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Measurement with VNA- role of IF Bandwidth settings - RF Cafe Forums
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| ashishbondia
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Post subject: Measurement with VNA- role of IF Bandwidth settings
Posted: Wed Aug 26, 2009 7:49 am
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| Colonel |
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Joined: Wed Aug 16, 2006
8:20 am
Posts: 28
Location: India
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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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Ashish Bondia,
Design
Engineer- RF
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Ming |
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Post subject: Re: Measurement with VNA- role of IF Bandwidth settings
Posted: Fri Aug 28, 2009 9:26 pm
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Joined: Tue Jan 13, 2004
8:42 am
Posts: 10 |
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Hi ashishbondia,
That is very unexpected.
Is there any chance that some DC is present on the
input ot outpt pins that could be biasing the detectors
in the VNA? I've seen really strange things happen
from that.
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SunshineDesign |
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Post subject: Re: Measurement with VNA- role of IF Bandwidth settings
Posted: Tue Sep 08, 2009 11:54 pm
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Joined: Sat Nov 08, 2008
11:35 pm
Posts: 30
Location: Ramona, CA
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Dear ashishbondia,
Let me get this straight,
you have 3 amps of 40dB gain each and have trouble
measuring when they are cascaded? Well first off
that is a LOT of gain. You are most likely oscillating.
The narrower BW eliminates some of the radiated
power and the wider BW gets more of it giving the
higher readings. Look at the Amp combo Input/Outputs
with a Spectrum Analyzer to see if I am correct.
Again, that is an unusually large amount of gain
for any purpose and certainly exceeds their IP points
in the 2nd and 3rd stages.
Sunshine Design
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Sunshine Design Engineering
Services
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Posted 11/12/2012
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