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 amplifier).
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