Common-Mode Suppression in Broadside Coupled Coplanar Waveguides

Abstract

Differential signaling is used in digital circuitry and high speed communication links due to its lower level of radiation and lower susceptibility to interference. Signal skew, amplitude differences and unequal parasitic electric or magnetic coupling to nearby structures can lead to common-mode signals being present on differential communication links which can result in unwanted electromagnetic interference and crosstalk. Common-mode filtering is often employed to suppress common-mode signal propagation in order to mitigate against these negative effects. In this paper broadside coupled differential coplanar waveguides are used which provide effective differential transmission from dc through 40 GHz. Simulation and measurement show that dipole-like common-mode filtering elements placed between the broadside coupled traces offer common-mode suppression of more than 10 dB over bandwidths greater than 5 GHz. A design equation is developed which can be used to estimate filtering frequencies from filter dimensions through 30 GHz. Filters can be cascaded to broaden filtering around a single frequency to filter at multiple frequencies. Simulation based registration studies were conducted which show stable filtering performance in the presence of layer-to-layer misregistration up to 0.254 mm.