Compact Cross-Coupled Bandpass Filters With Multiple Transmission Zeros

compact fourth-order quasi-elliptic quarter-wavelength bandpass filters are proposed. The first structure with cross-coupled mechanism is introduced to add a pair of transmission zeros (TZs), so as to improve the selectivity of frequency. In order to further increase the TZ, the non-resonating element is introduced in the second structure without increasing the physical size to enhance the performance of the upper stopband. Then the third structure introduces source/load coupling, not only can it create extra TZs comparing to the first structure, but it achieves wider stopband than the second structure. The simulated return loss and insertion loss of the proposed filters are all better than 18.2 dB and less than 0.7 dB in the range of 1.92 GHz to 2.08 GHz, respectively.