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Editor’s Comment on Paper ST184

Editor’s Comment on Paper ST184

February 10, 2010.  The paper by Xie et al. stimulated a vivid discussion between Dr. Dietmar Drung (PTB-Berlin) and the authors.  Dr. Drung (DD) pointed out they did not acknowledge the fact that in the early 1990s he invented and demonstrated not only the APF (additional positive feedback, a voltage feedback), but also BCF (bias current feedback) as means to suppress the preamplifier noise in direct readout SQUID electronics 1. The demonstrated circuit implemented both feedback schemes simultaneously.  Furthermore, in his comprehensive overview of SQUID readout electronics he explicitly stated that “Although not yet proven experimentally, BCF should be suited, as well, for any other type of SQUID using either current bias or voltage bias"2. Therefore, the SBC readout concept demonstrated in ST184 is not inherently new, but rather a specific new implementation of the general concepts presented in1,2.  The still ongoing discussion is contributing to the clarification of the preamplifier noise suppression problem in direct-coupled SQUID electronics without flux modulation.  Consequently, DD decided to write a discussion paper to be submitted to Supercond. Sci. Technol. (SuST).  The paper will be pre-published in Issue 12 of ESNF.

In his forthcoming paper DD will present an approach to understand and analyze direct readout schemes for SQUIDs. He plans to show that all the existing methods for suppression of room temperature amplifier noise are based on voltage and/or current feedback in the SQUID that were introduced in the original APF and BCF schemes. He also intends to show that the way the SQUID is biased (at constant current or voltage) does not affect the noise suppression.

1D. Drung and H. Koch, “An electronic Second-Order Gradiometer for Biomagnetic Applications in Clinical Shielded Rooms”, IEEE Trans. Appl. Supercond. 3, 2594 (1993).
2D. Drung, “Advanced SQUID Read-out Electronics”.  In SQUID Sensors: Fundamentals, Fabrication and Applications (Ed. H. Weinstock), Kluwer Acad. Publ. 1996, Dordrecht, p. 93.