Picosecond shock-wave generation on a GaAs nonlinear transmission line

A 3.5-ps falltime shock-wave signal has been generated on a nonlinear transmission line (NLTL). In this circuit a high-impedance transmission line is periodically loaded by Schottky varactor diodes, producing a synthetic transmission line with a voltage-dependent propagation velocity. As a negative-going input voltage transition propagates along the line, the falltime first decreases linearly with distance. As it decreases, dispersion arising from the Bragg periodic-line cutoff frequency and the varactor cutoff frequency competes with the compression arising from the voltage-dependent propagation velocity. A final limited falltime is reached at which the falltime compression per line section equals the falltime broadening per section, so that the resulting shockwave propagates unchanged. A full-scale NLTL design had 50 fF-diodes spaced 160 mu m apart. In a half-scale design, 25-fF diodes were spaced by 80 mu m. Nonlinear circuit simulations using SPICE indicate that the full-scale NLTL has a limiting falltime of 4.7 ps, while the half-scale line will compress to 2.7-ps falltime. A mixed design combined the full- and half-scale structure to utilize the lower minimum falltime of the half-scaled line after reaching the minimum falltime of the lower loss full-scale structure.<
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