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Homework Help: Mystery Circuit Components, Phase Angle, Confirmation and Clarification

  1. Apr 11, 2012 #1
    1. The problem statement, all variables and given/known data

    I just did a Signal Processing lab. The manual for the lab is attached, but it was essentially there's a circuit in a box with at most 3 separate linear circuit elements (resistor, capacitor, or inductor) in it. I had to figure out the components without opening it by only using a signal generator, an oscilloscope and a multimeter.

    2. Relevant equations

    Quality factor = Q = Resonant Frequency/Bandwidth = f0/Δf

    Q = energy stored / energy lost per radian

    In an RLC circuit,

    Q = ω0L/r = 1/(ω0Cr);

    Resonant Frequency = ω0 = 1/sqrt(LC)

    Where L is the inductance of the circuit, C is the capacitance, and r is the resistance.

    3. The attempt at a solution

    I'm going to refrain from giving the actual values unless it's necessary. I would appreciate it if someone could make sure I understand this all correctly, as I'm not incredibly comfortable with my knowledge of circuits. If this isn't something Physics Forums does, then I do have an additional question about the phase angle at the end.

    I hooked up the signal generator (I don't know if this is what its called, it can make a dc-current, sine waves, triangular waves, and square waves with frequencys varying from a few Hz to a few MHz) and the oscilloscope to the BNC Connectors in the diagram, and swept a large frequency range with a sine wave to see how the potential after Z1 behaves:

    There was a resonance at which the amplitude of the wave increased to several times the amplitude of the input

    The amplitude then approached 0 as I decreased the frequency from resonance; and Approached the same value as the input signal as frequency is increased past resonance

    So, I believe since we have a resonance like this, its likely a simple RLC circuit. I then used the multimeter to check the resistances of both Z's. It overloaded for Z1, which I understand to mean that there is a capacitor there. Z2 had a resistance of 175 ohms. Using the multimeter again, it told me that there was a capacitance in Z1.

    Knowing the resonant frequency I figured I could use the oscilloscope and calculate the bandwidth (difference between the frequencies in which the amplitude of the wave has dropped to 1/sqrt(2) of its maximum at resonance) and subsequently the quality factor of the circuit. I could then, knowing the capacitance of the circuit, isolate r in the third equation and solve for the total resistance in the circuit. This was larger than the resistance in Z2, so I can take it that there is an additional resistor in Z1, and that the resistance in Z2 is simply the internal resistance of the inductor.

    Finally, with the resonant frequency and capacitance at hand, I can use the fourth equation to determine the inductance in the circuit, which is in Z2, since the manual states there are no more than 2 parts in each section.

    So, there's the circuit. A strange thing happened with phase angles that I don't understand though.

    At resonance in the above procedure, the input wave from the generator was lagging behind the output from the right BNC connector. At low frequencies, the phase angle between these waves approached pi, and at high frequencies, they became in-phase.

    I don't understand how it becomes 180 degrees out of phase, my understanding was that at resonance, the waves should be in-phase, and then vary from pi/2 to -pi/2 as the impedances of the inductor/capacitor overwhelms each other. Some discussion into this would be greatly appreciated.

    Attached Files:

  2. jcsd
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