Electronically tunable and switchable filters using microstrip ring resonator circuits

Two microstrip ring resonator circuits are described: an electrically switchable filter and a tunable switchable filter, both using p-i-n diodes. In the first circuit, a microstrip ring resonator loaded with two p-i-n diodes is developed as a switchable filter. By replacing one p-i-n diode with a varactor diode, the switchable filter is made electronically tunable. Over 20-dB isolation with 9% tuning bandwidth has been demonstrated. Equivalent circuits have been developed for both circuits. The analysis was based on the transmission-line model including the effects of diode parasitics, coupling gaps, dispersion, and mounting gap capacitance. The experimental results agree very well with the theoretical calculation.<>