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Anyone know what Circut Bob Pease was going on about?

Discussion in 'CAD' started by Robert, Oct 5, 2007.

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  1. Robert

    Robert Guest

    From:
    An EDN Blog post about Analog Design tools.

    http://www.edn.com/blog/1480000148/post/240014624.html

    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.
     
  2. Jim Thompson

    Jim Thompson Guest

    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
     
  3. Phil Hobbs

    Phil Hobbs Guest

    He usually says that about the LM331.

    Cheers,

    Phil Hobbs
     
  4. Jim Thompson

    Jim Thompson Guest

    Anyone have a complete schematic?

    ...Jim Thompson
     
  5. John Larkin

    John Larkin Guest

    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
     
  6. 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.
     
  7. Phil Hobbs

    Phil Hobbs Guest

    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
     
  8. Robert

    Robert Guest

    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.
     
  9. Fred Bloggs

    Fred Bloggs Guest

    *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:
    http://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=1301&context=ecetr
     
  10. Phil Hobbs

    Phil Hobbs Guest

    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
     
  11. John Larkin

    John Larkin Guest

    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
     
  12. Phil Hobbs

    Phil Hobbs Guest

    Well, maybe...but using them instead of thinking is what he _ought_ to
    be complaining about. ;)

    Cheers,

    Phil Hobbs
     
  13. John Larkin

    John Larkin Guest

    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
     
  14. Jim Thompson

    Jim Thompson Guest

    On Sat, 06 Oct 2007 17:26:46 -0700, John Larkin

    [snip]
    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
     
  15. Phil Hobbs

    Phil Hobbs Guest

    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
     
  16. John Larkin

    John Larkin Guest

    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.
     
  17. John Larkin

    John Larkin Guest

    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
     
  18. Jim Thompson

    Jim Thompson Guest

    You don't nor do you care ;-)
    ...Jim Thompson
     
  19. John Larkin

    John Larkin Guest

    Oh. Thanks for straightening me out on that.

    John

    hmmm... I suppose you could heterodyne it against an ideal generator,
    and eyeball the difference.
     
  20. Ecnerwal

    Ecnerwal Guest

    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.
     
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