Capacitively tuned electrical coupling for reconfigurable coaxial cavity bandstop filters

A coaxial-cavity two-pole bandstop filter with continuously tunable center frequency and bandwidth is presented. The center frequency is tuned by capacitively loading the coaxial cavity with surface mount varactors. The bandwidth is reconfigured by controlling the energy coupled into the resonator by another surface mount varactor. Measured results show good agreement with simulation. The filter tunes from 0.77 GHz to 1.5 GHz. The 3-dB bandwidth tuning range is approximately 20MHz to 85MHz in the lower frequency range and approximately 50 MHz to 300 MHz in the upper frequency range. As an example, a frequency range of 0.77-1.25 GHz with constant bandwidth of 83 MHz is demonstrated with stopband attenuation of at least 18 dB. At larger 3-dB bandwidths, stopband attenuation up to 70 dB is measured.