A novel method is proposed to synthesize dual-band bandpass filters (BPFs) from a prototype lowpass filter. By implementing successive frequency transformations and circuit conversions, a new filter topology is obtained which consists of only admittance inverters and series or shunt resonators, and is thereby easy to be realized by using conventional distributed elements. A microstrip dual-band BPF with central frequencies of 1.8 GHz and 2.4 GHz is designed and fabricated using microstrip lines and stubs. The simulated and measured results show a good agreement and validate thereby the proposed theory.

A novel wideband bandpass filter with improved stopband characteristics is presented in this paper. Dual-mode square ring resonator is used in the proposed filter. New formulas based on the even- and odd-mode analysis are derived to facilitate the design of transmission zeros of the square ring resonator. A short-circuited stub and a piece of aperture-enhanced parallel-coupled lines are introduced to the input and output of the resonator to lower the passband return loss and widen the stopband of the filter significantly. The filter has a 50% fractional bandwidth, is compact in configuration, and shows remarkably improved performance compared with previously reported filters of the same kind. The measured filtering response shows a good agreement with the simulated result.