Multispurious suppression in parallel-coupled line filters by means of coupling control

In this work, a new method for stopband extension in both open-ended and interdigital parallel-coupled line filters is presented. This method is intended for the suppression of spurious bands which inherently appear in the stopband of this kind of filters. The multispurious suppression is achieved by means of a precise control of the coupling between resonators at unwanted resonant frequencies. The coupling section lengths between successive resonators are used as control parameters to obtain the required couplings. The dependence of the coupling coefficients on coupling section lengths at resonant frequencies is given by closed expressions, obtained following an energetic coupling approach. Several microstrip bandpass filters designed following the new method are presented as examples. These filters feature wide spurious-free bands with high rejection levels.