Tunable 500–1200-MHz Dual-Band and Wide Bandwidth Notch Filters Using RF Transformers

This paper presents a novel tunable dual-band bandstop filter based on doubly tuned RF transformers. This design results in two distinct notch frequencies in a single resonator using silicon varactor diodes. The 2-pole tunable dual-band bandstop filters are implemented using printed circuit board (PCB) transformers and air-coil transformers. With PCB transformers, the lower frequency can be tuned at 513-845 MHz while the higher frequency can be tuned at 715-1151 MHz with a notch level > 16 dB. With air-coil transformers, a single-band 2-pole notch filter with wide rejection bandwidth is achieved. The design results in 604-982 MHz tuning with a 20-dB rejection bandwidth of 27-45 MHz. Also, by implementing a series varactor in a transformer, the separation between two coupled frequencies can be changed. The topology can be easily extended to higher order filters and design equations are presented.