A Compact Tunable Dielectric Filter with Constant Absolute Bandwidth

This paper presents a compact tunable filter, consisting of two coaxial dielectric resonators and a pair of parallel-coupled microstrip lines. Coaxial dielectric resonators are directly coupled to each other through the aperture on the side surface of the resonators. The aperture provides magnetic coupling, the parallel-coupled microstrip lines produce electrical coupling, electrical coupling and magnetic coupling has canceling effect for the total coupling coefficient. Constant absolute bandwidth can be achieved by adjusting the gap of coupled lines and the size of the aperture simultaneously. A cross-coupling between the load and the source is created to introduce two transmission zeros in both the lower and the upper stopbands. Coaxial dielectric resonators are made of dielectric ceramic with dielectric constant of 88, parallel-coupled microstrip lines are made on Rogers RT/duroid 5880 substrate, the fabricated filter is measured, the results show that tuning range of center frequency is 0. 67GHz ~ 0.980GHz at 0 ~ 10V, 3dB bandwidth is 42 ~ 47MRz the volume of filter containing bias networks is 24×22×7mm
³
(0.052λ
₀
×0.048λ
₀
×0.015λ
₀
). The measured results meet with the requirement of constant absolute bandwidth within tuning range, and verified the proposed design approach.