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How can I set a frequency in a Wien bridge oscillator

Discussion in 'General Electronics Discussion' started by tburn, Apr 30, 2014.

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  1. Ratch

    Ratch

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    Mar 10, 2013
    Jeri says you are welcome.

    Ratch
     
    tburn likes this.
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    When researching lamp stabilised Wien bridge oscillators some time ago I came across a great article from HP describing issues they had in designing the original oscillators they made.

    They found that some oscillators were less stable than they should have been. Perversely, these were the ones with the best distortion figures. What they discovered was that it was the small non-linearities in the amplifying element that contribute to stability. I found this after making a rock-steady lamp stabilised sine wave oscillator using a cheap rubbishy op-amp, only to have the circuit become unstable and largely unusable when an ultra linear, low distortion, gold plated, hand assembled by nubile virgins, purchased at the cost of your first born child style of op-amp was transplanted into the circuit.

    I have a hard copy of that article at home, I can pull it out as an interesting reference as to why you might actually prefer to use "lame" parts in your Wien bridge oscillator.

    And curiously, after watching that video (I'm pretty sure I've watched it previously) we're actually talking about similar things.
     
  3. Ratch

    Ratch

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    So the inferior op amp had the right kind of nonlinearities that contributed to better stability in the circuit, and the circuit needed more or different nonlinearity than the lamp was able to provide when the superior op amp was used. Just speculating. Anyway, the article sounds interesting.

    Ratch
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Whilst I'm pretty sure this 1960 article from HP is going to go way beyond what tburn wants or needs, some of us might find it of interest.

    The most important conclusion is in the second paragraph:

    "It is shown that amplifier non-linearity so slight as to cause insignificant waveform distortion has a profound effect on envelope stability. In fact, if it were not for this slight non-linearity, it would be virtually impossible to build a simple lamp-stabilized RC oscillator with good stability over a wide frequency range."

    And it goes on to relate the correlation with low distortion and envelope instability seen in practice.
     
  5. Ratch

    Ratch

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    Mar 10, 2013
    That's a lot of heavy math and feedback control. Only a lamp was analyzed. What about other forms of AGC, such as back-to-back diodes, FET's, bridges, etc? Looks like making a Wien with very low distortion is not easy.

    Ratch
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    It is very interesting. My own tests indicate that the "modulating" frequency (the envelope instability) is related to the time constant of the lamp.

    If you use any other control device, there may also a time constant (such as a capacitor maintaining a voltage on gate of a jfet). I suspect that similar behaviour might be exhibited using such control elements, but I have not investigated it.

    As for other control elements (like back-to-back diodes) I can't comment other than to suggest that these don't produce a high purity sine wave.

    Incidentally the first op-amp I used was a reasonably well performing one and I did note that the Wien bridge took maybe a few seconds to stabilise after changing frequency, powering up, etc. I assumed that a better op-amp would improve this (and that lead me down a very interesting rabbit hole. (And I didn't expect to find WIlliam Hewlett and Charles De Gaulle along the way)
     
  7. LvW

    LvW

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    Apr 12, 2014
    It depends on the sign of the temperature coefficients.
    Your goal must be to LOWER the gain (which at t=0 must be larger than "3") for rising amplitudes leading to rising temperatures..
    That means either R2 must be INCREASED (light bulb) or R1 must be DECREASED (NTC thermistor).
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Getting back to the original question of setting the frequency, I found this page interesting when I was building up my oscillators.

    This page covers a heap of practical issues and explains how the oscillator works. It also suggests a method of frequency control (over a limited range -- a decade) by varying a single resistor.

    Of note is that the op-amp is given a beefier output stage to allow it to drive the low resistance feedback path (the part where the lightbulb is).
     
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  9. Ratch

    Ratch

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    Mar 10, 2013
    And here is another circuit that uses two op amps and one tuning resistor. A better AGC could be substituted for the two back-to-back diodes.
    Wien.jpg

    Ratch
     
  10. LvW

    LvW

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    Apr 12, 2014
    I think, the method as proposed in the referenced paper is very useful - in particular for fine tuning of the oscillator. That means, when the required tuning range is not too large (e.g. less than 10...20%).
     
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