An 550-650 MHz Digitally-Tunable Bandpass Filter: Design and Implementation

In this paper, a digitally-tunable band-pass filter is designed and implemented for applications of Cognitive Radio in middle Ultra high frequency television band from 550 to 650 MHz. This design is based on the 3th-order Chebyshev filter model in the combination with impedance inverters and series LC resonators, where capacitors are replaced by varactor diodes. The central frequency is tuned by means of a digital control board to adjust DC bias voltages of the diodes. Theoretical analysis, numerical simulation, and hardware implementation are described and carried out. The RF board is fabricated using SMV1736 varactor diodes with FR4-0.8mm material. The results show good match between simulations and measurements in terms of return loss, insertion loss, and fractional bandwidth over the operating range.