Reconfigurable Bandwidth Bandpass Filter With Enhanced Out-of-Band Rejection Using $\pi $ -Section-Loaded Ring Resonator

A novel ring resonator bandpass filter with reconfigurable bandwidth with central frequency at 2.4 GHz is demonstrated. Theoretical analysis for computing the resonant frequencies is shown, and the design approach of implementing π-section stubs connected to the ring to obtain the bandwidth reconfigurability is explained. The use of reconfigurable π-section allows the alteration of the stub's effective width and therefore the filter's bandwidth reconfigurability. p-i-n diodes are used as switching elements to achieve the narrowband/wideband response. Coupled line sections are used for suppressing the higher modes by generating out-of-band transmission zeros, resulting in a significantly enhanced out-of-band rejection. Measurements indicate that the 3-dB fractional bandwidth can be switched from 58.5% to 75% at a fixed center frequency of 2.4 GHz with an insertion loss better than 1.1 dB. Moreover, the -20-dB stopband performance is extended to 2.7 f
₀
.