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Question about Resonance

  1. Oct 31, 2004 #1
    Hi, I have a general question about resonance (specifically electrical). I have been working on a device dealing with this and I thought I might make sure I got the basics right before I continue. I have two scenarios that I'm hoping someone can help me out with:

    1. There is a basic oscillator circuit (ex: a battery, coil, and tunable capacitor). The battery of course powers the oscillator circuit and keeps it going at a set frequency. Now, if there is an outside electromagnetic source that is resonating at the same frequency as the inductor coil, would this outside source add power to the oscillator since they are both in resonance with each other?

    2. There is another basic oscillator setup, only this time; there is only a capacitor and a coil (no battery). Again, we have an outside electromagnetic source that we want to power the oscillator inductively. Now, assuming that this source by itself is not strong enough to excite the coil (there is no antenna, just the coil), if one wiped a magnet across the oscillator to get it moving (remembering that it is tuned to oscillate at the same frequency as the outside inductive source), would the oscillator keep oscillating at the frequency of the outside source since I have it some energy to start it going?

    In essence, my question is can an oscillator be powered by a weak inductive power source if it has been pre-excited to resonate at the same frequency as the power source? I have more questions about this but I'll leave it here for now. Any help is much appreciated.

    Jason O
  2. jcsd
  3. Nov 1, 2004 #2


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    Staff Emeritus
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    You'll need an active device of some kind as well as a battery and coil to make an oscillator.

    An outside source will tend to "phase-lock" the oscillator if it's close to the same frequency as the oscillator, due to stray coupling.

    A capacitor and a coil would be best described as a tuned circut rather than an oscillator. Stray coupling (capacitive or inductive) will induce a voltage in your tuned circuit even without an antenna, the amount of the voltage will depend on many factors, such as the amount of coupling, the "Q" factor of the resonant circuit, etc.

    Wiping a magnet across the tuned circuit won't do much - it's not an oscillator, it's just a tuned circuit. The changing magnetic field will induce some voltage across the inductor, causing the resonant circuit to "ring" a bit. You can think of a spring-mass-damper system in basic physics, the response will be similar - if you know how a spring-mass-damper system behaves, that is (I don't know your bakckground).

    Not really. Your "oscillator" is just a tuned circuit if it doesn't have a power source and an active device to provide gain.
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