R. Zakai wrote on Feb 14th, 2010, 9:21am:Just wondering if anyone knows where I can find formulas to manually compute s-parameters using nodal voltages from ac analysis.
It is very easy. You can find such formula in any basic circuit or filter theory text book.
See
http://www.designers-guide.org/Forum/YaBB.pl?num=1215827287/3#3As direct method, assume port impedances(=reference impedances), R
01 and R
02 are all real.
Available power of port1 is Pa=(|E|/2)
2/R
01Power delivered to port2 is Pload=|V
2|
2/R
02|S
21|
2=Pload/Pa=(2*|V
2/E|)
2*R
01/R
02S21=(2*V2/E)*sqrt(R01/R02)=(V2/(E/2))*sqrt(R01/R02)Here "E" is an open circuit voltage of port1. And E/2 is not equal to V
1 generally.
S
21 is different from conventional voltage gain V
2/V
1 even if R
01 is equal to R
02.
Also |S
21|
2 is different from conventional power gain Pload/Pin.
About meaning of "E", see the following and my related previous posts.
http://www.designers-guide.org/Forum/YaBB.pl?num=1237656418/3#3As another method, evaluate Y or Z parameters by AC analysis.
Then convert them to S parameters.
See
http://www.rfcafe.com/references/electrical/s-h-y-z.htm