AN INDEPENDENTLY TUNABLE DUAL-BAND BANDPASS FILTER USING A CENTER SHORTING-STUB-LOADED RESONATOR

This paper presents an independently tunable dual-band bandpass filter based on center shorting-stub-loaded resonators. The center shorting-stub-loaded resonator is a dual-mode resonator that generates odd-even modes approximately equal and coupled when the shorting stub is very short. Two different sizes of center shorting-stub-loaded resonators produce two separated resonant frequencies, which are mutually independent. The coupling between the source and load is introduced in the circuit by designing an appropriate coupling structure, and the skirt selectivity of the filter is greatly improved. Four varactor diodes are placed at the two open-circuit ends of the center shorting-stub-loaded resonator to control the two separated resonant frequencies. A prototype of a tunable dual-band filter with Chebyshev response is designed and fabricated. The measured results are in good agreement with the full-wave simulated results. Results show that the first passband varies in a frequency range from 0.81 GHz to 0.95 GHz with a 3 dB fractional bandwidth of 4.2% to 5%, whereas the second passband can be tuned from 1.51 GHz to 1.79 GHz with a 3 dB fractional bandwidth of 6.8% to 8%.