The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Measurements >> RF Measurements >> Significance of S21 plot in Smith chart
https://designers-guide.org/forum/YaBB.pl?num=1421296574

Message started by mixed_signal on Jan 14^th, 2015, 8:36pm

Title: Significance of S21 plot in Smith chart
Post by mixed_signal on Jan 14^th, 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?

Title: Re: Significance of S21 plot in Smith chart
Post by Frank Wiedmann on Jan 15^th, 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=1000002218:epsg:faq&nid=-11143.0.00&lc=eng&cc=US). 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.

Title: Re: Significance of S21 plot in Smith chart
Post by mixed_signal on Jan 15^th, 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

Title: Re: Significance of S21 plot in Smith chart
Post by Frank Wiedmann on Jan 16^th, 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.

Title: Re: Significance of S21 plot in Smith chart
Post by mixed_signal on Jan 17^th, 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.