Voltage Gain Determination - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues - primarily having to spend time purging garbage posts from the board. At some point I might start the RF Cafe Forums again if the phpBB software gets better at filtering spam.

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

-- Amateur Radio
-- Anecdotes, Gripes & Humor
-- Antennas
-- CAE, CAD, & Software
-- Circuits & Components
-- Employment & Interviews

-- Miscellany
-- Swap Shop
-- Systems
-- Test & Measurement
-- Webmaster

Measures

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.

fred47

Post subject: Re: Voltage Gain Determination

Posted: Fri Jan 22, 2010 11:49 pm

General

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!

Measures

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?

fred47

Post subject: Re: Voltage Gain Determination

Posted: Mon Jan 25, 2010 1:27 pm

General

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

Webmaster:

Kirt Blattenberger,

BSEE - KB3UON

RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The World Wide Web (Internet) was largely an unknown entity at the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps while tying up your telephone line, and a nice lady's voice announced "You've Got Mail" when a new message arrived...

All trademarks, copyrights, patents, and other rights of ownership to images and text used on the RF Cafe website are hereby acknowledged.

My Hobby Website: AirplanesAndRockets.com

My Daughter's Website: EquineKingdom