Theory and experiment of wideband tunable forced-mode ring resonator

For the first time a novel wideband tunable forced-mode ring resonator is proposed, which consists of a half guided-wavelength (λ_g/2) ring resonator and two loading capacitors with different values. By controlling the respective values of the two capacitors, the fundamental resonant frequency (f_fund) of the proposed ring resonator can be shifted up and down, resulting in a bi-directional tuning of f_fund. With this new design, the tuning range of f_fund can then be significantly extended to approximately double the range of the conventional tunable ring resonator. Meanwhile, no other resonance occurs in the wideband frequency-tuning range of f_fund. The design equations of the proposed ring resonator are derived by using odd- and even-mode analysis, and the operating principle is investigated and presented. For demonstration, the proposed tunable technique is verified by the simulation and experiment, and the measured tuning range of f_fund reaches 1.25 octaves, that is 0.68 → 1.53 GHz.