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Physica C Superconductivity

Non-constant bias current for dc SQUID operation

Y Zhang, N. Wolters, Raphael Otto, Hans‐Joachim Krause

Published Mar 1, 2002• 1 citation
Publisher
DOI
10.1016/s0921-4534(01)01163-7

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The flux modulation scheme (FMS) is the standard readout technique of dc SQUIDs, where a step-up transformer links the SQUID to the preamplifier. The transformer's primary winding shunts the SQUID via a large capacitor while the secondary winding connects it to the preamplifier. A modulation flux having a frequency of typically 100 kHz generates an ac voltage across the SQUID, stepped up by the transformer. The SQUID with FMS is customarily operated in the current bias mode, because a constant dc bias current flows only through the SQUID due to the capacitor isolation. With FMS, however, the transformer ac shunts the SQUID so that in reality the operating mode is neither purely current-biased nor voltage-biased but rather nominal current-biased or “mixed biased.” Our objective is to experimentally investigate the consequences of ac shunting of the dc SQUID in FMS and the transformer's transfer characteristics. For different shunt values we measure the change in the SQUID bias current due to the ac shunt using another SQUID in the two-stage readout scheme, and simultaneously monitor the SQUID output voltage signal. We then explain our measurements by a simplified graphic analysis of SQUID intrinsic current-voltage (I–V) characteristics. Since the total current flowing through the SQUID is not constant due to the shunting effect of the transformer, the amplitude of SQUID flux-to-voltage characteristics V(Φ) is less as compared to the direct readout scheme (DRS). Furthermore, we analyze and compare V(Φ) obtained by DRS and FMS. We show that in FMS, the transfer characteristics of the SQUID circuit also depend on the isolation capacitance and the dynamic resistance of the SQUID.
A high-resolution Josephson sampler using a single fluxoid is fabricated. The sampling pulse generator comprised a two-junction interferometer (DC–SQUID) with a damping resistance resistively coupled to a single junction as the sampling gate to reduce a turn-on delay. An ultra-fast sampling pulse was generated at a vortex-to-vortex transition in this sampler. The fabricated sampler had less than a 5ps time resolution and about a 5 µA current sensitivity. The dynamic waveform associated with two kinds of transitions was observed by the use of this sampler: a vortex-to-vortex transition and a vortex-to-voltage transition. The results show that the pulse corresponding to a single fluxoid has a nearly constant pulse height that is independent of the bias current and that setting the bias current near the critical point results in the long rise time of a DC–SQUID.