Wideband tunable stubs using phase shifters

A novel technique to realize a tunable λ/2 stub is presented. The technique can be applied in the realization of tunable filters with stubs. The effective length of the stub is made tunable by replacing part or whole of the line by a phase shifter. This technique can overcome limitations of tunable stubs using varactor diodes such as the limited C_max/C_min ratio of varactors which in turn limits the tuning range. In addition, in filter applications it is possible to decrease the size of a filter by using a phase shifter instead of several varactor diodes in series or in parallel. The amplitude response of the tunable stub is a function of the slope of the insertion phase of the phase shifter. If the insertion phase is a linear function of frequency, a tunable transmission line is mimicked best. The technique is demonstrated using the commercially available PS094-315 phase shifter from Skyworks and a tuning range of 35% is achieved. The tunable shunt stub can be further enhanced by introducing a transmission zero in the second passband using a transmission line and a varactor. The fabricated enhanced shunt stub has a tuning range of more than 35%. In addition, phase shifter with a larger bandwidth can increase the tuning range further. This can be realized for example with switched binary phase shifter. The phase shifter used here was not specifically designed for application in tunable stubs and it is anticipated that custom designed phase shifters can improve the performance substantially.