Differential microstrip lines with wideband common-mode rejection based on chirped-EBGs

This paper presents differential microstrip lines with a wide rejection band for the common-mode, achieved by means of chirped-EBGs. The idea is to apply a sinusoidal chirped modulation of the even (common-mode) characteristic impedance, keeping the odd (differential-mode) characteristic impedance invariant and equal to the reference impedance of the ports (Z₀). Thus, the even-mode characteristic impedance is modulated along the length of the differential microstrip line. This concept was applied successfully to the design of dispersive delay lines (DDL) in microstrip technology. The main contribution of this work is to apply this chirping-EBG concept for the suppression of the common-mode in differential lines. The authors have validated this concept by designing a differential microstrip line with a 5 GHz wide common-mode rejection band centred at 5 GHz.