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Physica B Condensed Matter

Comparative study of bias-reversing schemes for low frequency noise reduction in dc SQUIDS

James M. Lockhart, David Hipkins, G.M. Gutt, B. Muhlfelder, N. J. Jennerjohn

Published Feb 1, 1994• 1 citation
Publisher

Abstract

We have investigated the effectiveness for low frequency noise reduction of three different schemes of periodic bias current reversal in dc SQUIDs operated in the flux-locked mode. The three techniques incorporate different frequency and phase relationships between the bias current waveform and the flux modulation waveform. We find that some SQUIDs show significant low frequency noise reduction under bias reversing, while other SQUID devices of the same design inherently achieve the lower level of low frequency noise and hence do not benefit from bias reversal.

DOI
10.1016/0921-4526(94)90374-3

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We have investigated the effectiveness for low frequency noise reduction of three different schemes of periodic bias current reversal in dc SQUIDs operated in the flux-locked mode. The three techniques incorporate different frequency and phase relationships between the bias current waveform and the flux modulation waveform. We find that some SQUIDs show significant low frequency noise reduction under bias reversing, while other SQUID devices of the same design inherently achieve the lower level of low frequency noise and hence do not benefit from bias reversal.
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The effects of radio frequency radiation on the dc SQUID are examined. Simulations show how the shape of the SQUID transfer characteristic is distorted by radio frequency interference (RFI). How this affects three commonly used SQUID modulation methods is discussed, and the results explain why we experimentally observe the bias current reversing readout method to be the least susceptible to RFI. The commonly seen increase in the low frequency flux noise power spectrum of dc SQUIDs in unshielded environments is also explained.
The effects of radio frequency radiation on the dc SQUID are examined. Simulations show how the shape of the SQUID transfer characteristic is distorted by radio frequency interference (RFI). How this affects three commonly used SQUID modulation methods is discussed, and the results explain why we experimentally observe the bias current reversing readout method to be the least susceptible to RFI. The commonly seen increase in the low frequency flux noise power spectrum of dc SQUIDs in unshielded environments is also explained.
A detailed study has been made of the low-frequency excess noise of rf superconducting quantum interference devices (SQUIDs), fabricated from thin niobium films and operated at 4.2 K, with rf bias frequencies of 0.15, 1.7, and 3 GHz. When the SQUIDs were operated in an open-loop configuration in the absence of low-frequency flux modulation, the demodulated rf voltage exhibited a substantial level 1/f noise, which was essentially independent of the rf bias frequency. As the rf bias frequency was increased, the crossover frequency at which the 1/f noise power was equal to the white noise power moved to higher frequencies, because of the reduction in white noise. On the other hand, when the SQUID was flux modulated at 50 kHz and operated in a flux locked loop, no 1/f noise was observed at frequencies above 0.5 Hz. A detailed description of how the combination of rf bias and flux modulation removes 1/f noise due to critical current fluctuations is given. Thus, the results demonstrate that the 1/f noise observed in these SQUIDs is generated by critical current fluctuations, rather than by the hopping of flux vortices in the niobium films.
We have developed a radio frequency bias current scheme with reversal frequency up to 50 MHz for dc superconducting quantum interference devices (SQUIDs). The SQUID and the readout electronics, were connected via a quarter wavelength (/spl lambda//4) transmission line. This transmission line not only transported the SQUID signal, but also served as the resonant matching circuit. For a dc SQUID of inductance 115 pH (loop size 75/spl times/75 /spl mu/m/sup 2/), we measured a flux noise of 33 /spl mu//spl phi//sub 0///spl radic/Hz at 77 K in the flux locked loop configuration. The corner frequency of 1/f noise was less than 0.5 Hz.