, last year I had a few designs where super fast
pulse stuff happened all on SPICE. One client actually told me that they
do not believe in simulators and I guess were a bit surprised when my
cyberspace circuit produced roughly the same plots on their scope that
it did on my computer. Despite the fact that I used magnetics in an
unorthodox way.

Besides, messing with Dremels and soldering irons and test equipment
is fun.

Oh yeah, it sure is. Now if there was a xx GHz USB-sampler that would
make it even more fun. I've got a USB spectrum analyzer since a month
and that thing really kicks butt. I have to put da big machine on the
check list now so I won't forget to turn it on once in a while so it's
memory battery won't dry up.

 

[Joerg]

Phil Hobbs wrote:
Bill Sloman wrote:

[snip]
Real circuit design is a mental exercise, and mostly doesn't require
computers.
But computers can do the arithmetic a lot faster than our mental
machinery. We are much better at pattern recognition than computers,
and consequently much faster at coming up with configurations of parts
that may do what we want, but computers are a handy tool for exploring
what a particular configuration of parts might do in reality - if
mathematical models of the parts do a good job in the region we are
trying to explore.

--
Bill Sloman, Nijmegen
If you're merely claiming that computers can be useful, I agree. If you're
claiming that spending more time on computers and less time doing algebra and
bench work produces wizardly engineers faster, better, cheaper, or whatever, I
disagree 100%.

Full confession: That is my MO since years. I have done bench testing on
maybe 30% of projects last year that involved the design of new or
revised circuitry (not EMC works and such). The other 70% were born
totally in cyberspace, meaning on this here computation machine. And
they work, they are in production, they make my clients money.

I bet Jim's percentage of total cyberspace designs is 100% like it is
with almost any IC designer, or close to that.
Total cyberspace in that "testing" is done only on the simulator... no
more breadboarding. BUT most initial work-up is by pencil and paper
and Algebra.

Yeah, I had to buy Mathcad for a client's project last year and thought
about maybe using that for the math stuff at the beginning of a design.
But ... ain't happening, pencil and green engineer's pad are faster.

Years ago I bought Mathcad, but found it quite cumbersome to use. If
I can't solve an equation on paper, I use Excel or, quite often now,
simply use PSpice... the behavioral modeling capability is so good, I
can set up and solve a transcendental equation in minutes.

With the exception of
honey-do projects, of course
Pretty much the same way... except I, personally, am the "process
house" ;-)

Sometimes vendors are and then it gets ugly. Like my favorite job this
afternoon: Did the regular cleaning of the kitchen sink waste pipes and
sure enough now that all the gunk is out there's a tiny leak. Because
this stuff is all cheap plastic these days. Hurumph.

[...]

Yup :-( I just got an announcement that it's also wet under our
kitchen sink :-(

Could be worse. I was just outside and in the distance I heard a major
kablouie, made the windows rattle. Then you could hear stuff clanging
and shattering back to earth. But ... no sirens, nada, nobody seemed to
fuss about it. And I have no idea where it came from. No, not from my
lab

 

[Joerg]

John Larkin wrote:

[...]

Oh yeah, it sure is. Now if there was a xx GHz USB-sampler that would
make it even more fun. I've got a USB spectrum analyzer since a month
and that thing really kicks butt. I have to put da big machine on the
check list now so I won't forget to turn it on once in a while so it's
memory battery won't dry up.

There are some...

http://www.fastsampling.com/

Amazing! They don't give a risetime spec in the manual but at 8GHz
bandwidth it should be good enough.

http://www.picotech.com/picoscope9200-specifications.html

$10k, yikes ...

http://www.hyperlabsinc.com/?section=home&page=product&group=TDR Instruments&id=hl1100usb.txt

Well over $5k, seems to be geared towards the TDR market.

 

[Joerg]

Jim Thompson wrote:
Phil Hobbs wrote:
Bill Sloman wrote:

[snip]
Real circuit design is a mental exercise, and mostly doesn't require
computers.
But computers can do the arithmetic a lot faster than our mental
machinery. We are much better at pattern recognition than computers,
and consequently much faster at coming up with configurations of parts
that may do what we want, but computers are a handy tool for exploring
what a particular configuration of parts might do in reality - if
mathematical models of the parts do a good job in the region we are
trying to explore.

--
Bill Sloman, Nijmegen
If you're merely claiming that computers can be useful, I agree. If you're
claiming that spending more time on computers and less time doing algebra and
bench work produces wizardly engineers faster, better, cheaper, or whatever, I
disagree 100%.

Full confession: That is my MO since years. I have done bench testing on
maybe 30% of projects last year that involved the design of new or
revised circuitry (not EMC works and such). The other 70% were born
totally in cyberspace, meaning on this here computation machine. And
they work, they are in production, they make my clients money.

I bet Jim's percentage of total cyberspace designs is 100% like it is
with almost any IC designer, or close to that.
Total cyberspace in that "testing" is done only on the simulator... no
more breadboarding. BUT most initial work-up is by pencil and paper
and Algebra.

Yeah, I had to buy Mathcad for a client's project last year and thought
about maybe using that for the math stuff at the beginning of a design.
But ... ain't happening, pencil and green engineer's pad are faster.
Years ago I bought Mathcad, but found it quite cumbersome to use. If
I can't solve an equation on paper, I use Excel or, quite often now,
simply use PSpice... the behavioral modeling capability is so good, I
can set up and solve a transcendental equation in minutes.

With the exception of
honey-do projects, of course
Pretty much the same way... except I, personally, am the "process
house" ;-)

Sometimes vendors are and then it gets ugly. Like my favorite job this
afternoon: Did the regular cleaning of the kitchen sink waste pipes and
sure enough now that all the gunk is out there's a tiny leak. Because
this stuff is all cheap plastic these days. Hurumph.

[...]
Yup :-( I just got an announcement that it's also wet under our
kitchen sink :-(

Could be worse. I was just outside and in the distance I heard a major
kablouie, made the windows rattle. Then you could hear stuff clanging
and shattering back to earth. But ... no sirens, nada, nobody seemed to
fuss about it. And I have no idea where it came from. No, not from my
lab

Someone's propane tank ?

Then there'd have been a fireball and black smoke. But nothing could be
seen. The stuff must have been flying quite a bit because there were a
few seconds between the kaboom and the clanging.

 

[Joerg]

John Larkin wrote: [...]

Besides, messing with Dremels and soldering irons and test equipment
is fun.

Oh yeah, it sure is. Now if there was a xx GHz USB-sampler that would
make it even more fun. I've got a USB spectrum analyzer since a month
and that thing really kicks butt. I have to put da big machine on the
check list now so I won't forget to turn it on once in a while so it's
memory battery won't dry up.
There are some...

http://www.fastsampling.com/

Amazing! They don't give a risetime spec in the manual but at 8GHz
bandwidth it should be good enough.

http://www.picotech.com/picoscope9200-specifications.html

$10k, yikes ...

http://www.hyperlabsinc.com/?section=home&page=product&group=TDR Instruments&id=hl1100usb.txt

Well over $5k, seems to be geared towards the TDR market.

You can get a nice used 11801 and a 20 GHz sampling head for around
$1500. But it's not very portable.

That's why I got the SignalHound USB spectrum analyzer so I can schlepp
it in a carry-on bag. Plus the tracking generator, not so much for
filters but to be able to gauge the attenuation of undesired noise
intrusion paths.

If this IBZ sampling box really does 8GHz and the risetime is well under
100psec it should be at least half as good as the big old Tek. And at
$1k. Plus CTRL-PrintScreen gives you an instant snapshot.

 

[Joerg]

[...]

Sometimes vendors are and then it gets ugly. Like my favorite job this
afternoon: Did the regular cleaning of the kitchen sink waste pipes and
sure enough now that all the gunk is out there's a tiny leak. Because
this stuff is all cheap plastic these days. Hurumph.

We came back from Lyon to find that a 15mm copper pipe had pin-holed
through. We called a plumber who not only cut out the offending
section - and replaced it with new cooper pipe, hooked up with
compression fittings - but also noticed the coupling to the base of
the hot water tank was leaking.

When he pulled it apart, the hard rubber washer came out in two
segments that had obviously separated a while ago. A new washer
stopped the leak.

The stuff was probably installed fifty years ago. Cheap plastic may
start leaking earlier, but nothing lasts forever.

Unless the Romans built it, then it does.

 

[Tim Williams]

The Baxandall Class-D oscillator doesn't have any feedback, and Peter
Baxandall called what he was seeing "squegging" because he didn't have
a clue what was going on.

Could've fooled me, an oscillator without feedback.

Tim

 

[Joerg]

Phil Hobbs wrote:
Bill Sloman wrote:

[snip]
Real circuit design is a mental exercise, and mostly doesn't require
computers.
But computers can do the arithmetic a lot faster than our mental
machinery. We are much better at pattern recognition than computers,
and consequently much faster at coming up with configurations of parts
that may do what we want, but computers are a handy tool for exploring
what a particular configuration of parts might do in reality - if
mathematical models of the parts do a good job in the region we are
trying to explore.

--
Bill Sloman, Nijmegen
If you're merely claiming that computers can be useful, I agree. If you're
claiming that spending more time on computers and less time doing algebra and
bench work produces wizardly engineers faster, better, cheaper, or whatever, I
disagree 100%.

Full confession: That is my MO since years. I have done bench testing on
maybe 30% of projects last year that involved the design of new or
revised circuitry (not EMC works and such). The other 70% were born
totally in cyberspace, meaning on this here computation machine. And
they work, they are in production, they make my clients money.

I bet Jim's percentage of total cyberspace designs is 100% like it is
with almost any IC designer, or close to that.
Total cyberspace in that "testing" is done only on the simulator... no
more breadboarding. BUT most initial work-up is by pencil and paper
and Algebra.

Yeah, I had to buy Mathcad for a client's project last year and thought
about maybe using that for the math stuff at the beginning of a design.
But ... ain't happening, pencil and green engineer's pad are faster.

Years ago I bought Mathcad, but found it quite cumbersome to use. If
I can't solve an equation on paper, I use Excel or, quite often now,
simply use PSpice... the behavioral modeling capability is so good, I
can set up and solve a transcendental equation in minutes.

With the exception of
honey-do projects, of course
Pretty much the same way... except I, personally, am the "process
house" ;-)

Sometimes vendors are and then it gets ugly. Like my favorite job this
afternoon: Did the regular cleaning of the kitchen sink waste pipes and
sure enough now that all the gunk is out there's a tiny leak. Because
this stuff is all cheap plastic these days. Hurumph.

[...]

Yup :-( I just got an announcement that it's also wet under our
kitchen sink :-(

I tightened the big nuts under there to the point where I'd think any
more would break them and this #%^!! plastic junk still leaks. Unusual
shapes, tough to buy anywhere, the usual.

So I guess I'll have to try pipe dope now. Great. Just great.