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

The operating margins of a 10 V programmable Josephson voltage standard circuit using Operating margins of a 10 V programmable josephson voltage standard circuit using NbN/TiN
_x
/NbN/TiN
_x
/NbN double-junction stacks was investigated as a function of microwave frequency and operating temperature. The circuit contained 32 arrays of 5 120 Operating margins of a 10 V programmable josephson voltage standard circuit using NbN/TiN
_x
/NbN/TiN
_x
/NbN double-junction stacks. In other words, the circuit contained 327 680 Operating margins of a 10 V programmable Josephson voltage standard circuit using NbN/TiN
_x
/NbN/TiN
_x
/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 R
_n
and 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.