Accuracy of electro-optic measurements of coplanar waveguide transmission lines

Nonzero backside potentials in microwave and digital GaAs circuits lead to small errors in direct electro-optic sampling measurements. We present experimental studies of the magnitude of this error signal as a function of center conductor width for 50 Ω coplanar waveguide transmission lines on a 20 mil substrate. Backside voltages 20 dB below the applied frontside signal are found for typical microwave circuit transmission lines. Calculation of this backside signal with finite difference techniques shows excellent agreement with experiment. Analytic estimates of the backside potential also may be derived in terms of the spatially periodic eigenfunctions for Laplace’s equation in the substrate.