Common-mode suppression for balanced bandpass filters in multilayer liquid crystal polymer technology

A new multilayer differential transmission line for common-mode suppression fabricated using an organic liquid crystal polymer multilayer substrate is proposed and analysed. The structure consists of a pair of parallel strip lines printed on the top layer of the substrate that are capacitively coupled to conductor patches periodically etched in an intermediate interface. These patches are symmetrically connected to a solid ground plane by means of short-circuited high-impedance microstrip line sections and metalised via holes. The differential signal supported by the coupled strips is not significantly affected by the presence of the middle layer layout, whereas the common-mode signal propagation is strongly suppressed within a certain frequency band. When this structure is cascaded with a simple differential bandpass filter exhibiting poor common mode rejection, a good common mode response is achieved within the unperturbed differential passband. The measurements confirm a significant enhancement of common-mode rejection thanks to the use of the proposed differential lines.