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Resonance confusion

Discussion in 'Electronic Design' started by Sim Joo Khai, Jul 24, 2003.

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  1. Sim Joo Khai

    Sim Joo Khai Guest

    A piece of specially-designed metal strip assembly resonates at 58 kHz. If I
    have an LC tank of resonant frequency 58 kHz too and I move the material
    near to the tank, will the potential difference across the tank increase or

    Please help me with this question. I strongly assert that it is NOT a
    homework or whatever crap question. It is for the design of an EAS
    tag-activated air conditioning access controller.

    Thanks in advance.

    Sim Joo Khai
    (Delete the obvious)
  2. Not much of either, I think. The metal movement will alter either the
    total inductance or capacitance or both of the tank, but those changes
    will go through a complete cycle once every electrical cycle, so will
    have little net effect on the fundamental frequency. I think the
    strongest effect will be that the second harmonic of the tank
    frequency will be increased.
  3. I would expect an initial decrease in the sense tank, but then, due to
    inevitable tunning differences, there would be beats as the energy
    sloshes between the tanks. To make matters worse, there will be doppler
    effects according to relative velocity.

    So why not just put a detector, differentiator/high-pass filter and
    amplifier on the sense tank to detect changes in power level?
    I hope whomever has the magic tag doesn't try to go shopping, lets he
    set off shoplifting detectors wherever he goes =)



    DIY Piezo-Gyro, PCB Drill Bot & More Soon!

  4. Jim Thompson

    Jim Thompson Guest

    Sounds like a homework problem ;-)

    But if I really wanted to pick up a signal from a resonant metal strip
    I'd make the strip part of the tank *capacitance*.

    ...Jim Thompson
  5. R.Legg

    R.Legg Guest

    You don't make explicit what the intended coupling mechanism is.

    There is no first-order coupling mechanism between an isolated
    mechanical part and an isolated electrical tank circuit.

    Does tank current flow in the mechanically resonant part?

    Does the tank circuit L have an open magnetic field?

    Is the mechanical part composed of a magnetic material, or even
    conductive material? You say metal, so perhaps we could assume the

    What is the basis for active resonance in the mechanical part?

  6. Tom Bruhns

    Tom Bruhns Guest

    I don't think you have given enough information. As I see it, you
    have two resonators (one electrical, one mechanical) which may or may
    not be coupled. If they are coupled, I suppose it is through the
    magnetic field. In addition, metal in the vicinity of the electrical
    resonator will likely change its resonant frequency and lower its Q.

    I can imagine in one case, with the mechanical resonator excited and
    magnetic and the electrical resonator not excited except through the
    mechanical one that the potential across the electrical resonator
    would increase with increased coupling to the mechanical resonator.

    I can imagine in another case, with the electrical resonator excited
    and the mechanical resonator loaded (low Q) that as you brought the
    mechanical resonator close to the electrical one, it would remove
    energy from the electrical one and possibly decrease the potential
    across it, unless it was excited from a voltage source. It would be
    rather like a "grid dip meter".

    Why not simply do an experiment in the configuration you have in mind
    and see what happens? Develop your model from measurements you make
    on your experiment.

  7. If you're talking about one of these store security devices, then I
    would imagine it *draws* energy from the tank (which forms part of the
    visible loops one walks through) thereby *lowering* the potential
    across the tank in the same manner as a high-impedance grid dip
    oscillator functions to show resonance.
    Caveat: I am not an Engineer so this is just a hunch.
  8. Jim Thompson

    Jim Thompson Guest

    The cheap tags are nothing but a loop plus a diode. Emersed in a
    field they produce second harmonic which is detected.

    ...Jim Thompson
  9. Marc H.Popek

    Marc H.Popek Guest

    thats the ticket if you want some vibrational resonant gain for sure.
  10. Are you sure, Jim? That implies UHF and I was under the impression
    that these devices worked at about 9Mhz.
  11. I don't know about the frequency, but I believe Jim is correct.
    Try taking a diode hooked to a wire hoop through one. Worked
    for me a while back. Also, it makes sense that they'd need
    "real cheap" units to apply to the tens of thousands of items
    which go through the doors of some shops. Can't be complicated.
    And most folks don't go carrying diode-loops as jewelry. So it
    mades sense.

  12. It's a capacitance diode with a tuned dipole, I gather. Take a
    look at one of those very carefully and peal the parts away from
    each other. When I do so, I've gotten a pair of foils. Not
    just one.

    Anyway, read:

    I think this will help some. Figure 3.5, for example.

    (In my case, I just tried a bunch of different small loops.
    Some seemed to work, some didn't. I was just curious, but not
    willing to spend a lot of my friend's time, so that was all I

  13. John Larkin

    John Larkin Guest

    Some of these are a strip of permalloy or something, magnetized. These
    are often used in books. Exposed to an AC magnetic field at the door,
    they generate a second harmonic field which the gadget picks up. They
    are deactivated at the checkout by an AC demagnetizer. A video tape
    will often set off the detector by accident.

  14. That explains why a lot of shoplifters these days are taking
    aluminium/aloominum foil into shops with them. They presumably wrap
    the stolen goods in the foil and thereby 'foil' this security measure,
    if you'll excuse the pun.
  15. Ben Bradley

    Ben Bradley Guest

    In, Paul Burridge
    Oh, so a lot of plain old, ordinary-looking kooks are actually
    shoplifters in disguise:
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