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Exp so slow [LTSpice]

Discussion in 'CAD' started by Chris Carlen, Jan 22, 2004.

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  1. Chris Carlen

    Chris Carlen Guest

    Hi:

    This simple thing:

    * Y:\spice\LT-SWcad\impulse.asc
    V1 N001 0 EXP(0 10 1n 1n 10n 1m)
    ..tran 10m
    ..backanno
    ..end


    Simulates at a snail's pace.

    Why is that?

    I simply want to make an exponentially decaying source, with effectively
    a step on the rising edge, which is why the rise tau is so short.

    But it sims real slow.

    Any suggestions?


    Thanks.


    Good day!

    --
    ____________________________________
    Christopher R. Carlen
    Principal Laser/Optical Technologist
    Sandia National Laboratories CA USA
     
  2. Jim Thompson

    Jim Thompson Guest

    Those sharp edges probably force small simulation steps over that
    whole 10ms.

    How large is the data file ?:)

    ...Jim Thompson
     
  3. Chris,
    It's making a mistake about the timestep size it needs to take to
    not outrun the time constant specified for rise time constant,
    the 4th parameter.
    Use

    V1 N001 0 EXP(0 10 1n 1n 10n 1m)

    for the time being.

    --Mike
     
  4. Chris,
    OK, this problem is fixed on the version currently on
    the web. LTspice looks at the details of the sources
    to pick timesteps that allow it to run both faster and
    more accurately than standard SPICE's. The error in
    the algorithm used for exp() sources has been fixed so
    LTspice will be able to run your deck more accurately
    and faster than anything other SPICE out there.

    --Mike
     
  5. Chris Carlen

    Chris Carlen Guest



    Thanks Mike! Geez, this new version seems to compute results
    *instantly* compared to the one I was using (from about 09/03/03), even
    for stuff that wasn't giving me any trouble.

    Good day!


    --
    ____________________________________
    Christopher R. Carlen
    Principal Laser/Optical Technologist
    Sandia National Laboratories CA USA
     
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