Frequency-tuned microwave ring resonator using varactor diodes

Four identical varactor-based tuning circuits are proposed to tune the centre frequency of a microwave quarter-wavelength side-coupled ring resonator (QWSCR). Each of the tuning circuit is placed at every corner of the ring to give a shunt capacitor effect. The overall electrical length of the resonator can be varied according to the desired resonant frequency through the biased voltage applied to the varactor-diode that controls the value of the resultant shunt capacitor. The complete tunable resonator circuit is simulated using the integration of microstrip technology and lumped elements on FR4 substrate. The proposed concept of the tunable QWSCR filters allows a tuning up to 15% of the nominal frequency of the filter.