Characterization of picosecond electric-pulse propagation on CPW components by transient near-field mapping

Abstract

For the first time, the picosecond electric-pulse propagation characteristics of a few coplanar waveguide (CPW) components, including transmission lines, capacitive open and short terminations and a right-angle bend, have been successfully measured by mapping the time-dependent tangential near-electric-field distributions. The measurement was performed using a two-dimensional photoconductive (PC) near-field mapping system incorporating a recently developed novel PC electric-field probe, which could measure the separate picosecond orthogonal electric-field components with minimal loading effects. The resulting field-images mapped over the structural capacitive open termination and the short termination, showed previously unknown, and quite remarkable, transient picosecond electric-pulse reflection phenomena, which cannot be measured using conventional test instruments. In addition, the images showed that reflection-phenomenon itself required about 1 ps of propagation delay, and that the pulse-reflection from the capacitive open-termination required additional time delay matched to the length of the open-gap between the two slots of the CPW. We also found that at a CPW right-angle bend, comparable part of the energy of the guided signal leaks from the bend to the outer ground plane as radiating energy, and that two series-connected CPW discontinuities can remove the coupled-slotline mode generated at one of the discontinuities without using any additional component, such as an air-bridge.