Britt Larsen, NKT Research Center, Priorparken 878, DK-2605 Brøndby, Denmark, Phone: +45 4348 3527, Fax: +45 4363 0099, e-mail: b.larsen@nkt-rc.dk, WWW: http:\\www.nkt-rc.dk

An amorphous photoconducting GaAs switch is deposited by laser ablation at room temperature onto a high-T_c circuit grown on an MgO substrate. Subpicosecond electrical pulses can be generated and detected using a sampling technique when this switch is irradiated with femtosecond laser pulses. We present measurements on a YBa₂Cu₃O_x circuit consisting of two closely coupled coplanar transmission lines with different types of terminations and excitation possibilities. The electrical pulses and reflections are analyzed along with their Fourier spectra. In order to compare to room temperature measurements, part of the circuit has gold on top of the YBa₂Cu₃O_x layer. A short in the transmission line has dimensions similar to a grain-boundary Josephson junction, and the high frequency properties of the short is thus important in the consideration of whether the subpicosecond electrical pulses on the superconducting transmission line can change the magnetic flux in an interferometer.