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Voltage Gain Determination - RF Cafe Forums

Because of the high maintenance needed to monitor and filter spammers from the RF Cafe Forums, I decided that it would be best to just archive the pages to make all the good information posted in the past available for review. It is unfortunate that the scumbags of the world ruin an otherwise useful venue for people wanting to exchanged useful ideas and views. It seems that the more formal social media like Facebook pretty much dominate this kind of venue anymore anyway, so if you would like to post something on RF Cafe's Facebook page, please do.

Below are all of the forum threads, including all the responses to the original posts.


 Post subject: Voltage Gain Determination
Posted: Wed Jan 20, 2010 1:35 pm 
 
Captain

Joined: Tue Dec 15, 2009 10:22 am
Posts: 9
Hello

I'm trying to obtain the voltage transfer function employing an E5062A VNA.

From pages 24, 23 in:
http://cp.literature.agilent.com/litweb ... 2-1087.pdf

I see that it can be obtained in terms of the S parameters and the load reflection coefficient.

My question is: is it correct to assume that the source and load impedances are equal to the reference impedance (Zl=Zs=Zo=50 Ohm)?
In this case, the expression in page 24 of the document would reduce to Av=S21/(1+S11). Is it correct?

Thank you very much.


 
   
 
 Post subject: Re: Voltage Gain Determination
Posted: Fri Jan 22, 2010 11:49 pm 
 
General
User avatar

Joined: Wed Feb 22, 2006 3:51 pm
Posts: 113
When you measure a device with a VNA, the source impedance from the VNA is 50 Ohms (normally, unless you're in the TV industry, where they use 75 Ohms a lot). Likewise, the load impedance seen by the device you're testing is 50 Ohms, again due to the VNA.

Remember to use complex arithmetic when you use the equations!

Good luck!


 
   
 
 Post subject: Re: Voltage Gain Determination
Posted: Mon Jan 25, 2010 6:24 am 
 
Captain

Joined: Tue Dec 15, 2009 10:22 am
Posts: 9
So... can I assume that the reflection coefficients of the source and the load are zero and, then the referred expression is simplified as I said?


 
   
 
 Post subject: Re: Voltage Gain Determination
Posted: Mon Jan 25, 2010 1:27 pm 
 
General
User avatar

Joined: Wed Feb 22, 2006 3:51 pm
Posts: 113
Basically, yes.

But it's more precise to say that the source and load impedances (50 Ohms + j0 Ohms) determine the reference impedance.

If you connect a short piece of low-enough loss transmission line of characteristic impedance 50 Ohms between the two ports of the VNA, then calibrate out the length of the line, you'll get zero for S11 and S22, and one for S21 and S12 - which I think is the answer you're looking for, stated precisely.

Good luck!





Posted  11/12/2012
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