Novel SQUID Current Sensors With High Linearity at High Frequencies

We have developed novel SQUID current sensors consisting of a sensitive 16-SQUID input stage followed by an output stage of 40, 160, or 640 low-inductance SQUIDs. A highly gradiometric design of the individual SQUIDs with approximate to 5 mu m maximum linewidth makes the devices insensitive to external magnetic noise and allows cool-down and operation in the Earth magnetic field. Output current feedback (OCF) is used to increase the linear signal range. When driving a terminated 50 Omega coaxial cable, the sensor with 160 output SQUIDs exhibits a signal range of one flux quantum Phi(0) at a power dissipation of about 100 nW. At 10 MHz and 4.2 K, a total flux noise level of 0.34 mu Phi(0)/root Hz achieved corresponding to a dynamic range of +/-1.5 x 10(6) root Hz. At higher frequencies, interaction of preamplifier voltage noise with the output cable impedance increases the sensor noise (a general problem in high-speed SQUID systems). The small-signal bandwidth is 200 MHz. A very high slew rate of > 50 Phi(0)/mu s is achieved allowing one to use the full signal range at frequencies up to 16 MHz with a total harmonic distortion below approximate to 1%.