DC-SQUID electronics based on the noise cancellation scheme

We have constructed a compact dc SQUID electronics unit, in which the noise of the room-temperature amplifier is measured by the SQUID and cancelled out. This makes it unnecessary to use a transformer and a modulation scheme to achieve noise matching between the SQUID and the amplifier. The electronics unit contains a PI/sup 3/2/-controller and it has demonstrated a bandwidth of 100 kHz, a 1.2 /spl mu//spl Phi//sub 0///spl radic/(Hz) white noise level and a 1/f noise corner of 0.3 Hz in a flux-locked loop mode. The design criteria for the noise-cancelled electronics are discussed, including the problem of excess noise.<
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