Coplanar-waveguide-fed microstrip bandpass filters with capacitively broadside-coupled structures for multiple spurious suppression

Coplanar-waveguide (CPW)-fed microstrip bandpass filters are proposed with capacitive couplings suitably introduced at the input/output (I/O) ports, as well as between the resonators for spurious suppression. By adopting these capacitive couplings, several open stubs are established so that adjustable multiple transmission zeros may independently be created to suppress several unwanted spurious passbands, thereby extending the stopband and improving the rejection level. In this study, the capacitive couplings required at the I/O ports, as well as across the resonators, are realized by the broadside-coupled transition structures between the top microstrip layer and the bottom CPW layer so that the I/O ports may properly be matched and the spurious responses may effectively be suppressed. Specifically, a fifth-order bandpass filter, centered at f(o) = 1.33 GHz with a stopband extended up to 8.67 GHz (6.52 f(o)) and a rejection level better than 30 dB, is implemented and carefully examined.