Design of wide stopband filter with tunable center frequency

A dual passbands filter with the function of varactor tuned is proposed in this paper, which has a wide stopband. Stepped impedance resonators (SIRS) are used at the filter. The varactor is loaded on the T-shaped resonators, while changing the bias voltage across the varactor, the second passband frequency is tuned from 1.56 GHz to 2.25 GHz; the first passband´s center frequency is fixed at 1.02 GHz and maintained though out the simulation. The first passband bandwidth is 14.7%, the insertion loss value is 0.2 dB and the value of the return loss is 19 dB. The second passband is tuned between 1.56 GHz and 2.25 GHz where the tuning range is maintained at 36.2%, relative bandwidth remains unchanged. The insertion loss changes from 0.2 dB to 0.8 dB while the return loss shift from 16 dB to 30 dB. The propose filter achieves a great harmonic suppression characteristic of í20 dB up to 5 GHz. The designed filter has two advantages, one is that to depress stopband, the filter network is incorporate in the output input coupling line, the other advantage is that the performance of the filter is optimize by adding spur lines (spurlines) at the input and output coupling line which is useful to increase zero between the two passbands. Compared to other filters, the proposed filter is more efficient, and it has compact in size, low insertion loss, and wider rang harmonic suppression. Simulations show that the filter proposed is valid and has a good performance.