Double Relaxation Oscillation SQUID Magnetometer with Superconducting Digital Flux-Locked Loop

采用超导数字磁通锁环（FLL）电路实现了片上双弛豫振荡SQUID（DROS），实现了高灵敏度磁通传感器和SQUID放大器。本文研究了两种超导数字FLL电路：一种是由8位可逆计数器和8位R-2 R梯形D/A转换器组成，采用4JL门电路，由频率高达4GHz的两相电源驱动。另一种是由基于约瑟夫森计数器的片上FLL电路和基于SFQ逻辑门的6位增/减计数器组成，其中增/减计数器采用二进制超前进位（BCL）电路实现高速运算。采用2.5kA/cm^2 Nb/Al-AlO_x/Nb结工艺，对带有BCL电路的4JL 8位加减计数器进行了逻辑模拟，结果表明，在4GHz时钟频率下，加减计数器工作正常。我们预期采用超导数字FLL电路的DROS具有高达10-7Φ_0/s的大摆率。使用SFQ逻辑门的6位加/减计数器，逻辑模拟显示了高达33 GHz的正确加/减计数操作，我们预期采用SFQ数字FLL电路的DROS可以工作在10 GHz。

On-chip double relaxation oscillation SQUID(DROS) with a superconducting digital flux locked-loop(FLL) circuit is demonstrated far high sensitive magnetic flux sensor and SQUID amplifier. In this study, we examined two kinds of superconducting digital FLL circuits : one was composed of a 8-bit up/down counter and a 8-bit R-2R ladder-type D/A converter designed using 4JL-gates which are driven by a two phase power supply at a frequency up to 4GHz. The other one was composed of an on-chip FLL circuit based on a Josephson counter and a 6-bit up/down counter using SFQ logic gates.In the up/down counter, we used binary carry lookahead(BCL) circuits for high-speed operation. Logic simulations for the 4JL 8-bit up/down counter with BCL circuits showed correct up-and down-counting operation at 4GHz clock frequency assuming a 2.5kA/cm^2 Nb/Al-AlO_x/Nb junction technology. We expected that a DROS with the superconducting digital FLL circuit has large slew rate as high as 10-7Φ_0/s.Using the 6-bit up/down counter using SFQ logic gates, logic simulations showed the correct up and down counting operation up to 33GHz and we expected that a DROS with the SFQ digital FLL circuits can be operated at 10GHz.