Performance of DC SQUID Second-Order Axial Gradiometer Response to Radio Frequency Interference

A low-Tc dc SQUID placed in a superconducting and magnetic shield is insensitive to magnetic interference, but not completely immune to the effects of radio frequency interference (RFI). Here, we investigated the effect of two orthogonal RF fields on the second-order axial gradiometer's transfer characteristics and noise characteristics. RF field induced phase shift of the V – $\Phi$ curve. The modulated spikes of RF field on the V – $\Phi$ curve and noise spectrum became significant with the RF field increasing. The white noise of the system increased to 12.5 $fT/\sqrt{\text{Hz}}$ with +10 dB axial RF field, nearly one and a half times of 8.5 $fT/\sqrt{\text{Hz}}$ without RFI. The working point jumping occurred under the radial RF field of +5 and +10 dB, and the white noise increased to 17.2 and 165.8  $fT\sqrt{\text{Hz}}$ , respectively. The reason of work point drift related to the magnetic flux coupling between the SQUID and pickup coil needs further studies.