WinSmith is a Smith chart match calculating program. The starting point is 8 – j16 ohms @ 915MHz. Both the blue and the green curves start at this point at the lower left of the curves. Click on the image to see a bigger version
What is interesting about the above plot is that the elements work orthogonally. That is to say for any point selected by component of move #2 or move #3 work independently.
The matching circuit is shown below:
Nominal values
- TL1: 45 degrees of 50 Ohm transmission line @ 900MHz
- C2: 25pF
- C1: 7pF
The left side is 50 Ohms match and the right side is what presents a conjugate match to the FET IC drain circuit. Move #1 on the Smith chart is the 45 degrees of 50 Ohm characteristic impedance transmission line.
Tuning Procedure- theoretical
- The output load of the FET MMIC amplifier will vary somewhat. In spite of this the 45 degrees will transform the load to a region in the charge where the two caps can move the load to the close to the center of the chart.
- C2 has one job only: move the load to a 0.02 conductance circle on the Smith chart. This is the conductance circle that corresponds to 50 Ohms in shunt. Once you know the value of this capacitor you solder in place and leave it.
- C1 is used in shunt to bring the whole thing to the center of the chart. A calculated value can be used for a first try. You may need to cut and try to get absolute optimum on your lab model.
Input Match
The input match is the same topology. It starts much closer to 50 Ohms. The nominal values are
- TL1: 60 degrees of 50 Ohm line at 900MHz
- C2: 22pF
- C1: 0.5pF – This is placeholder in case of surprises. The match looks accomplished without it.
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