I want to design a Phase Shifter at 10 GHz using FR4.
I am using branchline coupler followed by two varactors at port 2 and 3 (port 1 is input and 4 is output).
Although FR4 is not suitable for high frequencies but I am getting S11 and S22 below -14db at 10 GHz
The problem is in Phase of S21,
I am getting no phase change at 10 Ghz in whole biasing range
Instead I am getting phase change at 2 GHz (around 40 degrees phase shift)
The shape in the picture is not a branch line coupler.
To bias the varactors, instead using an inductor connected to the middle of the coupler, try using two resistors (~100k)
Its is a 180 degree coupler/ rat race coupler (branch line not circular!)
I am using microstrip inductor for blocking ac and otherwise it will travel to my power supply
Looks like it is a version of a Schiffman phase shifter.
**broken link removed**
The 100k resistor (or two) would block the AC also, and more than this, do not have cut-off frequencies or parasitic resonant frequencies as an inductor.
Initially I tried Standard Branch line coupler but i was unable to achieve desired S11 at 10 Ghz
Standard Branch Line Coupler has less bandwidth, my design is a modification of standard branch line coupler.
By adding one segment to it, its bandwidth increases
So my modified design performs same (with some compromise on coupling)