From: An EDN Blog post about Analog Design tools.
Bob Pease at a Panel session on DFM went on about a circuit Spice said couldn't work and held a breadboard up with the circuit he said was in production.
Anyone know what it was?
Robert H.
I think Pease has gone senile. Did everyone see his _religion_ rant in the last issue?
I went to one of his "seminars", didn't learn a thing, saw a lot of application notes and data books on National's products.
As for "...Spice said couldn't work...", what does that mean... Spice couldn't converge on an initial solution? Happens all the time, doesn't mean that the circuit can't work.
Show me the circuit schematic and I assure you I can Spice it.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
America: Land of the Free, Because of the Brave
He usually says that about the LM331.
Cheers,
Phil Hobbs
Anyone have a complete schematic?
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
America: Land of the Free, Because of the Brave
Pease has the problem common to columists, out of ideas but deadlines keep coming. He's become an embarassment, but Electronic Design has, too.
HoJo seems to be straining, too.
John
Inadequate and incomplete models, of course. And unknown or unaccounted-for parasitic elements in the circuit. Happens all the time out there in the real world (i.e., the world that's not IC design). :-)
The solution is a painful process to characterize the part, etc., and create a spice model or subcircuit that matches over the range of interest. I also find myself making test PCBs, and taking inductance, capacitance etc. measurements on that. Lots of bench work, serious bench equipment, lots of computer work, plenty of knowledge and experience about what to look for. Given the commonplace absence of this approach, it's rather easy to throw stones at "spice".
But given the careful, time-consuming approach, I find spice extremely useful in my push-the-envelope projects.
Spice is a reasonable general-purpose program for solving systems of nonlinear ordinary differential equations. There are some systems that no program is going to be able to solve, but they're quite rare in applications. Thus just about anything that can be modelled accurately with a set of coupled ODEs can be simulated accurately with Spice.
On the other hand, not everything can be modelled with ODEs, not even every circuit. Some better known non-ODE things, e.g. transmission lines, have been put into Spice by hand. (A transmission line's circuit properties aren't given by an ODE because they're nonlocal, i.e. the output undergoes a true time delay.)
Other classes of problem that can't be modelled as systems of ODES are transport equations--e.g. the Boltzmann equation for electron transport, or any problem that involves convective motion, such as the air in a heat sink.
It isn't great at multiple scale analysis, either, so for instance it would be very poor at modelling the turn-on behaviour of a laser diode, in which time scales from sub-femtosecond (the E & H fields) to hundreds of milliseconds (the thermal transient) all contribute very significantly and nonlinearly. There are codes for this kind of problem, but Spice isn't one of them--it has to follow each cycle laboriously, because a linearized AC analysis won't get the right answer.
And anyway, Pease's main complaint is that people use (generally poor) computer analyses as a substitute for thought--and have tried to beat him up with the results over the years. Most of the rest of his posturing is for fun, I think. You can't really make a serious critique of computer simulation by throwing a computer off the roof of the NSC parking garage. Widlar envy, perhaps.
Cheers,
Phil Hobbs
Thanks.
As Win said, "inadequate Models...", could cover a multitude of sins.
I wonder how well something like a Flash device's charge storage is modeled in Spice.
Robert H.
*Nonlinear* ODE's? Sounds oxymoronic, didn't know there was anything ordinary about nonlinear DE's.
Those are actually PDE's...
Actually that is what Will Gear's integration mehod was intended to handle, widley varying time scales.
SPICE probably could be configured to model this, but it's not going to hand it to you on a silver platter.
Nope, Pease's main complaint is not that people use poor computer analyses, it is that they use computer analyses poorly.
SPICE is not about solving, it is about producing numbers, numerical integration:
An ODE is a DE with one independent variable, which for Spice is usually taken to be time, and they can be linear or nonlinear, with constant or time-varying coefficients. YCLIU.
They're integral equations, because they're nonlocal. PDEs, e.g. Maxwell's equations and the heat equation, are just as local as an ODE--they just have more than one independent variable. I was careful to say that *the circuit properties* of a transmission line aren't given by ODEs, because they aren't. The fields inside the transmission line obey Maxwell's equations, which as you say are PDEs. I'm talking about the two-port circuit behaviour.
No, it was designed to handle stiff systems, i.e. those whose eigenvalues are very different in size. Multiple scale is something quite different. YCLIU again.
It could, if you could wait long enough for it to integrate 10**14 cycles, and could handle the resulting extreme roundoff problems. There are better methods, e.g. multiple scale analysis. Bender & Orszag's book on asymptotic analysis is an excellent read on this sort of stuff--and it has lots of great pictures.
Sure, that's what I said--Spice is an ODE integration package, and what Pease doesn't like is that many people use it as a substitute for thought. Anyway, what's the operational difference between wrong answers from bad models and wrong answers from stupid modellers?
Cheers,
Phil Hobbs
The horrible electrical case is time-domain modelling of a crystal oscillator. You need picosecond time steps (or fs, maybe) to resolve the actual frequency, and you may have to run for seconds until it reaches steady-state. Besides, Spice is terrible for measuring frequency.
I just model them in the frequency domain, and estimate behavior based on open-loop phase-amplitude plots.
Nope, Pease's complaint is that people use computers.
John
Well, maybe...but using them instead of thinking is what he _ought_ to be complaining about. ;)
Cheers,
Phil Hobbs
Pease lives not far from us. It's funny, his front yard, and sometines the street in front of his house, is always full of rustbucket old Beetles and VW Microbusses. I guess he's just a traditional sort of guy.
I've met him a couple of times. He's very nice, one-on-one.
John
You just need to know how ;-) I regularly simulate start-up in crystal oscillators because that amount of time is part of the specification.
[snip]The best (most stable) crystal oscillators are "sort of" AGC'd.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
America: Land of the Free, Because of the Brave
I've met him a couple of times at NSC seminars on Long Island. The first time, he was plugging my book (not knowing I was there, of course) and I introduced myself. We got on fine. Of course, in SF people would have VW microbuses on blocks in their front yards...I bet they wouldn't let you have Chevys like us more staid folks. ;)
Cheers,
Phil
Most SF "front yards" might have room for one bicycle up on blocks. RAP lives in a neighborhood that has actual detatched [1] houses and real grass. One of his Electronic Design columns related his skills in repairing rain gutters.
John
[1] the standard lot, like mine, is 24 feet wide, and houses and roofs are in total contact. One could walk almost my entire block, end to end, on the roofs.
So tell me? How can I measure the frequency to, say, 1 PPM in a time-domain sim?
The envelope response is reasonable to simulate in time domain. Phase noise and tempco aren't.
Yup. Crystal drive matters. But I mostly just buy oscillators now, even OCXOs. OCXOs and TCXOs have really gotten cheap lately.
I need a new universal frequency counter. The low-end Agilent looks nice, but it has a crappy XO, and they want another THOUSAND DOLLARS for a mere TCXO.
John
You don't nor do you care ;-)
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
America: Land of the Free, Because of the Brave
Oh. Thanks for straightening me out on that.
John
hmmm... I suppose you could heterodyne it against an ideal generator, and eyeball the difference.
Has it got an external input? I've got an ancient (or is that well-aged and "even more stable"?) GenRad OXCO which I can plug into any generator or counter that will take an external reference to make it more accurate.
With your rather more serious (than mine) operations, I'd think you could put in one rubidium or cesium reference (and or some new-ish method tied to GPS I'm not overly familiar with) and distribute it throughout your labs/plant/wherever, so long as your counters and/or generators will take an external reference frequency. Assuming decent cable termination and appropriate drivers, I don't see that you'd need more than one "really good" reference.
-- Cats, coffee, chocolate...vices to live by
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