Tunable bandpass filter with symmetric attenuation characteristics

This paper proposes a novel tunable resonator and bandpass filter that have symmetric frequency characteristics around the resonant frequency, and that can tune their resonant/center frequency, bandwidth, and filter order to configure the attenuation characteristics for out-of-band frequencies. The proposed resonator consists of switches, a pair of transmission lines, and variable reactance circuits. The resonant frequency does not change even when the bandwidth is tuned. The variable reactance circuits can tune the attenuation characteristics to be symmetric with respect to the resonant frequency by configuring attenuation poles at the upper and lower frequencies regarding the resonant frequency. The resonator is fabricated using a capacitive patch array as variable reactance circuits. The measured resonant frequency can be changed from 2.6 GHz to 6.5 GHz. The fractional bandwidth is tuned from 5% to 22%. Symmetrical attenuation characteristics are obtained by setting the capacitance of the capacitive patch array to an appropriate value. A two-pole bandpass filter is also formed on the same structure to show the reconfigurability in terms of the filter order.