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coils, pulse trains and resonance

Discussion in 'Electronic Design' started by Matt, Jan 4, 2004.

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  1. Matt

    Matt Guest

    I have been doing some testing with finding out what the resonant
    frequency of a parallel connected L-C circuit is.

    I connected the L-C circuit to a 1k resistor which one end was
    connected to ground.

    I then supplied a sine wave accross the whole circuit.

    I then connected one trace from an oscilliscope to the top of the L-C
    circuit and the another trace from the top of the 1k resistor


    Sine wave input >---------------------------------> To Trace 1
    | |
    | ---
    Capacitor --- --- Coil
    --- ---
    | ---
    | |
    -------------------------> To Trace 2
    |
    |
    ---
    | | 1K Resistor
    | |
    ---
    |
    |
    -------------------- GND
    I compared the two traces on the oscilloscope, starting from a low
    frequency
    I started increasing the input frequency. Trace two bacame smaller and
    smaller until the point of resonance where the trace stated to
    increase in size.
    This is all normal and what I would expect to see.
    The voltage drop accross 1k decreases because less current is flowing
    through the resister due to the resonant circuit regecting the current
    at resonance.

    Now when I change the input wave form from a sine to a pulse of say
    20% duty at the same frequency both trace one and trace two look
    pretty identical. So this would seem that no current reduction through
    the 1k resister is seen. Which in turn means that the L-C circuit is
    not rejecting any current.

    So heres the question
    Does the resonant frequency change in an L-C circuit based on the
    input wave form?
     
  2. Talking about frequency in AC we mean sine waveform as opposed to rectangle
    in digital electronics. A pulse consists of multiple sine+cos signals (cos
    is a shifted sin, see how to convert any signal into this form in Fourier
    chapter of math.) of different amplitudes. This means, there is no specific
    frequency you can tune your filter on.
     
  3. No.

    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.

    That which is mostly observed, is that which replicates the most.
    http://www.anasoft.co.uk/replicators/index.html
     
  4. Matt

    Matt Guest

    So in theroy if the circuit was in resonance with a sine wave it should be
    in resonance at the same frequency with a pulse train then?

    Is this what you are suggesting?
     
  5. James Meyer

    James Meyer Guest

    Dear Kevin,

    It is seldom necessary to quote an entire posted question simply to add
    a single word reply.

    Jim
     
  6. James Meyer

    James Meyer Guest

    Dear Matt,

    It is seldom necessary to quote a single word reply and the question
    that prompted it in order to make a reply of two sentences.

    Jim
     
  7. Fred Bloggs

    Fred Bloggs Guest

    A pulse train is actually the superposition of an infinity of sinusoidal
    voltages at all integer harmonics of the pulse repetition frequency.
    Resonance of linear circuits relates to the response to sinusoidal input
    functions.
     
  8. Matt

    Matt Guest

    Dear Jim,
    It is obtuse to make such remarks in the context of this discussion.

    Thanks for you input!
     
  9. Resonance is a property of the system, not of the input signal.
    Technically, if it is non-linear the circuit resonance can change, e.g.
    if the capacitance or inductance changes with the input signal
    instantaneous voltage, but by default it is usually assumed that one is
    discussing a linear circuit.

    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.

    That which is mostly observed, is that which replicates the most.
    http://www.anasoft.co.uk/replicators/index.html
     
  10. Followed by a dozen lines of sig. :-(
     
  11. Reg Edwards

    Reg Edwards Guest

    No. Why should it?
     
  12. Yes - However, you must remember that a square wave is the sum of many
    components - the fundamental frequency (where your circuit is
    resonant) and all odd harmonics (where it isn't).
     
  13. Wim Lewis

    Wim Lewis Guest

    And a pulse train with a 20% duty cycle will have less of its energy at
    the pulse repetition frequency than a 50-50 square wave would. Still,
    I would expect to see some distortion as the input signal is tuned
    through the resonance frequency. (Or as any of the harmonics pass
    through the resonance.)
     
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