questions regarding optimization/tuning of transmission line
I would like to know if I can change the width of the series transmission lines in my matching networks when i optimize for better results. Can I change the width or it should stay related to 50 ohm?
Also when using balance stubs for the matching networks, should the length of the stubs be the same after optimization or I can change one or both? What does "balance" mean? does it mean equal length for the stubs?
thanks
ayhz2002
When you transform one impedance to another with transmission line networks the lines between the two points can be any impedance or length that makes the transformation work. That is, within limits. If the line is too high of an impedance you get into manufacturing variation problmes. If it is too low impedance it takes up a lot of board space.
As far as stubs off opposite sides of a line, whatever works is acceptable. They are made equal usually to have equal currents to minimize losses.
Changing the line width and length is OK, often you will do this by hand
to get the best performance. But be careful, you might get better gain
but your S11, S22, phase or output power may no longer not be acceptable. Also the circuit you are tuning / matching is most likely not 50 Ohms so different widths might be necessary
hope this helps
Cheers
One final thing. Another reason for using two lines off opposite sides is to lower the impeance by an easier way than lowering the impedance of one line. Since the impedance is a log function, less board area is take up by two lines of twice the impedance than one line of the desired impedance.