Angle of the impedance must be within [90 , -90] in degree, right?

Discussion in 'General Electronics Chat' started by Dong-gyu Jang, Feb 13, 2017.

  1. Dong-gyu Jang

    Thread Starter Member

    Jun 26, 2015
    115
    4
    Hello.

    The impedance is expressed as Z = R + iX where R is a resistance and X is a reactance.

    Since R can not be negative, angle of Z, θ in Z = exp(iθ), must be within [90°, -90°] for any impedance, right?

    However, when I naively measured a voltage and current at a point on RF circuit by using HV (High Voltage) probe and CT (Current Transformer), θ derived from these profiles is ~ 187°. RF signal frequency is 13.56 MHz. I'm guessing that a signal propagation time from the HV probe to an Oscilloscope is different from that for the CT, so this unrealistic impedance angle is measured.

    Could you tell me whether or not I'm right? If I'm not right, could you give me possible causes for this wrong measurement?
     
  2. BR-549

    AAC Fanatic!

    Sep 22, 2013
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    At resonance.....there should be no reactance. Voltage and current are in phase. The inversion is probably coming from the current transformer.
     
  3. Dong-gyu Jang

    Thread Starter Member

    Jun 26, 2015
    115
    4
    Thanks for giving some comment.

    I need to ask what resonance you mean. My apparatus was actually ICP (Inductively Coupled Plasma) and the measuring point is an output of impedance matching box. So there is no reason reactance is cancellled out at this point.

    Yes, I was also suspecting CT but I don't know exactly how CT gives wrong phase of the current. For example, Is there some time delay from CT that should be compensated in measurement?
     
  4. BR-549

    AAC Fanatic!

    Sep 22, 2013
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    The CT does not cause a time delay........it causes a current inversion. The CT output current is 180 degrees from it's input current.

    Sorry, I assume all rf signals are in resonance.
     
    Dong-gyu Jang likes this.
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