Linearity of sensitive YBa2Cu3O7-x dc superconducting quantum interference device magnetometers

The linearity of sensitive superconducting quantum interference device (SQUID) magnetometers fabricated from high critical temperature (high-T-c) superconductors and operated in a direct-coupled flux-locked loop with bias reversal 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 the simultaneous measurement of the harmonic distortion of a single magnetometer and of a first-order electronic gradiometer formed by two individual magnetometers. 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 character is taken into account. At signal frequencies above about 0.1% of the system 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. For 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 channel and -100 dB for an electronic gradiometer were obtained. (C) 1999 American Institute of Physics. [S0021-8979(99)03818-9].