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Can anyone explain this effect?

Discussion in 'CAD' started by Paul Burridge, Sep 15, 2004.

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  1. Hi guys,

    May I direct your attention to the following:

    http://www.burridge8333.fsbusiness.co.uk/triangle.gif

    Where you will find a very simple common-source FET stage that gives
    some rather odd results when simulated. It feeds a 0.5mV AC sine
    voltage of 10Khz to the gate of the FET. The output is a reasonable
    replication of the input signal shape, but upon removing the inductor
    (which I had to insert to prevent this problem) the output turns into
    a perfect triangle wave. This gross distortion of the input signal
    only seems to happen at very small input levels, but nevertheless, I
    can think of nothing in the real world that might explain it. Could it
    be some sort of peculiarity with the intricacies of spice signal
    sources of which I am ignorant? If not, how is this sine-to-triangle
    conversion taking place?

    THanks,

    p.
     
  2. Your obviously not using SS, cos it works fine here.

    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
     
  3. analog

    analog Guest

    Turn off waveform compression and try again.
     
  4. Hello Paul,
    you have two chances to do that.

    1.
    Control Panel -> Compression
    Disable any compression


    2. The better method is to control the compression mode in your schematic.

    Add the following command line to your schematic.

    ..options plotwinsize=0


    Compression is a great feature of LTSPICE. It can reduce your output
    file size by a decade. And that counts a lot when you reach the
    hundreds of megabytes in file size.


    Best regards,
    Helmut
     
  5. Leon Heller

    Leon Heller Guest

    It works fine for me, using Pulsonix SPICE.

    Leon
     
  6. Thanks, guys. That's sorted the problem out pretty damn quick! But it
    does beg the question of whether one should run LT with compression on
    by default. In what circumstances is it safe to leave it on and when
    should one disable it? I'm a bit worried now about getting false
    results (not quite so obvious as this!) and not realising it could be
    the compression feature causing it. :-/ It's not a problem I've
    *knowingly* encountered before...
     
  7. Hi Kev,
    I did *try* simulating it with SS, but came up against the familiar
    old problem of the size of the graphs it produces. The plot was too
    small to make out its shape. I thought you'd have addressed that
    niggle by now but maybe I'm the only one who has a problem with it.
    Running on a laptop didn't help though, I must admit! BTW, if there is
    a way to get a full-screen view of the graph by toggling a function
    key, then please let me know!

    p.
     
  8. Personally, I think its bloody daft having compresion on by default. In
    fact, its insane. The only time to have it on, is when you *really* need
    it, like running out disk space. Its like cleaning the house up by
    sweeping all the shit under the carpet.


    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
     
  9. This makes little sense to me.
    Send me a gif screen shott of what you have.

    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
     
  10. Fred Bartoli

    Fred Bartoli Guest

    I use another spice but have LTspice installed and tried this, just out of
    curiosity.
    Wave compression is a nice feature but I guess I would have been caught on
    this one. Well, maybe not, since I'd know the expected output frequency.
    Anyway, at least in this case, compression has the same effect as
    undersampling and gives plausible waveforms to the inattentive user.
    Wouldn't it be feasible to detect such "undersampling" conditions and either
    automatically adapt the compression or issue a warning message ?
     
  11. Hehe! Great analogy, Kev. :)
    Anyway, about the graph problem: is there a way to copy the graph to
    clipboard (as in LT) or do I have to photograph the screen with me
    digicam? I'm quite happy to photograph it if there's no quick way.

    p.
     
  12. Good idea. I iniitally suspected it was a time-step problem, but after
    trying various combinations of frequency and time-step, it was
    evidently something else, hence my query.
    Do the graph files really need to be so big, anyway?? IFAICS, LT saves
    the plots from *every* node, even if you're only interested in one or
    two points in the circuit. Seems an unnecessary waste of disk space
    IMHO. :-/
     
  13. analog

    analog Guest

    In the case of Paul's circuit, the small size of the signal coming
    from the voltage source seems to slip in under the compression
    algorithm's radar. I suppose Mike E could modify the algorithm to
    restrict compression based on the a priori knowledge of the presence
    of sources with periodic waveforms.
    Yes it can be under certain circumstances. That is why there already
    is a "dot" command to address this very issue. See if you can pick
    it out from the "Dot Commands" sections of the help file. :)
     
  14. Leon Heller

    Leon Heller Guest

    Just capture it with the Prt Sc key, and then paste it into MS Paint.

    Leon
     
  15. There aint on my system. Send me a screen shott so I can determine what
    your problem is.
    Yes. Click on its invisible title bar to select it. Use the menu
    "Edit\copy to clipboard". This copies it as a standard bit map. You can
    then paste to any other app.

    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
     
  16. Will do. As soon as I get back from the pub. (abt. 3 hrs.)
     
  17. Thanks. I'll take a look...
     

  18. Hello Paul,
    you can limit the nodes beeing saved.
    Just add a command line withe nodes you are interested.

    Example: It will save only two items in the output file.

    ..save V(out) I(V1)


    Best regards,
    Helmut
     
  19. analog

    analog Guest

    Hi Helmut,

    More than he needs the answers, Paul needs to learn how to find
    the answers for himself. I may be wrong, but I think you are
    enabling a grown bird to stay in the nest going "peep, peep, peep".
    He should be finding at least some of his own worms by now.

    Regards :)
     
  20. JeffM

    JeffM Guest

    is there a way to copy the graph to clipboard (as in LT)
    A work-around (screen capture utility):
    http://www.google.com/search?&q=ScreenRip-32
     
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