Linearity of sensitive YBa2Cu3O7-x dc superconducting quantum interferencedevice magnetometers

The linearity of sensitive superconducting quantum interference device (SQU ID) magnetometers fabricated from high critical temperature (high-T-c) supe rconductors and operated in a direct-coupled flux-locked loop with bias rev ersal was investigated. The system nonlinearity was determined by applying a sinusoidal test signal and measuring the output signal harmonics using a setup which ensures the elimination of nonlinearities arising from the test signal source and the spectrum analyzer. The experimental setup enables th e simultaneous measurement of the harmonic distortion of a single magnetome ter and of a first-order electronic gradiometer formed by two individual ma gnetometers. Test signal amplitudes and frequencies were chosen to simulate typical laboratory interferences. An analysis of the results was performed using analytical equations in which the SQUID's inherent nonlinear charact er is taken into account. At signal frequencies above about 0.1% of the sys tem bandwidth, the system nonlinearity was found to be mainly caused by the nonlinear behavior of the SQUID magnetometers, whereas at low frequencies nonlinearities arising from read-out electronics components predominate. Fo r signal peak-to-peak amplitudes up to 1 mu T and frequencies up to 0.5 kHz , total harmonic distortions below about -105 dB for a single sensor channe l and -100 dB for an electronic gradiometer were obtained. (C) 1999 America n Institute of Physics. [S0021-8979(99)03818-9].