Weird Wien bridge oscillator behavior: Bad dual potentiometer issue?

Discussion in 'The Projects Forum' started by Félix Thouin, Feb 6, 2016.

  1. Félix Thouin

    Thread Starter New Member

    Aug 14, 2015
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    1
    Hi all,

    I am a physics student who started to dabble in electronics two years ago. I have built myself my own lab tools including a wicked cool power supply (at least I think so) and a low distortion sine wave generator.

    The later was based on the circuit of figure 3.4.11 of http://www.learnabout-electronics.org/Oscillators/osc34.php. It worked (38 dB of contrast between the FH and the SH according to the FFT on my scope) very well but was limited in bandwidth because of a poor opamp choice (a 741: I realised ''industry standard'' does not mean ''appropriate for all circuits''). I also used capacitor rated for 1.6kV (for some reason I still can't figure out) and an unsuitably very slow rectifier diode rated for 1.5A.

    I thus decided to upgrade my trusty sine wave generator now that I had a better understanding of opamps, FETs and diodes. I changed the op amp to a TL081, the diode to a 1N914BCT, and the capacitors to acceptable voltage tolerances. I also tried to reduce the number of long wires in my enclosure and reduced the time constant of the AGC circuit to match that of the aforementionned reference....

    The result was very disappointing: the oscillator only works when the dual potentiometer is in one of its extreme positions! If it is in one of these positions, I can instantly change the frequency by changing the values of the capacitor pairs (using a switch) without a problem. However, as soon as I change a tiny bit the position of the pot, the oscillations stop!

    The reference suggests that a badly matched dual potentiometer may cause these problems. How bad is bad? How can I know if I have a decent enough potentiometer? Is there any you would suggest?


    Thanks a lot!
     
  2. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    A schematic could save lots of words, I mean the one you actually implemented.

    Any pictures?

    /Edit
    Maybe one of the pots is wrongly connected.
    Edit/
     
  3. Picbuster

    Member

    Dec 2, 2013
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    make sure that the gnd is also the gnd for + and 1 power supply.
    http://www.eeweb.com/blog/circuit_projects/op-amp-based-wien-bridge-oscillator
     
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
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  5. Félix Thouin

    Thread Starter New Member

    Aug 14, 2015
    6
    1
    Hi all,

    Attached is the EagleCAD schematic I made for this project.

    The power supply and oscillating circuit do indeed share the same ground. The power supply is a linear regulated one with a 7805 and 7905.

    I hardly see how I could have messed up the wiring of the pots. These devices are symetric around the wiper pin right?

    Are dual ganged capacitors more accurate than potentiometers? For some reason, I always thought they would be less reliable than potentiometer: people talk about them so very little.
     
  6. Félix Thouin

    Thread Starter New Member

    Aug 14, 2015
    6
    1
    FIXED IT!

    Despite having the same nominal value, the 10K resistor I used had a low value (9.7K), which caused the gain to be just above 3 (3.04). This did not allow oscillations to start in many cases. I added a trimmer pot in series with it and that solved the issue.

    Thanks a lot for your help! It was very appreciated.
     
  7. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    Congratulations.
     
  8. atferrari

    AAC Fanatic!

    Jan 6, 2004
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  9. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    atferrari likes this.
  10. Veracohr

    Well-Known Member

    Jan 3, 2011
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    I've been doing some tests on oscillator circuits including Wien Bridge in the audio range. I found that the oscillation is somewhat unstable when adjusting a dual pot (20k in my case, it's the only dual pot I have). I tried 3 different variations and they all had trouble with the pot.

    If you're interested I have a different sine wave oscillator design, based on all-pass filters, that is more stable with a dual pot. It uses a few more components though, and I haven't yet looked at the distortion characteristics of any of the circuits I've tested.
     
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