How to Design a Matching Network (Given VSWR) - RF Cafe Forums
Post subject: How to design a matching network (given
Unread postPosted: Wed Apr 27, 2005 12:05 pm
Joined: Wed Apr 27, 2005 11:51 am
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
frequency range: 1920 - 1980 MHz
Is this solved
by use of the Smith chart? As you can tell, I am a newbie to RF
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
Unread postPosted: Wed Apr
27, 2005 5:50 pm
Unread postPosted: Wed Apr 27, 2005 9:49 pm
Joined: Tue Mar 15,
2005 11:43 pm
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
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
G.Gonzales," Microwave Transistor Amplifiers,
Analysis & design," 2nd Ed, Prentice Hall,1997
I am not sure what kind of matching networks you need to design,
is it input or output matching networks?
Unread postPosted: Thu Apr 28, 2005 9:04 am
Are you making
a power match or a gain match? This is not the same.
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?
Unread postPosted: Thu Apr 28, 2005
Were talking about input match...
Sorry disregard my comment about output power match.