Improvement of a bit error rate measuring system for high-temperature superconducting circuits

We have evaluated and tried to improve the measurement precision of a bit error rate (BER) measuring system for superconducting circuits equipped with a magnetic shield and a cryocooler. A noise model has suggested that the most effective way of improving BER is to reduce the noise figure of the wideband pulse amplifier (WPA). BER measurement using a 3-Gbps, 2¹⁵-1 pseudo random binary signal showed that by using low-noise WPA (noisefigure=4 dB) a 12.0-mV output voltage is required to obtain a minimum BER of 1.19×10^-15 (a 10^-7 improvement over the previous system) for a 50 Ω standard microstrip. A BER of a La_0.2Yb_0.9Ba_1.9Cu₃O_x/MgO/Au microstrip-line to coplanar-waveguide transition-line converter (MCC) was also measured. It was found that the minimum BER was 4.87×10^-13 (a 10^-5 improvement over the previous system) when the output voltage was 23.8-mV. However these measured BER values were larger than those expected by the noise model. The cause of the larger measured BER is discussed. A minimum BER of 3.64×10^-55 in 14.8-mV output voltage for a 50 Ω standard microstrip was obtained when the cryo-cooler was not operated (without the noise from a cryo-cooler).