A loop resonator tunable filter using phase shifters

A novel tunable bandpass loop resonator filter with three tunable loop resonators is presented. Each resonator is tuned through a phase shifter. This phase shifter not only is the tuning component of the quasi-elliptic filter, it also reduces the total size of the filter by reducing the size of the resonators. The size of the fabricated resonators is only 36% of the size of the traditional square loop resonators at the center of frequency range (850 MHz). The fabricated filter has a tuning range of 20% (765 MHz-939 MHz). The filter insertion loss is from 4.1 dB to 1.8 dB and the return loss is better than 20 dB for all the states. The 3 dB bandwidth is changing from 166 MHz to 171 MHz which is almost constant. In addition, the filter shape is constant for all states and the filter has a high out of band rejection.