Apparatus for reducing low frequency noise in dc biased SQUIDS
First Claim
1. A low noise superconducting circuit for measuring magnetic flux comprising a superconducting device including a pair of Josephson junctions and superconductors connecting the junctions in a closed loop, means applying a bias current through the two junctions in parallel, means reversing the bias current direction through the junction periodically at a first frequency, means for generating magnetic flux linking the closed loop, means modulating said flux at a second frequency which is different from said first frequency, demodulating means responsive to reference signals at said first and second frequencies, and means coupling the voltage across said junctions to said demodulating means, the demodulated output providing an output signal whose amplitude is determined by the level of magnetic flux linking the loop.
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Abstract
A circuit for reducing low frequency noise in a direct current biased superconducting quantum interference device. A squarewave bias signal having no dc component is used to bias the two junctions of the dc SQUID. At the same time, the magnetic flux in the SQUID is modulated to heterodyne the input signals up to some convenient frequency where they may be amplified without concern for drift. Final demodulation automatically adjusts for the fact that the relative phase is reversed each time the squarewave bias changes polarity states.
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Citations
15 Claims
- 1. A low noise superconducting circuit for measuring magnetic flux comprising a superconducting device including a pair of Josephson junctions and superconductors connecting the junctions in a closed loop, means applying a bias current through the two junctions in parallel, means reversing the bias current direction through the junction periodically at a first frequency, means for generating magnetic flux linking the closed loop, means modulating said flux at a second frequency which is different from said first frequency, demodulating means responsive to reference signals at said first and second frequencies, and means coupling the voltage across said junctions to said demodulating means, the demodulated output providing an output signal whose amplitude is determined by the level of magnetic flux linking the loop.
- 6. A control circuit for a dc SQUID having a pair of Josephson junctions connected in a superconductive loop, comprising means for applying a bias current through the pair of junctions in parallel forming said loop, means reversing the polarity of the bias current periodically at a first predetermined frequency, modulation means for producing modulation flux linking the loop at a second predetermined frequency, and means coupled across the junctions in parallel for generating an output signal that varies in magnitude in response to changes in the voltage across said two junctions in parallel due to changes in the level of flux linking the loop.
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12. Apparatus for measuring magnetic flux comprising:
a superconducting device including a pair of Josephson junctions and superconductors connecting the junctions in parallel to form a closed loop, means connecting a periodically reversing bias current through said junctions at a first frequency, means generating magnetic flux linking the closed loop, means periodically changing the level of said flux at a second frequency, demodulation means responsive to the changes in voltage across said junctions, and means mixing signals at said first and second frequencies for providing a reference signal to said demodulating means, whereby the output of the demodulating means is a measure of the level of magnetic flux linking the closed loop. - View Dependent Claims (13, 14, 15)
Specification