Varactor-based frequency-tuned microwave quarter-wavelength side-coupled ring resonator

A center-frequency-tuned microwave quarter-wavelength side-coupled ring resonator based on the varactor diodes is presented. Four identical varactor-based tuning circuits are applied at every entry point of the coupled-lines in the ring resonator to give capacitance effects and cause an increment of electrical length and thus results in tuned center frequency at the desired frequency band. The complete tunable resonator circuit is simulated using EM circuit simulator. The proposed filter is then realized and fabricated on FR4 substrate.