TIME-DOMAIN CHARACTERIZATION OF BENT COPLANAR WAVE-GUIDES

The aim of this paper is to present a comprehensive experimental study of the propagation characteristics of picosecond step-like transients on bent coplanar waveguides (CPW's). Our samples were made on GaAs substrates, an important material for today's microwave electronics. Bent CPW's with different degrees of bend-smoothing were investigated with the aid of a subpicosecond electrooptic sampling system. Our observations indicated that the physical length of the bent CPW had to be taken into account in order to evaluate the signal propagation velocity correctly. Also, smoothing of the bends significantly improved the high-frequency performance of the lines. After each bend, we observed a substantial time separation between the transients propagated on the two sides of the line and attributed it to the bend-induced excitation of even and odd CPW propagation modes. We found that the even mode could be suppressed by the implementation of a second, oppositely oriented band. Another way to suppress the even mode was to wirebond the two ground lines across the bend. Our results shed new light on picosecond pulse propagation on these transmission line structures, and demonstrate that transients with a bandwidth in excess of 100 GHz suffer only limited signal distortion when they are propagated over substantial distances on bent CPW's.