Home Forums Bug Reports Impedance Smith Chart Plotting Error

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• #624

I created a simple Qucs schematic to illustrate this problem.   I am using a voltage controlled dependent current source to drive a transmission line and load resistance.  The idea is to drive the transmission line and load with a 1 Amp current, then plot the impedance (voltage when I=1A) on a Smith Chart.  However, when I use the formula Gamma = (ZL-Z0)/(ZL+Z0)  the smith chart plots the reflection coefficient outside of the unit circle.  When I plot the formula for 1/Gamma = (ZL+Z0)/(ZL-Z0)  the plot looks correct.  This is just a problem with using the dependent sources; Driving the load with a swept-frequency AC current source does not exhibit this problem.

I need to compare the frequency If several slightly different circuits, and to do that, I need to use dependent current sources that are all controlled by the same swept-frequency voltage source.

I have a workaround (by plotting 1/Gamma) and am not stuck at the moment, but I feel this is a bug that needs fixing.

I am using QucsStudio version 3.3.2 on Windows 10, 64-bit.

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#628

The input current has the wrong polarity! It doesn’t flow into the transmission line, because the arrow at the input of the voltage-controlled current source points upwards and therefore, gets a negative voltage.

You can either flip the voltage source, set the voltage to -1V or the transconductance to -1S.

#637

QucsStudio,

Thanks, that makes sense.  So is the voltage arrow on the voltage controlled current source drawn the correct way? The passive sign convention draws the voltage arrow from negative to positive.  I’m in the United States, and we don’t use voltage arrows much.  We use + and – to indicate voltage polarities.

Jess Stuart

#639

The voltage arrow is correct. It always goes from positive to negative potential. This is the same in passive and in active components.

Yes, maybe it’s clearer to draw + and – signs here. Perhaps I will change this.

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