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 impe
dance of the SQUID through a twisted pair to a room-temperature amplif
ier, The amplifier uses active feedback techniques to flatten the resp
onse of the signal path, When operated in conjunction with the high fr
equency 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 1.9x10(6) Phi(0)s(-1), a closed-lo
op performance of 5 MHz and a noise level of 5.5 mu Phi(0)/(Hz)(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 per
formance is limited by cabling and expect significant improvements in
a miniaturized design.