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

#### Dong-gyu Jang

Joined Jun 26, 2015
115
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?

#### BR-549

Joined Sep 22, 2013
4,928
At resonance.....there should be no reactance. Voltage and current are in phase. The inversion is probably coming from the current transformer.

#### Dong-gyu Jang

Joined Jun 26, 2015
115
At resonance.....there should be no reactance. Voltage and current are in phase. The inversion is probably coming from the current transformer.
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?

#### BR-549

Joined Sep 22, 2013
4,928
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.