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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
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
solved by use of the Smith chart? As you can tell, I am a newbie to
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
postPosted: Wed Apr 27, 2005 5:50 pm
Unread postPosted: Wed Apr
27, 2005 9:49 pm
Tue Mar 15, 2005 11:43 pm
Regarding your questions on designing matching network of a given
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.
maybe you could check out this textbook:
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?
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
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?
Thu Apr 28, 2005 9:14 am
Were talking about input
match... Sorry disregard my comment about output power match.