# How to Design a Matching Network (Given VSWR) - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Post subject: How to design a matching network (given VSWR)
Unread postPosted: Wed Apr 27, 2005 12:05 pm
Offline
Lieutenant

Joined: Wed Apr 27, 2005 11:51 am
Posts: 1
Can anyone explain how to design a matching network given the VSWR of a device?
Given:
an RF power amp module "optimized for 50 Ohm system".
the data sheet says the the module incorporates matching networks optimized for output power and linearity in a 50 Ohm system.
the only spec that the manufacturer lists is input impedance VSWR of 2:1

a duplexer with VSWR of 1.4 on the TX side
frequency range: 1920 - 1980 MHz

Is this solved by use of the Smith chart? As you can tell, I am a newbie to RF

Thanks,
Rich

Top
Profile

Guest
Post subject:
Unread postPosted: Wed Apr 27, 2005 1:30 pm

Hello,

You have to measure the output impedance of your module, Once you have this information, you can design your matching network. The output impedance of your module will be the starting point on the Smith chart from which you have to "move" towards 50 ohm load (the prime-center).

To measure the output impedance: Terminate the input of the device with 50-ohm, bias it and measure the output impedance (S22) with Network Analyzer

Top

Guest
Post subject:
Unread postPosted: Wed Apr 27, 2005 5:50 pm

WRONG!

Top

Jeanalmira
Post subject:
Unread postPosted: Wed Apr 27, 2005 9:49 pm
Offline
General
User avatar

Joined: Tue Mar 15, 2005 11:43 pm
Posts: 65
Location: Singapore
Hi,

Regarding your questions on designing matching network of a given input VSWR.

Basically VSWR represents impedance and return losses, which the best value is 1:1.

I think what you can do is, measure the RF PA small signal by VNA, and plot in Smith Chart for S11 and S22. at this point you can see whether VSWRin is 2:1.

To design input matching networks, you have to move the curve to the nearest point of 50 Ohm (center) by adding either low-pass or high-pass configurations.
It depends which directions it should be moved.

For details, maybe you could check out this textbook:

G.Gonzales," Microwave Transistor Amplifiers, Analysis & design," 2nd Ed, Prentice Hall,1997

However I am not sure what kind of matching networks you need to design, is it input or output matching networks?

Regards,
Jean

Top
Profile

Guest
Post subject:
Unread postPosted: Thu Apr 28, 2005 9:04 am

Are you making a power match or a gain match? This is not the same.

Top

Guest
Post subject:
Unread postPosted: Thu Apr 28, 2005 9:12 am

If you see that the manufacturer is claiming the device to be already optimaly matched and that you have a bad VSWR from measurement, then it is a power match. You will not improve things by trying to conjugate match the module. You will in fact decrease its power output capacity.

You have to choose, gain or power?

Top

Guest
Post subject:
Unread postPosted: Thu Apr 28, 2005 9:14 am

Gee...

Were talking about input match... Sorry disregard my comment about output power match.

Posted  11/12/2012

RF Cafe Software

Wireless System Designer

RF & EE Symbols Word
RF Stencils for Visio
Calculator Workbook
RF Workbench
Smith Chartâ„¢ for Visio
Smith Chartâ„¢ for Excel

1996 - 2022
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 Internet was still largely an unknown entity at the time and not much was available in the form of WYSIWYG ...

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

Try Using SEARCH
to Find What You Need.
There are 1,000s of Pages Indexed on RF Cafe !