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

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.