AIP Digital Archive
Electrical Engineering, Measurement and Control Technology
A fast SQUID (superconducting quantum interference device) magnetometer system using the latest multiloop magnetometer W7A with additional positive feedback has been built. A 3-dB bandwidth of 5 MHz has been achieved in a simple direct-coupled flux-locked loop. The feedback range is ±620 Φ0 or ±290 nT, the white-noise level 3.4×10−6 Φ0/(square root of)Hz or 1.6 fT/(square root of)Hz, and the 1/ f corner frequency (approximately-equal-to)7 Hz. Above 1 MHz the noise level increases slightly, approximately with (square root of)SΦ ∝f1/6. At high signal frequencies a slew rate of about 8×105 Φ0/s has been measured. Below the pole frequency of the second integrator (480 kHz) the slew rate increases with decreasing frequency, having a maximum of 3×107 Φ0/s at 7.6 kHz. The total harmonic distortion (THD) decreases strongly with the signal amplitude below the slew rate limit. For a sinusoidal signal with half the maximum amplitude, the THD is (approximately-equal-to)4% at 1 MHz, (approximately-equal-to)0.04% at 100 kHz, and (approximately-equal-to)2.7 ppm (1 ppm=10−6) at 10 kHz. Below about 2 kHz the THD becomes almost frequency independent and lies between 0.4 and 7 ppm for signal amplitudes between 2.5 and 620 Φ0. © 1995 American Institute of Physics.
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