Spurious-Response Suppression in Microstrip Parallel-Coupled Bandpass Filters by Grooved Substrates

Application of the grooved substrate is presented to suppress the first spurious response of microstrip parallel coupled-line filters. The grooves are oriented parallel to all coupled lines and just next to them. It is shown that the grooves equalize the even- and odd-mode phase velocities in microstrip coupled lines and, therefore, suppress the spurious response. Sets of design graphs are derived for various depths of the grooves and the curves of equal even/odd-mode phase velocities are shown. Design equations based on well-known arbitrary image impedances are reviewed and the image impedances are optimized according to the design graphs. This eventually makes it possible to select the even/odd-mode characteristic impedances for each section of the filter as close as possible to the phase velocity equalization curve. Finally, an equal ripple filter of order 3 is designed using the proposed method. The grooves were simply machined and the measurement shows a rejection of more than 35 dB in the first spurious response, as predicted in simulated results.