SQUID linearization by current-sampling feedback

Local negative feedback derived within the cryogenic stage from the output current of a voltage-biased SQUID series array is used to linearize the flux response and to simultaneously approach the noise matching resistance of the room temperature readout amplifier. The flux noise level of the SQUID array was 0.5 μΦ₀ Hz^−1/2 in open loop and 0.8 μΦ₀ Hz^−1/2 in the feedback arrangement having a 2.2 Φ₀ peak-to-peak flux locking range. The noise level degraded to 2 μΦ₀ Hz^−1/2 in an arrangement with a 7Φ₀ locking range. Very good linearity was observed in the feedback system regardless of the modest loop gain, owing to the open-loop SQUID characteristics which are more linear in the voltage-biased case than in the current-biased case. Upward and downward slew rates of 3.4 and 1.2Φ₀ µs⁻¹ were recorded which, however, do not represent ultimate limits of the approach. Local feedback schemes are reviewed and their effect on the linearity of a SQUID system is discussed.