I was characterizing a amplifier chain consisting
of three amplifiers each with a gain of 40 dBa nd
inpula nd output return loss better than 14 dB.
When I measure the two ports S-parameters for each
amplifier individyually I get the results as simulated
(simulations done using S-parameter files of the
However when I cascade them I get
to see that although my S21 is same as simulated,
the return loss S11 and S22 and reverse isolation
S12 are do not agree with the simulated results.
In fact both the return loss S11 and S22 are positive
(in dB scale)
The IF bandwidth settings of the
VNA is 70 KHz. When I reduce the IF bandwidth settings
of the VNA to 7 KHz and repeat the measurements
I see that the readings improve. On further reduction
of IF bandwidth the readings improve further.
I was unable to understand the reason for sucha
behavior. If anyone has encountered such situations
or has any suggestions then please help.
you can find the plots here
http://picasaweb.google.com/ashishbondi ... directlink