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Measurement with VNA- role of IF Bandwidth settings

Question asked by ashishbondia on Aug 27, 2009
Latest reply on Aug 27, 2009 by Dr_joel
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
http://picasaweb.google.com/ashishbondi ... directlink

Thanks  

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