A tightly coupled dc SQUID with an intermediary transformer

We have developed a de SQUID in which the input inductance is coupled via an on chip integrated intermediary transformer. The overall coupling factor between the 0.367 muH input coil and 30 pH dc SQUID was as large as 0.77. In order to eliminate transmission line resonances. due to both the increased coupling factor and the larger input inductance. we introduced a microwave double stage low pass filter into the intermediary transformer. No distortions were observed on the V-Phi curve with modulation depth and steepness as large as 125 muV and 0.6 mV/Phi(v) respectively. The measured values of the coupled energy resolution of the SQUID were 1.25 x 10(-31) J/Hz with a room temperature directly coupled preamplifier and about 10(-32) J/Hz (16 h) in a double stage SQUID configuration at a temperature of 4.2 K in the white frequency range. This corresponds to the current noise of approximate to0.8 and 0.25 pA/Hz(1/2) respectively. The 3 dB bandwidth of the single stage SQUID with closed feedback loop is more than 2.5 MHz. (C) 2002 Elsevier Science B.V. All rights reserved.