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Phase shift oscillator

  1. Feb 13, 2016 #1
    1. The problem statement, all variables and given/known data
    upload_2016-2-13_18-16-57.png
    upload_2016-2-13_18-17-20.png
    Gain & phase response of TL072 op-amp
    upload_2016-2-13_18-19-1.png

    2. Relevant equations
    'Frequency of oscillation was observed to be about 15 % lower'

    Why ???
    3. The attempt at a solution
    upload_2016-2-13_18-24-39.png
    so at this frequency the op-amp has 45 phase shift, but why will this impact the oscillator and lower the frequency.

    PSpice simulation
    upload_2016-2-13_18-32-10.png
    analysis
    upload_2016-2-13_18-33-34.png
    why is my frequency 22 Hz
     
  2. jcsd
  3. Feb 13, 2016 #2
    how can i find an 'ideal op-amp' in PSpice ?
     
  4. Feb 13, 2016 #3

    LvW

    User Avatar

    An ideal opamp model in PSpice is simply called up with "OPAMP".
    Regarding your result - I suppose the critical parameter is the limited slew rate which drastically lowers the frequency (and causes a triangle form).
     
  5. Feb 13, 2016 #4
    but would the slew rate effect the frequency or only distort the waveform,
    the phase shift oscillator should producesa sine wave output.
     
  6. Feb 13, 2016 #5

    LvW

    User Avatar

    Yes - the slew rate SR lowers the frequency because SR causes in addition a negative phase shift (delay corresponds with phase shift).
     
  7. Feb 13, 2016 #6
    65 kHz is my worked out frequency but PSpice is 22 Hz
     
  8. Feb 14, 2016 #7

    LvW

    User Avatar

    I have simulated your circuit - and the result is at 55 kHz.
    What are the time steps you have chosen?
    My choice: End of simulation 1ms and resolution 1µs.
    Why such a large simulation time for 55 kHz(1s)?

    Comment: A much better waveform can be achieved if you split the feedback resistor in two (33k=23k +10k) and connect two diodes (antiparallel) across the 10k resistor.
     
    Last edited: Feb 14, 2016
  9. Feb 14, 2016 #8
    you got 55 kHz that is what it should be about 15% lower.
     
  10. Feb 14, 2016 #9
    upload_2016-2-14_17-36-57.png
    i think something is wrong with this iam still getting 22 Hz
     
  11. Feb 14, 2016 #10
    i have to write a report about why the circuit is 15% lower from designed frequency, when the TL072 op-amp was used.

    have simulated your circuit - and the result is at 55 kHz.
    What are the time steps you have chosen?
    My choice: End of simulation 1ms and resolution 1µs.
    Why such a large simulation time for 55 kHz(1s)?

    Comment: A much better waveform can be achieved if you split the feedback resistor in two (33k=23k +10k) and connect two diodes (antiparallel) across the 10k resistor.


    did you use PSpice to simulated the circuit? can you see any differences between your circuit and my one ?
     
  12. Feb 14, 2016 #11
    could you email me your simulated circuit ?
     
  13. Feb 14, 2016 #12

    LvW

    User Avatar

    I have simulated exactly your circuit using a realistic model for TL072.
    Why didn`t you answer my question regarding simulation time amd time resolution??
     
    Last edited: Feb 14, 2016
  14. Feb 14, 2016 #13
    the time i used was 1 Second
    upload_2016-2-14_21-4-47.png
    but time resolution i have no idea.
    upload_2016-2-14_21-5-42.png
     
  15. Feb 14, 2016 #14
    the run time should not change the frequency
     
  16. Feb 14, 2016 #15

    LvW

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    OK - if you know better I have no problems.
    It seems you are very familiar with simulation programs.

    I have again removed my explanation for the frequency variation in my answer#12.
    I see no necessity to give you hints if you know better.
     
  17. Feb 14, 2016 #16
    but i dont understand what is wrong with my circuit, why is it not getting 55 kHz
     
  18. Feb 14, 2016 #17
    i have chanced the run time to 1m the freq is now 21 kHz
    upload_2016-2-14_21-35-13.png
    but where can i change time resolution ?
     
  19. Feb 14, 2016 #18

    NascentOxygen

    User Avatar

    Staff: Mentor

    What is the DC value of your circuit's output, and what value were you expecting it to be?

    What pk-pk amplitude are the oscillations you are seeing?
     
  20. Feb 15, 2016 #19
    i have no idea how to work out the amplitude of the waveform
     
  21. Feb 15, 2016 #20
    upload_2016-2-15_14-55-14.png
    why when the stop time is change dose it effect the frequency.
     
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