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Design of digitally tunable bandpass filter for spectrum sensing application in the TVWS

This paper presents a digitally tunable bandpass filter(BPF) that can tune its center frequency with constant absolute bandwidth for spectrum sensing applications in the TVWS(TV White Space). The proposed two-pole BPF is implemented using lumped elements and the frequency tuning is realized by PIN diode and varactor diode. Due to the L-C parallel circuit as a J-inverter, constant absolute bandwidth is achieved. 4 PIN diodes in each resonator and the capacitor bank are used to cover the desired frequency tuning range from 480 MHz to 687 MHz with 21 MHz bandwidth. The complete design procedures are described. The fabricated filter has wide tuning range of 42.5% with insertion loss varying from 2.8dB to 4.6dB, while the 1-dB bandwidth of the filter being kept as 21 MHz.