This paper presents the design and implementation of a tunable bandpass filter (BPF) for Cognitive Radio, which aims at improving the efficiency of utilizing allocated spectrum in lower Ultra high frequency (UHF) television range, i.e., (450-550)MHz. Unlike the conventional designs which mainly rely on shunt LC resonators in combination with varactor diodes, our design uses series LC resonators whose capacitors are replaced by the varactor diodes. The central frequency is tuned by adjusting DC bias voltages of the diodes through a digital control board. The design steps are described in detail by means of theoretical analysis and numerical simulation. Hardware implementation is carried out and measured parameters are shown in comparison to the simulated ones in order to validate the design. Obtained results prove that the tunable filter exhibits low return loss and insertion loss with a nearly constant fractional bandwidth over the tuning range
