low frequency oscillator with air capacitors?

Discussion in 'The Projects Forum' started by 1680sean, Jul 23, 2012.

  1. 1680sean

    Thread Starter New Member

    Jun 24, 2012
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    I'd like to control the frequency of a low frequency sine oscillator with air core variable capacitors. Is there a type of sine oscillator circuit that could be controlled by such low value capacitors at very low frequencies? Big resistors? capacitance multipliers? I haven't figured out the values of the air capacitors in the old radios i got. One radio is an atwater kent model 55c and it has a triple air core. The other radio is much smaller and has two sections with different numbers and sizes of plates. I assume they're all under 500p. Can I somehow do this?
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,420
    3,355
    Variable capacitors from old radios are typically around 300 to 500pF at their max.
    You will not be able to get this to adjust low frequencies. Do the math
    Z = 1/ωC
     
  3. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    My Wien Bridge audio oscillator has a dual 365pF variable air capacitor. For the 10Hz to 100Hz range its resistors are 20M I think (or maybe 40M).
    Its opamp is a TL072 which has extremely high input inpedance because it has Jfets.
     
  4. 1680sean

    Thread Starter New Member

    Jun 24, 2012
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    Ok, I'm using tl082, which is close enough, right? I've built an audio range wien bridge oscillator with it. I'm trying to get as close as i can to 1hz. Will I run into issues as I bring the resistance way up?
     
  5. takao21203

    Distinguished Member

    Apr 28, 2012
    3,577
    463
    Yes issues will arise. The large air capacitor from the old radio will pick up EMI easily having resistors in the 10s M Ohm range inside the circuit.

    I made a SMPS using a small control transformer, which is so sensible, if I turn on my soldering station, it will actually lock up and saturate.

    And this does not even need resistors in the K Ohm range.

    Try to place some electric appliances near the circuit, and switch them ON/OFF.

    I guess it's for research, not professional application?
     
  6. 1680sean

    Thread Starter New Member

    Jun 24, 2012
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    Yes, this is for research. I can get a sine wave off my laptop if I need it. I'm trying to design something clunky and dated that can do the same thing. what if i tried shielding the device with copper or aluminum sheet metal? I kind of like the Idea of having to turn off nearby appliances but this oscillator is meant to be used with my ring modulator, so maybe some sort of shielding is in order? No size restrictions. Or perhaps I should just find something else to use these caps for.
    edit: there is already shielding between the cap sections.
     
  7. #12

    Expert

    Nov 30, 2010
    16,252
    6,750
    Filtering out interference from a 1 Hz oscilator is not all that difficult. The interference will all be at much higher frequencies.

    Perhaps you could examine a Theramin circuit. They use a beat frequency to get the frequency of interest and use air as the dielectric.
     
  8. 1680sean

    Thread Starter New Member

    Jun 24, 2012
    10
    0
    Thanks for the ideas, everyone. As far as filtering, do you mean passive rc low-pass? I've simulated some beat frequency based sine waves using a ring mod, so I understand the math behind it.
     
  9. #12

    Expert

    Nov 30, 2010
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    Yes. That's what I meant when I said it's not all that difficult.
     
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