Phase Shift Oscillator Design

Discussion in 'Homework Help' started by artmaster547, Jan 6, 2016.

  1. artmaster547

    Thread Starter Member

    Jan 6, 2016
    89
    1
    Hi
    I have a design task of creating a Phase Shift Oscillator with a frequency of 5kHz, which I have done now (using a 3 stage RC ladder network and an inverting operation amplifier (with gain of 29)), as an extension to this I would like to remove unwanted frequencies from this circuit, which are visible when viewing a FFT (fast fourier transform) any tips/guidance/circuit topologies in order to add to this oscillator would be much appreciated .

    I personally would also love to know how could I adjust the oscillation so that it occurred between two voltage this more out of curiosity
    Thanks
    Sunday
     
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    Do you have any circuit automatic gain control circuit for amplitude stabilization? And do you build this oscillator in real life?
     
  3. artmaster547

    Thread Starter Member

    Jan 6, 2016
    89
    1
    Hi at the moment this is in the simulation phase of design I am using a software called: LT Spice. That's interesting that you bring up automatic again control circuit I currently do not have this in my design will this assist in creating what I am after? Do you know any examples of topology for this?
     
  4. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
  5. artmaster547

    Thread Starter Member

    Jan 6, 2016
    89
    1
    Sorry I am a little unsure about what you mean, I have got the same structure as
    Figure 14: Phase-Shift Oscillator (Single Op Amp) in the document that you referenced
     
  6. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    Is your op amp saturated ??
     
  7. artmaster547

    Thread Starter Member

    Jan 6, 2016
    89
    1
    yeah I believe so, after analysing it closer sorry for the not understanding the first time
     
  8. artmaster547

    Thread Starter Member

    Jan 6, 2016
    89
    1
    will using automatic gain control be my solution also just to let you know I have a power supply of 9V hope that helps
     
  9. atferrari

    AAC Fanatic!

    Jan 6, 2004
    2,648
    764
    Could you post the .asc file of your test in LTSpice?
     
  10. artmaster547

    Thread Starter Member

    Jan 6, 2016
    89
    1
    yes I will post it give me a couple of minutes
     
  11. artmaster547

    Thread Starter Member

    Jan 6, 2016
    89
    1
    I think this is the correct file
     
  12. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
  13. DickCappels

    Moderator

    Aug 21, 2008
    2,656
    632


    [​IMG]

    Good call, Jonny130.
     
  14. artmaster547

    Thread Starter Member

    Jan 6, 2016
    89
    1
    may I ask exactly what I am doing wrong should I only have one voltage input at rails? I connect the negative rail to ground? Also still unsure where to progress from there in order to remove unwanted harmonics as they will still be present I understand? Is this what you mentioned about automatic gain? Do you use a JFET for this?
     
  15. atferrari

    AAC Fanatic!

    Jan 6, 2004
    2,648
    764
    Your power supplies are a weird "pulse" pulse instead of a steady DC value (the usual way). There is clipping as you were told already.

    Some time ago when playing with different circuits I used this among many. Warning: I know there is a better (and proper way) to make the oscillator start in the simulator. I used brute force.

    Just in case be sure you do not confuse circuits with op amps powered with dual supply (+/- voltages) and those powered with a single polarity supply as below. In this case, you set Vref = +V/2.
     
  16. artmaster547

    Thread Starter Member

    Jan 6, 2016
    89
    1
    Hi thank you for providing this resource and some more explanation about supplies this aids a lot with my understanding. I understand the staggered approach of using buffers between the RC stages has an effect on the loading. Upon analysising the FFT of the this circuit I can see that that are still quite a lot of unwanted frequencies, in order to remove this should I be using filters or is there a more elegant approach I can be taken I have seen with Wein Bridge Circuits in a lot cases they employ diodes, or in others they employ JFet's but have not seen such additions to Phase Shift circuits which is why I'm having some difficulty
     
    • FFT.jpg
      FFT.jpg
      File size:
      705.5 KB
      Views:
      5
  17. artmaster547

    Thread Starter Member

    Jan 6, 2016
    89
    1
    also what method would you use in order to make the oscillation occur between two voltages i.e. V max: 4V and V min: 1V
     
  18. atferrari

    AAC Fanatic!

    Jan 6, 2004
    2,648
    764
    To shift the signal, (if the amplitude is already what you need) I would add a voltage in Vref that would bring it up as much as you need.

    Anyway, as jony130 suggested, download that .pdf and try some of those circuits, if at all possible with the same pair of R & C values. LTSpice is good to become familiar with the basics.
     
  19. artmaster547

    Thread Starter Member

    Jan 6, 2016
    89
    1
    OK yes thank you, if I did need to change the amplitude however what should I do?
     
  20. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    From where did you get this circuit. For sure this is not a classic Phase-Shift Oscillator. And your circuit oscillate "by accident".
     
Loading...