Improved high-Tc superconductor sampler circuits using Josephson transmission line buffers

We report on an increase of the measurement bandwidth of a high-T_c superconductor sampler circuit. The circuit composes a waveform measurement system, in which it is cooled by a compact single-stage cryocooler and operates at a temperature of 35-40 K. In the sampler circuit, there exists a sampling jitter due to thermal fluctuation in the critical current of the pulse generator trigger junction. To reduce the jitter, one can overdrive the trigger junction. However, leakage current into the adjacent sampler element due to parasitic inductance is an obstacle to the use of a larger trigger current. We attempted to suppress the leakage current by inserting a Josephson transmission line (JTL) buffer into the sampler circuit. It was found that a trigger current two-times larger than that of the conventional circuit became possible with the addition of a four-stage JTL and that the frequency at which the measured amplitude decreases by 3 dB increased from 7 GHz to 14 GHz. This result suggests that the jitter was suppressed in the sampler circuit with the JTL buffer.