The circuit of the unit is fairly simple, but is a bit irksome to set up. The reason is that obtaining matched FETs is not easy, so I had to make sure that the circuit would work with off-the-shelf FETs. The phase shifter is a standard opamp circuit, and has been used for this sort of application many times. After experimenting with alternative va
riable resistors, I decided that the FET was still the best choice, although they are fairly critical to set up, and have linearity problems.