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Significance of S21 plot in Smith chart (Read 5722 times)
mixed_signal
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Significance of S21 plot in Smith chart
Jan 14th, 2015, 8:36pm
 
Hi,

What is the significance of plotting S21 in Smith chart? I know that it plots normalized (S21-Z0)/(S21+Z0), what is its significance?

Plotting S11 makes sense because in that case it represents reflection co-efficient and hence it is helpful in matching etc. But why S21?
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Frank Wiedmann
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Re: Significance of S21 plot in Smith chart
Reply #1 - Jan 15th, 2015, 5:54am
 
Plotting S21 in a Smith chart does not make sense (see http://www.keysight.com/main/editorial.jspx?ckey=1000002218:epsg:faq&id=10000022...). You can plot S21 in a polar chart if you want to see the gain together with the phase. In most cases, however, gain (usually in dB) and phase (usually in degrees) as a function of frequency (usually in logarithmic scale) are plotted in separate rectangular charts, just like it is generally done for gain functions.
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mixed_signal
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Re: Significance of S21 plot in Smith chart
Reply #2 - Jan 15th, 2015, 7:14am
 
Hi Frank,
Thank you very much for the reply.

I just want to make a correction of my question. Plotting S21 does NOT mean plotting (S21-Z0)/(S21+Z0) but it is plotting of S21 in magnitude and phase in polar form. Therefore, Smith plot is just like a polar plot and may be useful if both magnitude & phase need to be visualized together
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Frank Wiedmann
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Re: Significance of S21 plot in Smith chart
Reply #3 - Jan 16th, 2015, 12:44am
 
But the lines of a Smith chart are not very useful for interpreting S21 data, whereas the lines of a polar chart are useful for this.
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mixed_signal
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Re: Significance of S21 plot in Smith chart
Reply #4 - Jan 17th, 2015, 10:34am
 
Yes you are correct. However, it is still seen to be a common practice where S21 is plotted in Smith chart.....may be because ADS and many VNAs provide the chart.
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