Resonance analysis using a high frequency simulation method for dc-SQUID amplifiers

ABSTRACT We performed high-frequency simulation to optimize the geometric size of dc-SQUID amplifiers and to remove their intrinsic resonance characteristics from the operating range. The aim is to eliminate distortions in the IVΦ characteristics that ensures low noise and stable operation over a large range of bias current. With this method, we designed and fabricated a dc-SQUID amplifier with two series-connected octagonal loops to achieve a robust operating performance. The measured results not only show smooth IVΦ characteristics without any distortion, but also demonstrate that the swing voltage is much larger than the corresponding voltage realized without this method at the same bias current. In addition, the white flux noise level as low as 1.7 μΦ0/√Hz was achieved at 0.1 K corresponding to a low current noise level of 15 pA/√Hz.