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Measurement with VNA- Role of IF Bandwidth Settings - RF Cafe Forums

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Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement


ashishbondia
 Post subject: Measurement with VNA- role of IF Bandwidth settings
Posted: Wed Aug 26, 2009 7:49 am 
 
Colonel
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Joined: Wed Aug 16, 2006 8:20 am
Posts: 28
Location: India
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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Design Engineer- RF


 
   
 
Ming
 Post subject: Re: Measurement with VNA- role of IF Bandwidth settings
Posted: Fri Aug 28, 2009 9:26 pm 
 
Captain
User avatar

Joined: Tue Jan 13, 2004 8:42 am
Posts: 10
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.


 
   
 
SunshineDesign
 Post subject: Re: Measurement with VNA- role of IF Bandwidth settings
Posted: Tue Sep 08, 2009 11:54 pm 
 
Colonel

Joined: Sat Nov 08, 2008 11:35 pm
Posts: 30
Location: Ramona, CA
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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