LTSpice: How do make a Schmitt oscillator run?

Discussion in 'CAD' started by Joerg, Sep 16, 2008.

  1. Joerg

    Joerg Guest

    Ok, simple circuit: Schmitt inverter, 10K resistor from output to input,
    1000pF capacitor from input to ground. Vh set to about 20% of Vhigh.
    Should happily oscillate but I get the error message "Time step too
    small". Also there is a noted dip in voltage down to about 80%,
    something which should even be possible with a digital behavioral device.

    What gives? How can I fix that and make it run? Does it need some kind
    of kicker?
     
    Joerg, Sep 16, 2008
    #1
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  2. Hallo Joerg,

    I don't know what you did wrong.
    Have you set the trigger levels vt and vh?

    hysteresis: vh
    trigger levels:
    v1 = vt+vh
    v2 = vt-vh

    Example: Right-mouse-click on teh schmitt device.
    Vhigh=5 Vt=2.5 Vh=1
    td=5n

    "osc.asc"-file:

    Version 4
    SHEET 1 880 680
    WIRE 144 0 96 0
    WIRE 288 0 224 0
    WIRE 96 96 96 0
    WIRE 128 96 96 96
    WIRE 176 96 128 96
    WIRE 288 96 288 0
    WIRE 288 96 240 96
    WIRE 320 96 288 96
    WIRE 96 112 96 96
    WIRE 96 192 96 176
    FLAG 96 192 0
    FLAG 320 96 out
    FLAG 128 96 rc
    SYMBOL Digital\\schmtinv 176 32 R0
    SYMATTR InstName A2
    SYMATTR Value2 Vhigh=5 Vt=2.5 Vh=1
    SYMATTR SpiceLine Td=5n
    SYMBOL cap 80 112 R0
    SYMATTR InstName C1
    SYMATTR Value 1n
    SYMBOL res 128 16 R270
    WINDOW 0 32 56 VTop 0
    WINDOW 3 0 56 VBottom 0
    SYMATTR InstName R1
    SYMATTR Value 10k
    TEXT 80 -104 Left 0 !.tran 0 100u 0 100n
     
    Helmut Sennewald, Sep 16, 2008
    #2
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  3. Joerg

    Joerg Guest

    Thanks, Helmut. I did have everything in there except td since it wasn't
    mentioned in the LTSpice manual. That did it. But I still have a weak
    output, dips about 3V from the rails with Vhigh=12 with some serious
    inductive load and 2ohms in series. Somehow that doesn't look normal. Do
    they have a finite output current capability?

    When I tried to open your file I got a syntax error.
     
    Joerg, Sep 16, 2008
    #3

  4. Hello Joerg,

    I copied my attached circuit into a file "osc.asc" and it worked.
    You should copy the text from line "Version 4" to the end.
    (The line "Version 4" is already part of the schematic file osc.asc.)


    Version 4
    .....
    TEXT 80 -104 Left 0 !.tran 0 100u 0 100n


    I think it has 1 Ohm output resistance.
    Please add an E-source with gain 1 at the ouput to get a low output
    impedance.

    Best regards,
    Helmut
     
    Helmut Sennewald, Sep 16, 2008
    #4
  5. Joerg

    Joerg Guest

    That is what I tried.

    Aha! Thanks. That explains it.

    Well, a 1ohm Z Out is pretty much what my final driver will have so
    it'll be ok this time.

    Thanks for your help, Helmut.
     
    Joerg, Sep 16, 2008
    #5

  6. Hello,

    "ref" is the threshold for normal digital devices like AND and OR.
    If you don't specify "ref", it's (vhigh+vlow)/2

    The threshold for Schmitt trigger devices is "vt" and "vh", but not "ref".

    Helmut
     
    Helmut Sennewald, Sep 17, 2008
    #6
  7. Joerg

    jonathanscottjames

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    schmit trigger oscelator. is there some kind of update that can fix all that?

    i was using the free trial of multisim and it's schmit trigger works normaly. but lt threw me.
    i tried t o uninstall it from xp pro sp3 in add/remove and it just blinkes. restarted and same..
    at least now i can try to rtemodel its schmit trigger. thanks.
    is there any other models that have to be altered t o make them do their functions?
     
    jonathanscottjames, Jul 25, 2010
    #7
  8. Joerg

    jonathanscottjames

    Joined:
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    5
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    i meant to say ltspice4.08 works but won't uninstall. reinstalling didn't help

    i meant to say ltspice4.08 works but won't uninstall. reinstalling didn't help
     
    jonathanscottjames, Jul 26, 2010
    #8
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