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L oscillator stability

Discussion in 'Electronic Design' started by J. Hunter, May 23, 2005.

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  1. J. Hunter

    J. Hunter Guest

    Hi !

    I'm using the oscillator part of a 74HC4060 in a home made inductance
    meter. One cap to ground at both input and output of the CMOS oscillator
    gate. The inductance to be measured between the input and output in series
    with a reference inductance of 10uH. A microcontroller reads the frequency
    from one of the output of the HC4060, calculates the inductance value and
    displays it on a LCD screen. It should read between 1uH (129Mhz) to about 1H

    It seemed to work well until I found out I has a slight oscillator
    instability when measuring a 4700uH coil.

    The oscillator runs about 60Khz with that inductance. If I connect the coil,
    it can oscillate at say 58Khz and stay at that frequency. If I disconnect
    and reconnect the coil, it ma oscillate at a sligthly higher or lower rate
    and stay at that new frequency. Weird!

    I experimented a bit to correct that problem and found out that connecting a
    small network in series with the indcutance to be measured (a 100 ohms in
    parrallel with a 1uF, values not critical) stabilizes the reading.
    Smaller inductance values are unaffected by this network. Higher inductance
    values, I don't know yet...

    My questions are:

    What could be involved in the instablity?
    Is my solution (which is empirical) a good one?
    Is there a better solution if any?

    I know this is lot of questions.

    J. Hunter
     
  2. Not really, it is simple.
    Th inductance is involving an area. The connecting
    wires are part of the enclosed area.

    Rene
     
  3. J. Hunter

    J. Hunter Guest

    I don't quite understand as the setup is the same from one reading to the
    next. Even with few uH coils, it is stable. Wiring should be less of a
    problem with higher inductance values.
    Shouldn't be...?
     
  4. Yes. Have you looked at the waveform with an oscilloscope? Maybe the
    distributed capacitance of that relatively high value is causing some
    problems-- you might see high-frequency hash during parts of the
    oscillator wavform, for example, so you could have an oscillator on
    top of an oscillator.


    Best regards,
    Spehro Pefhany
     
  5. J. Hunter

    J. Hunter Guest

    Hi !
    That's a good point. I'm gonna check this very carefully.
    If this is the case, maybe the small RC I'm adding in series lowers the
    oscillator gain.

    J. Hunter
     
  6. Rich Grise

    Rich Grise Guest

    Good Answer. I wanted to say "Parasitics," and I wanted to suggest hanging
    a couple of feet (1/2-1 meter) of wire off one or the other end of the DUT
    and wave your hands around, to see if what you actually have is a theremin.
    :)

    Good Luck!
    Rich
     
  7. Rich Grise

    Rich Grise Guest

    If it works, leave it. There's nothing wrong with empirical design. The
    PHDs want to wrap higher-order equations around everything, but if it
    works, it works. :)

    Especially if it's repatable, and especially especially if you're getting
    the "right" reading while poking around the circuit with your fingers. :)

    Cheers!
    Rich
     
  8. john jardine

    john jardine Guest

    [clip for brevity]

    The way I read it, you have the inductor(s) wired in the same position as
    the oscillator crystal would normally sit.
    I was initially going to reply that it could not possibly oscillate at low
    frequencies due to loss of phase shift in the fixed caps. Then realised I
    was assuming a cap in // with the inductor. Then realised there wasn't one.
    Then realised series LC resonance would be needed anyway . Then realised
    stray C around the L's would give a number of additional oscillation modes.
    Then figured the inverter output impedance and input limiting diodes will
    also give trouble.
    Then gave up and simmed it :).
    Surprisingly it can work all the way upto those 1H inductors but you now
    have an oscillator that readily insists on oscillating but for all manner
    of awkward reasons. AC voltage levels vary all over the place as the
    frequency varies and as the other guys mention, stray capacitance can have a
    sizeable frequency effect even with the big inductors (the circuit will
    "squegg").
    The circuit functions but not really well enough as a measuring meter.
    It's far better to resonate the Ls against a known, fixed, decent sized, C
    value. Use the arrangement in the original AADE meter and it's clones
    (LM317?) and -then- feed into the counter chip for subsequent counting. Even
    then, there will be problems trying to get 'low Q' coils to resonate.
    A final solution needs more complex oscillator electronics, resulting in an
    even bigger headache than wading through those resonant frequency sums
    inside the micro :)
    regards
    john
     
  9. doug dwyer

    doug dwyer Guest

    Your system has more than one frequency which meets the zero phase+ loop
    gain ^1 criteria.
    The oscillator has a varying gain with amplitude as it goes into
    limiting so once its going it will lock also oscillation builds per
    cycle so that the higher frequency if the gains the same will build
    first.
    All this means reduce capacitive coupling due to the self capacity on
    the inductance best done by reducing the termination resistance both
    sides on the inductance, this probably means a better oscillator.
    Finally You want most or nearly all the oscillation current through the
    inductance so it could usefully be placed in one arm of an AC bridge
    with a compensating capacitor and resistor in the other arm and set this
    into the oscillator circuit.
     
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