Microstrip Dual-Band Bandpass Filter Design With Closely Specified Passbands

An efficient and practical design method for a dual-band bandpass filter (BPF) is presented. The electrical specifications of the filter, such as the center frequency, bandwidth, and transmission zero location, are controllable and adjustable. Once, the desired ratio of the resonant frequency of the two bands is given, the characteristic impedance of the line corresponding to the specific ratio can be accurately determined from the design curve. Compared with the conventional half- and quarter-wavelength stepped-impedance resonators (SIRs), the proposed unequal-length shunted-line stepped- impedance resonator (shunted-line SIR) can provide an efficient way to realize the dual-band filter, especially when two passbands are closely spaced. Furthermore, the multitransmission zeros and one controllable transmission zero are generated near the passbands to improve the out-of-band rejection. The design examples of microstrip dual-band BPFs operating at 2.2/3.45 and 1.95/2.65 GHz with equal absolute bandwidths and high isolation are demonstrated to validate of the design method. In the illustrated cases, a small frequency ratio of 1.3 between two resonant frequencies can be achieved. The proposed design method is very useful for the dual-band BPF design, especially when the two passbands are very close.