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Miniature Ultrasensitive Superconducting Magnetic Gradiometer and Its Use in Cardiography and Other Applications
1.D. Cohen, E. A. Edelsack, and J. E. Zimmerman, Appl. Phys. Letters 16, 278 (1970);
1.see also J. E. Zimmerman, J. Appl. Phys. 42, 30 (1971).
2.J. E. Zimmerman, P. Thiene, and J. T. Harding, J. Appl. Phys. 41, 1572 (1970).
3.We are indebted to Miss Nicola Kamper for assistance with this experiment.
4.A. H. Silver and J. E. Zimmerman, Phys. Rev. 157, 317 (1967).
5.In this context, the most obvious development would be to further increase the bias frequency. For a variety of reasons the inherent signal to noise will not increase indefinitely with bias frequency, but it may be that the optimum frequency will lie in the microwave region.
6.The noise level of given here is (as in Ref. 2) the equivalent fluctuation level within the SQUID. An optimally designed flux transformer with an aperture a few cm across should easily acheive an equivalent fluctuation level (referred to the transformer aperture) of The flux transformer in the gradiometer of Fig. 1 gave only a factor of 2 enhancement in sensitivity, although with the dimensions indicated it can be shown that the optimum enhancement factor should be about 10. The discrepancy is presumably due to the low coupling coefficient between the SQUID and flux transformer coupling coil, stray lead inductance, and imprecise matching of and
7.K. W. Gray and W. N. Hardy (unpublished) have shown that an order‐of‐magnitude improvement in noise figure can be achieved by cooling a germanium junction FET (a type no longer manufactured, apparently).
8.R. Forgacs and A. Warnick, Rev. Sci. Instr. 38, 214 (1967).
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