High speed normal and superconducting coplanar transmission lines

The authors present measurements of picosecond pulse propagation on coplanar strip transmission lines for which speed (i.e., group velocity) as well as phase and amplitude information is measured. Electrode effects are studied using transmission line loops 1 mm in diameter with cumulative propagation distances as long as 10 cm. The intrinsically low dielectric constant of coplanar-air transmission lines is shown to result in high signal speed and low attenuation. The results are compared to the case of superconducting electrodes. For superconducting electrodes one sees an apparent negligible loss for frequencies well below the gap and propagation distances up to 10 cm. For higher frequencies almost complete attenuation is found, even for very short propagation distances. The effects of substrate removal are shown to influence not only the speed of a given transmission line, but also the attenuation, due to strong reduction of radiation losses and through lowering of the surface impedance. The results obtained suggest that digital, intrachip communication on length scales of order 1 cm with pulses as short as one picosecond and velocities as high as 0.9c is readily achievable using electrical signals.<>