Operating Margins of a 10 V Programmable Josephson Voltage Standard Circuit Using ${rm NbN}/{rm TiN}_{x}/{rm NbN}/{rm TiN}_{x}/{rm NbN}$ Double-Junction Stacks

摘要

The operating margins of a 10 V programmable Josephson voltage standard circuit using ${rm NbN}/{rm TiN}_{x}/{rm NbN}/{rm TiN}_{x}/{rm NbN}$ double-junction stacks was investigated as a function of microwave frequency and operating temperature. The circuit contained 32 arrays of 5 120 ${rm NbN}/{rm TiN}_{x}/{rm NbN}/{rm TiN}_{x}/{rm NbN}$ double-junction stacks. In other words, the circuit contained 327 680 ${rm NbN}/{rm TiN}_{x}/{rm NbN}$ junctions. It was found that the operating margins (the heights of the constant-voltage step) of the arrays varied largely with microwave frequency and operating temperature. The microwave-frequency dependence was due to the resonance of the microwaves in the circuit. The temperature dependence of the circuit was due to the temperature dependence of the $I_{c}R_{n}$ product of the junctions, where $I_{c}$ was the critical current and $R_{n}$ was the normal resistance. Fortunately, however, the operating margin was maximized by changing the temperature of the chip, mounted on the cold head of a cryocooler.