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

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

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?
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



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


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


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



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

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

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?



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.


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?


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


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

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