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?

    --
    Regards, Joerg

    http://www.analogconsultants.com/

    "gmail" domain blocked because of excessive spam.
    Use another domain or send PM.
     
    Joerg, Sep 16, 2008
    #1
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  2. "Joerg" <> schrieb im Newsbeitrag
    news:f1Vzk.498$x%...
    > 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?
    >
    > --
    > Regards, Joerg
    >
    > http://www.analogconsultants.com/
    >
    > "gmail" domain blocked because of excessive spam.
    > Use another domain or send PM.




    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

    Helmut Sennewald wrote:
    > "Joerg" <> schrieb im Newsbeitrag
    > news:f1Vzk.498$x%...
    >> 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?
    >>
    >> --
    >> Regards, Joerg
    >>
    >> http://www.analogconsultants.com/
    >>
    >> "gmail" domain blocked because of excessive spam.
    >> Use another domain or send PM.

    >
    >
    >
    > 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
    >


    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.

    --
    Regards, Joerg

    http://www.analogconsultants.com/

    "gmail" domain blocked because of excessive spam.
    Use another domain or send PM.
     
    Joerg, Sep 16, 2008
    #3
  4. "Joerg" <> schrieb im Newsbeitrag
    news:tyVzk.365$...
    > Helmut Sennewald wrote:
    >> "Joerg" <> schrieb im Newsbeitrag
    >> news:f1Vzk.498$x%...
    >>> 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?
    >>>
    >>> --
    >>> Regards, Joerg
    >>>
    >>> http://www.analogconsultants.com/
    >>>
    >>> "gmail" domain blocked because of excessive spam.
    >>> Use another domain or send PM.

    >>
    >>
    >>
    >> 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
    >>

    >
    > 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.
    >
    > --
    > Regards, Joerg
    >
    > http://www.analogconsultants.com/
    >
    > "gmail" domain blocked because of excessive spam.
    > Use another domain or send PM.



    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

    Helmut Sennewald wrote:
    > "Joerg" <> schrieb im Newsbeitrag
    > news:tyVzk.365$...
    >> Helmut Sennewald wrote:
    >>> "Joerg" <> schrieb im Newsbeitrag
    >>> news:f1Vzk.498$x%...
    >>>> 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?
    >>>>
    >>>> --
    >>>> Regards, Joerg
    >>>>
    >>>> http://www.analogconsultants.com/
    >>>>
    >>>> "gmail" domain blocked because of excessive spam.
    >>>> Use another domain or send PM.
    >>>
    >>>
    >>> 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
    >>>

    >> 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.
    >>
    >> --
    >> Regards, Joerg
    >>
    >> http://www.analogconsultants.com/
    >>
    >> "gmail" domain blocked because of excessive spam.
    >> Use another domain or send PM.

    >
    >
    > 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.)
    >


    That is what I tried.

    >
    > Version 4
    > ....
    > TEXT 80 -104 Left 0 !.tran 0 100u 0 100n
    >
    >
    > I think it has 1 Ohm output resistance.



    Aha! Thanks. That explains it.


    > Please add an E-source with gain 1 at the ouput to get a low output
    > impedance.
    >


    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.

    --
    Regards, Joerg

    http://www.analogconsultants.com/

    "gmail" domain blocked because of excessive spam.
    Use another domain or send PM.
     
    Joerg, Sep 16, 2008
    #5
  6. "john jardine" <> schrieb im Newsbeitrag
    news:RHVzk.60287$2.easynews.com...
    >
    > "Joerg" <> wrote in message
    > news:f1Vzk.498$x%...
    >> 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?
    >>
    >> --
    >> Regards, Joerg
    >>
    >> http://www.analogconsultants.com/
    >>
    >> "gmail" domain blocked because of excessive spam.
    >> Use another domain or send PM.

    >
    > Add something like "vhigh=5 trise=10n". Essentially the 'trise' adds a
    > bit
    > of time reality to the gate. The help file is particularly obtuse in this
    > area. I've still to discover what that "ref" thing is there for.



    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

    Joined:
    Jul 25, 2010
    Messages:
    5
    Likes Received:
    1
    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:
    Jul 25, 2010
    Messages:
    5
    Likes Received:
    1
    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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