TY  - JOUR
AU  - Hong, Tao
AU  - Wang, Hai
AU  - Zhang, Yi
AU  - Krause, Hans-Joachim
AU  - Braginski, Alexander
AU  - Xie, Xiaoming
AU  - Offenhäusser, Andreas
AU  - Jiang, Mianheng
TI  - Flux modulation scheme for direct current SQUID readout revisited
JO  - Applied physics letters
VL  - 108
IS  - 6
SN  - 1077-3118
CY  - Melville, NY
PB  - American Inst. of Physics
M1  - FZJ-2016-02506
SP  - 062601 -
PY  - 2016
AB  - 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.
LB  - PUB:(DE-HGF)16
UR  - <Go to ISI:>//WOS:000373056300029
DO  - DOI:10.1063/1.4941665
UR  - https://juser.fz-juelich.de/record/809116
ER  -