Design of highly selective open-loop tunable bandpass filters with wide tuning range

This paper presents an open-loop tunable filter with a continuous tunability for both center frequency and bandwidth. Electric and magnetic mixed coupling is utilized to achieve one transmission zero in the lower stopband, while the source/load coupling introduces another zero in the upper stopband which totally improve the selectivity. The bandwidth tunability is achieved by placing a transmission line with one end shorted to ground and with a varactor loaded at the opposite end. Moreover, this new filter with novel input and output structure without any via-hole grounds which avoid parasitic inductance can reduce the design error validly. A two-pole filter with wide simultaneous and continuous tunings of center frequency (from 500MHz to 1025MHz, tuning range of 68.9%) and bandwidth (from 30 to 47 MHz around 800MHz) is presented. Measured results agree with simulated predications excellently.