Help ..function generator

Discussion in 'The Projects Forum' started by marco_26, Mar 2, 2013.

  1. marco_26

    Thread Starter New Member

    Feb 28, 2013
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    I'm trying to build a function gen with variable frequency and amplitude using another schematic diagram (attached below). However it doesn't seem to work in multisim. I also attached the multisim file. I wonder what is wrong. Any ideas? The diagram is from the book of Floyd. Thanks a lot!
     
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  2. t06afre

    AAC Fanatic!

    May 11, 2009
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    Sometimes the simulator is to perfect. So the oscillation will not start. You should also replace the 741 opamps with more modern ones. If you build a real one. You will gain a lot by doing this.
     
  3. marco_26

    Thread Starter New Member

    Feb 28, 2013
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    But other schematics seem to be working fine with multisim.
     
  4. patricktoday

    Member

    Feb 12, 2013
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    I have Multisim 11 so I can't open the file. Try setting an initial condition of 1V to the output wire of IC1 in the schematic. Double-click the wire then Simulation Settings -> Use IC for Transient Analysis. This wouldn't be needed in a real-world circuit but will provide the simulator a means of starting the oscillation in place of the random noise which normally occurs, amplifies itself and starts the oscillation cycles in a real circuit.
     
    Last edited: Mar 2, 2013
  5. marco_26

    Thread Starter New Member

    Feb 28, 2013
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    Thanks@patricktoday. I did what you said, but the output still gives me s straight line instead of a function, as before.
     
  6. marco_26

    Thread Starter New Member

    Feb 28, 2013
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    Here is the image for the multisim file I created. Are the connections right?
     
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  7. patricktoday

    Member

    Feb 12, 2013
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    I'd try troubleshooting the 1st op amp section first until you get it going, just up to it's output. I don't see any wiring errors in that section. Make sure the variable pots are set to the same value and that two matched caps are selected with the cap switches. Replace the 741 with maybe a TL084 (it's more modern and I'm not sure if leaving the null offset pins unhooked might cause any issues in the simulation). I _think_ the path below C3 is connected but can't be sure from your image. Spot check some voltages to make sure you're getting power. And remember to leave an initial condition present; I have an oscillator diagrammed that won't start without it.
     
  8. marco_26

    Thread Starter New Member

    Feb 28, 2013
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    Hey thanks, I actually get it to work, though the outputs are not that great. I'm getting a semi-square wave for the sine wave and more like a sine wave for a triangular wave as the frequency increases.
     
  9. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Try swapping out the 741 op amps with TL071 or better yet MC34071, which are the same price or less when purchased online.

    741s have many shortcomings, the artifacts you are seeing in the output (a mix of slew rate, noise floor, and Gain Bandwidth Product).
     
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