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dc SQUID readout electronics with up to 100 MHz closed-loop bandwidth

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6 Author(s)
Drung, D. ; Phys. Tech. Bundesanstalt, Berlin, Germany ; Assmann, C. ; Beyer, J. ; Peters, M..
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High-speed readout electronics for sensors based on dc superconducting quantum interference devices (SQUIDs) are presented. The SQUID sensor involves a series array of 16dc SQUIDs and an intermediate transformer to enhance its current sensitivity. By using a highly gradiometric design and ≤5 μm linewidth for the SQUID array, the device can be cooled down in fields of up to 85 μT and be operated magnetically unshielded. A special feedback coil design minimizes the parasitic coupling between feedback and input coil. The SQUID sensor is directly connected to the room temperature electronics. A composite preamplifier is used consisting of a slow dc amplifier in parallel with a fast ac amplifier. A virtual 50 Ω input resistance with negligible excess noise is realized by active shunting. Two types of high-speed readout electronics were developed. The first was designed for optimum dc performance, high flexibility, and user-friendliness. It is fully computer controlled. The white voltage and current noise levels are 0.3 nV/√Hz and 3 pA/√Hz, respectively, resulting in an overall system noise level of 0.4 μΦ0/√Hz or a coupled energy sensitivity around 500h (Φ0 is the flux quantum and h is Planck's constant). The maximum flux-locked loop (FLL) and open-loop bandwidths are about 20 MHz and 50 MHz, respectively. The second readout electronics is an ultra-high-speed prototype which was designed for maximum speed at the expense of dc performance. A very low intrinsic signal delay of 1.7 ns and a high open-loop bandwidth of 300 MHz were measured. Using a novel FLL scheme, a very high signal bandwidth of 130 MHz was achieved with 0.8 m distance between SQUID and electronics.

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Applied Superconductivity, IEEE Transactions on  (Volume:15 ,  Issue: 2 )