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Phase Shift Oscillator Distortion

Discussion in 'Electronic Design' started by David White, Mar 4, 2012.

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  1. David White

    David White Guest

    I have been studying phase shift oscillators, and built a few up
    today. The circuit linked below has a slight problem I can't figure
    out.

    It is intended to output 10Hz, but the tops of the sinewaves are
    slightly skewed to the left, and there is a "kink" as they approach 0
    volts before progressing to the next cycle.

    http://www.4shared.com/photo/mtkJplbI/Phase-Shift-Oscillator.html?

    If I crank the trimmer back to just before the oscillation dies,
    everything comes right, but the amplitude drops and wavers around
    (unstable). There does not appear to be any in-between adjustment.

    I have tried substituting components, to no avail. Can anyone please
    explain what is happening, and if there is a fix?

    Does it perhaps have to do with the relatively low frequency involved?

    Thank you,

    David White
     
  2. miso

    miso Guest

    Is this supposed to e a known working circuit. It looks to me like the
    battery connects right to the load through a diode. I would be
    suspicious of this circuit since the standard 1n4002 is drawn like a zener.

    Note where you got the circuit and never go there again.
     
  3. David White

    David White Guest

    OK. Is this solved by adjusting the R3 and R4 ratio? What would you
    suggest?
    Would this be particularly pronounced at low frequencies? The waveform
    irregularity seems to even out as things go higher.
    The "1N4002" is not a zener, in spite of the symbol. Just diode
    protection. Couldn't find a regular diode in the DipTrace library. At
    least it's free.

    BTW the "470R" is just a resistive load ... checking to see how much
    the circuit will drive.

    Thank you for looking at my circuit.

    David White
     
  4. Phil Allison

    Phil Allison Guest

    "David White = TROLL "


    ** FFS - it is only loads the damn BATTERY !!!!


    ** You are an utter imbecile.

    Piss off.
     
  5. David White

    David White Guest


    As you point out, I drew the circuit wrong. Sorry folks. The top of
    the load resistor goes to emitter Q2.

    Here is the corrected version:

    http://www.4shared.com/photo/1Y6WR3dP/Phase-Shift-Oscillator.html?

    My original questions still apply.

    **********

    I have been studying phase shift oscillators, and built a few up
    today. The circuit linked below has a slight problem I can't figure
    out.

    It is intended to output 10Hz, but the tops of the sinewaves are
    slightly skewed to the left, and there is a "kink" as they approach 0
    volts before progressing to the next cycle.

    If I crank the trimmer back to just before the oscillation dies,
    everything comes right, but the amplitude drops and wavers around
    (unstable). There does not appear to be any in-between adjustment.

    I have tried substituting components, to no avail. Can anyone please
    explain what is happening, and if there is a fix?

    Does it perhaps have to do with the relatively low frequency involved?

    **********

    Any advice on these points would be appreciated.

    David White
     
  6. Phil Allison

    Phil Allison Guest

    "David White"

    ** Replace D1 with a link and see what changes.

    You are still an imbecile.
     
  7. Tauno Voipio

    Tauno Voipio Guest


    To get pure signal from the oscillator, you need some kind
    of low-distortion amplitude control, a kind of automatic
    electronic version of you tuning the gain resistor. For
    clean stable oscillation, you need a gain of exactly 1
    around the amplifier-phase shift loop.

    One source of distorsion is the resistor-zener-LED chain
    in the output. Pick it off and re-check the signal.
     
  8. Phil Allison

    Phil Allison Guest

    "Robert Bare Faced Liar"

    ** No fooling ..........

    ( Apologies to FZ )




    ..... Phil
     
  9. Tauno Voipio

    Tauno Voipio Guest

  10. Jamie

    Jamie Guest

    Another spam site... all garbage on the screen and nothing there that I
    can see that has anything to do with what you are talking about.

    Just another garbage dump site.

    Jamie
     
  11. Jamie

    Jamie Guest

    I can offer you a modification but you'll need to add an additional
    stage on the end of all that if you need a large output swing..

    If you intend to keep the selected frequency of design around 10hz
    do the following.

    Change R6 to 10k and place a cap of 1.5uf at the base of Q2 to
    common. Also reduce R5 to 3.3k to help in the loss.

    That network will give you a Q of 1 and supply a better symmetrical
    wave to the base.. This offsets the base drive from what it was to ~
    *0.707 now. You'll get a little less than 7v at the base and - one diode
    drop at the emitter.

    All of this breaks down to around 6V Pk-Pk DC on the final out come
    and it'll give you a nice looking wave form.


    jamie
     
  12. David White

    David White Guest

    And so it does. Yes, the gain was the problem. Thank you for taking
    time to provide this learning experience.

    David White
     
  13. miso

    miso Guest

    David White
    There is a circuit there. Beware of a 4-way way. I had some trouble
    seeing the feedback. I make it a rule to never make a 4-way connection
    on a schematic, even if it makes the circuit look messy.
     
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