Differential transmission line for common-mode suppression using double side MIC technology

A novel microstrip differential transmission line for broadband common-mode rejection is proposed. The basic unit cell of the artificial transmission line consists of a pair of coupled lines with a slot in the ground plane in which a conductor patch has been etched. This patch is symmetrically series connected to the ground plane by means of high impedance lines that act as inductors. The structure exhibits all-pass behavior under differential-mode operation because the etched pattern in the ground plane side is a virtual ground. This is because the symmetry plane is an electric wall. However, under common-mode operation the symmetry plane acts as a magnetic wall and the structure exhibits elliptic low pass characteristics. Therefore, the structure prevents common-mode propagation above a specified frequency that can be easily controlled by adjusting the length of the high impedance lines. The common-mode rejection level can be increased by cascading unit cells of this type. As an example of practical application, the structure is combined with a simple balanced bandpass filter based on folded stepped impedance resonators. The introduction of the proposed differential lines in the I/O sections significantly improves the common-mode rejection level of the conventional filter. Measured results are provided for both the differential lines and the filter.