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What is a lock-in amplifier with a forcing grid ?

  1. Aug 25, 2011 #1
    What is a "lock-in amplifier with a forcing grid"?

    The context of the title is in an undergrad paper, concerning plasma waves in an glow discharge column.

    The paragraph which mentioned this is in the section of the paper, Suggestions for Further Research:

    The most important step remaining to be done is to get a conclusive estimate
    of the wavelength. A rough estimate of k was obtained, but a steel
    cylinder attached to the anode to shorten the length of the discharge column
    would allow to measure the variation in frequencies with tube length.
    Experiments using a lock-in amplifier with a forcing grid and a capacitive
    antenna have been used to measure the wavelength.​


    The abstract of the paper, "Discrete Modes in an Glow Discharge Column" if needed:

    The frequency landscape of waves in the ion acoustic range of frequencies
    (IARF) was studied as a function of voltage and pressure in a constricted DC
    glow discharge plasma. Capacitive antennae were used to measure density
    and temperature oscillations in the plasma. Four different gases (helium,
    neon, argon and air) were examined. Discrete modes, with harmonics up to
    n = 26, were observed at frequencies in the ion acoustic range, 1-100 KHz.
    The optical spectra emitted by the discharges show that air contamination
    is a possible cause of the wave. For some pressures and voltages, the normal
    modes transition to chaos though a period doubling route. The possibility
    of an ionization wave parametrically decaying into other ionization waves or
    into an ion acoustic wave was explored by adding a small audio frequency
    (AF) component to the high voltage. Sideband frequencies around each naturally
    occurring discrete harmonic were observed. The turbulence caused
    by parametric excitation is compared with the period-doubling phenomena
    leading to chaos. The AF component succeeded in stabilizing a semi-chaotic
    signal for a small window of the driving frequency. Thresholds for driving
    the plasma with the AF component were measured​

    Thanks.
     
  2. jcsd
  3. Aug 25, 2011 #2

    jim hardy

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    Re: What is a "lock-in amplifier with a forcing grid"?

    I am going to venture a GUESS here ---

    "Lock In" amplifier i would suspect might mean same thing i learned by name of "Phase Locked".

    See if a google on term "Synchronous Demodulation" opens any windows in your analysis.
    Here's a device used for such applications:
    http://www.analog.com/static/imported-files/data_sheets/AD630.pdf
    its datasheet is informative on the technique

    and here's the guys who "wrote the book" on phase locked loops:
    http://www.scribd.com/doc/55127135/1972-Signetics-PLL-Applications
    (at least it's the same book where i learned about them - still have my ancient hardcover copy someplace)

    From the tidbits you copied it sounds like they're measuring standing waves in a tube of ionized gas. That'd be a logical approach, capacitive coupling through glass walls of the tube.

    But that is a guess on my part. Reason for my guess is this - way back in vacuum tube days we did something similar in my high school electronics class to observe standing waves in an air dielectric transmission line.

    Perhaps somebody closer to today's technology will have better advice for you.
    old jim
     
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