LC oscillators

Discussion in 'General Electronics Chat' started by bearblock, Oct 24, 2008.

  1. bearblock

    Thread Starter Member

    Oct 24, 2008
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    Hi,

    I'm having some trouble getting an LC tank oscillator to work. Basically I need to wind some inductors for a theremin project but I don't have an LC meter... Since LC meters are fundamentally just an LC osc and a frequency counter (AFAIK) I thought I could just breadboard a simple tank oscillator circuit, measure the frequency and plug it into the resonant frequency equation.

    Unfortunately I can't get anything to oscillate i.e. I can't detect any useful amounts of AC with my multimeter. Specifically I'm thinking of Question 9 on this worksheet:

    http://www.allaboutcircuits.com/worksheets/opamp10.html

    TL70x is an OK part here, right? The nominal resonant frequency of the tank network I'm testing it with works out at just under 9KHz which I don't think is too fast for this opamp. What would be appropriate resistor values to make it work? Is my breadboard just broken? (possible)

    Thanks for reading! :)
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    Try posting a schematic, we can't even begin to help unless we have a starting point.
     
  3. bearblock

    Thread Starter Member

    Oct 24, 2008
    18
    1
    [​IMG]

    Here's the image from the link in my first post. No oscillations with a 10K resistor and pot...
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    What freq are you aiming for?

    **********************

    I'll assume 9Khz, since you mentioned it ealier.
     
    Last edited: Oct 25, 2008
  5. Wendy

    Moderator

    Mar 24, 2008
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    Give us a discription of the values, and how you made the coil (physical dimensions).
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    I whipped up the attached simulation using an LF353, which is basically the equivalent of a TL072.
    Note that R1 shows 90%, which means that the side to ground is 1k, and the feedback path is 9k. With the values shown, it oscillates at around 1.6kHz.

    10mH is a pretty large inductor. For an air core inductor, one possible configuration would be a 1" bobbin that has 1/4" space between the flanges, and wind on 466 turns of AWG 30 magnet wire. That would take nearly 150 feet of wire. Of course, changing wire gauge, bobbin diameter, or flange spacing would drastically change the inductance, as would adding a ferrite core.
     
  7. bearblock

    Thread Starter Member

    Oct 24, 2008
    18
    1
    Hi Bill,

    At the moment I'm using a commercial 22μH inductor and a 15μF cap. The idea was to get it working with a coil of a known value, then switch them for my home-made efforts. There's no special reason why I need 9KHz, that's just what the formula gives as the resonant frequency for this cap and inductor (8761Hz actually) so I need to be seeing AC at close to that frequency before I can trust my coil measuring 'system'.

    One thing I don't understand at all is how to choose the size of the resistors. I know they don't directly affect the frequency but will certain values only work over certain frequency ranges?

    Thanks!
     
  8. bearblock

    Thread Starter Member

    Oct 24, 2008
    18
    1
    Thanks for the simulation Sarge! That does sound like a huge inductor, but I have a whole spool of magnet wire here so I could try and make one.

    How much can you change the L and C values and still expect it to work? For instance, a 100μF cap and 100μH coil have the same resonant frequency as the 1μF and 10mH in your example... on paper anyway, but are there other factors you have to take into account in the real world?
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    Yes, there are other factors - but I'm really tired at the moment.

    I plugged in your values; it wouldn't run in the simulation either. Decreasing C and increasing L works, though. 100uH won't be enough. You really need to increase it quite a bit. I started off with 50mH, which worked pretty well.

    With a Theremin, you'll be working with very low values of capacitance.
     
  10. Wendy

    Moderator

    Mar 24, 2008
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    Thinking about it, you might also incorporate a light bulb, similar to how a Wien Bridge Oscillator does it. Think of it as an AGC.
     
  11. bearblock

    Thread Starter Member

    Oct 24, 2008
    18
    1
    no probs - you've been very helpful already. looks like I might just have to acquire an LC meter. I've never worked with inductors before (can you tell?!) so I should have expected things to be a bit more complex than they first appear...
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    OK, if you reduce R1 to 1k Ohms, it should oscillate with your inductor.

    If you reduce it to 500 Ohms, the circuit will experience "phase reversal" aka "latchup"; the negative swing on the noninverting input will fall down to the negative rail due to the reactance of the tank circuit, causing the output to reverse phase.
     
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