Wideband front end for high-frequency SQUID electronics

We report the development of a wide-bandwidth, low-noise front end to facilitate the high frequency operation of both LTS and HTS DC SQUIDs. A transformer of novel design non-resonantly matches the dynamic impedance of the SQUID through a twisted pair to a room-temperature amplifier. The amplifier uses active feedback techniques to flatten the response of the signal path. When operated in conjunction with the high frequency SQUID feedback electronics (developed by IBM T.J. Watson Labs) with a modulation frequency of 16 MHz, and a Quantum Design DC SQUID, the system achieved a slew rate of 1.9/spl times/10/sup 6/ /spl Phi//sub 0/ s/sup -1/, a closed-loop performance of 5 MHz and a noise level of 5.5 /spl mu//spl Phi//sub 0//(Hz)/sup 1/2/. The response from SQUID input to amplifier output was relatively flat from 2 MHz to 25 MHz, with usable signal to 45 MHz. We believe the performance is limited by cabling and expect significant improvements in miniaturized design.