Microstrip parallel-coupled bandpass filters with source-load coupling

The compact coupled-line bandpass filters with two capacitive cross-couplings to create four transmission zeros are proposed. The locations of transmission zeros can be adjusted by varying the values of cross-coupled capacitances so as to improve the filter selectivity. To demonstrate the concept of multiple cross-couplings, a 4th-order microstrip coupled-line bandpass filter with four transmission zeros is implemented and examined. To further improve the stopband response, the coupled sections are shortened so that the spurious response may be suppressed. Specifically, another 4th-order microstrip coupled-line bandpass filter with spurious suppression around triple the center frequency (3f0) is also implemented.