FLUX-LOCKED CURRENT SOURCE REFERENCE

The quantization of flux in a closed superconducting circuit is used t o provide a stable reference current. A 10 mA current source is couple d through a toroidal transformer to a dc SQUID input, and the resultin g signal is fed back as an error current. The result is a net flux lin kage that exhibits short-term stability of 1 part in 10(9)/h. The net current is quantized with a step size of 59.4 nA, and it will exhibit the same stability as the flux provided the mutual inductance of the t ransformer remains constant. This current is passed through a precise 100 OMEGA resistor and compared against Zener diode references. The ob served temperature coefficient for the flux transformer is 28.5 +/- 3 ppm/K at 4.2 K. Possible sources for the temperature dependence are di scussed.