Precise Design of a Bandpass Filter Using High-Q Dielectric Ring Resonators

A precise design is presented for a bandpass filter constructed by placing TE_01delta dielectric ring resonators coaxially in a TE₀₁ cutoff circular waveguide. On the basis of a rigorous analysis by the mode- matching technique, the interresonator coupling coefficients are determined accurately from the calculation of two resonant frequencies f_sh and f_op when the structurally symmetric plane is short- and open-circuited. For the TE_01delta ring resonator,the resonant frequency f₀, the temperature coefficient tau_f, the unloaded Q(Q_u), and the other resonances are also calculated accurately in a similar way. From the calculations, the optimum dimensions are determined to obtain the maximum Q_u, as F_r = f_r /f₀ is kept constant, where f_r is the next higher resonant frequency the ring resonator using low-loss ceramics (epsilon_r = 24.3, tan delta = 5 x 10^-5) has Q_u = 16800 at 12 GHz and tau_f = 0.1+-0.5 ppm/° C, while the rod one has Q_u = 14700. A four-stage Chebyshev filter having ripple of 0.04 dB and equiripple bandwidth of 27.3 MHz at f₀ =11.958 GHz is fabricated using these resonator; the measured frequency responses agree well with theory. The insertion loss is 0.9 dB, which corresponds to Q_u = 9800.