Constant absolute bandwidth third-order tunable bandpass filter with reconfigurable bandwidth using single λ/2 resonator

This paper presents a third-order bandpass filter with bandwidth tuning and constant bandwidth center frequency tuning. Based on the analysis of modal voltage waveforms of a varactor-terminated half-wavelength microstrip line, a varactor is loaded at the center to control even mod es, four varactors are loaded between two symmetric specific positions to tune, only, odd modes and two varactors at the ends to tune both the even and odd modes. Using these varactors, the passband is formed by bringing the first even and second, third odd modes close to each other. To keep the bandwidth of the passband constant, these three modes are controlled separately. The S-matrix has been derived to analyze and the coupling matrix has been synthesized to design the filter for a specification. An example filter has been designed and fabricated whose band can be tuned from 1.0 to 1.45 GHz (36.73%) with absolute bandwidth of 260 MHz and bandwidth is tunable from 130 to 490 MHz.