Amplitude modulation in Wienbridge oscillator

Discussion in 'The Projects Forum' started by hrs, Sep 25, 2015.

  1. hrs

    Thread Starter Member

    Jun 13, 2014
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    Hi,

    I made a Wienbridge oscillator that is almost the same as the one I found >here<. It has two frequency ranges. The high frequency range (460 - 67500 Hz) uses 22nF polyester film box capacitors and it works fine. The lower range (30 - 4750 Hz) uses 330nF
    multilayer monolithic ceramic capacitors, in case that matters. It's unstable from about 80 Hz and up where it shows some kind of amplitude modulation. Two pictures of this at the same frequency but different time scales are attached as well as the circuit diagram that I used.

    I noticed that when I heat up the ceramics capacitors with my fingers the modulation slowely disappears. Also, when starting the circuit from cold at a problematic frequency it takes a few seconds before modulation becomes apparent.

    Does anyone know what might cause this? Are ceramic caps a bad choice for this and/or known to be thermally unstable?

    amplitude1.jpg amplitude2.jpg diagram.png board.jpg
     
  2. Lestraveled

    Well-Known Member

    May 19, 2014
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    What frequency is the modulation and is it stable?
     
  3. BR-549

    Well-Known Member

    Sep 22, 2013
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    Fluorescent lights.
     
  4. hrs

    Thread Starter Member

    Jun 13, 2014
    82
    7
    The modulation becomes stable after a few minutes in that the amplitude variance doesn't change anymore. But the frequency of the modulation doesn't appear to be stable or fixed if that is what you mean.

    It's a bit of guess work but here's what I found:
    Code (Text):
    1. Sine wave frq                     modulation frq
    2. 1390 Hz                             44 Hz
    3. 4760 Hz                             22 Hz
     
  5. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Does it use an incandescant lamp for feedback regulation? If so, it could be a time constant issue with the lamp or associated circuitry
     
  6. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Ahh...yes it does, "L1". Try a few different lamps here and see if that helps. You might try a couple in series or parallel. They should be BARELY glowing in normal operation
     
  7. Lestraveled

    Well-Known Member

    May 19, 2014
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    Since the AM is not fixed at 60 or 50 Hz, it most likely inherent to the design. Try a different set of resistance and capacitance values to get the frequency range. You can also try adding a ground plane by taking a piece of PCB stock, connect the copper to the ground, and position it close to the board. These are always interesting problems to solve.
     
  8. hrs

    Thread Starter Member

    Jun 13, 2014
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    I thought it is just a non-linear resistor that rises exponentially with temperature. Could you point me to some resource that discusses the time constant on a fundamental level?

    I should have mentioned that it runs on two 9V batteries with ground in the middle. I tried to add a ground plane but I cant get it very close to the board because of the component leads sticking out. It doesn't seem to make any difference though.

    I'll try to swap some resistors.
     
  9. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    It may be a mechanical resonance. I see the two AM freq are simple multiples and your finger may be dampening those vibrations.

    In that case a change of mass or physical support with hot glue or RTV silicon might fix
     
  10. Veracohr

    Well-Known Member

    Jan 3, 2011
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  11. hrs

    Thread Starter Member

    Jun 13, 2014
    82
    7
    I replaced the ceramic capacitors with film box capacitors. This fixes the problem.

    Thanks everyone for the tips and suggestions.
     
  12. ian field

    Distinguished Member

    Oct 27, 2012
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    Apparently the capacitance of some ceramic types can also change with applied voltage.

    With disc ceramic capacitors, there are various types, grades and temperature coefficients and the correct type has to be selected for a given application.

    AFAIK: the multilayer ceramic chip SMD variety have only "several" dielectric types to chose from.
     
  13. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    There's a certain thermal mass of the filament, which is a function of how quickly it heats up or cools down.
     
  14. ian field

    Distinguished Member

    Oct 27, 2012
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    If you view the output on a scope and swing the frequency control rapidly between the end stops - you can see the amplitude swell and dip as the compensation lamp tries to keep up.
     
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