Introduction

According to the type of superconducting material used, Superconducting QUantum Interference Devices (SQUID) can be divided into two groups: the low-temperature superconducting and the high-temperature superconducting SQUID. Josephson junction (JJ) is the key element of SQUIDs. JJs are analyzed with the resistively and capacitively shunted junction (RCSJ) model, thus introducing two important parameters: the Stewart–McCumber parameter and the thermal rounding parameter. For readout electronics, the DC SQUID is regarded as dynamic resistance Rd modulated by the flux threading into the SQUID loop. In the case of a direct readout scheme where the SQUID directly connects to a preamplifier, the electronics noise is usually much larger than the SQUID intrinsic noise. The techniques of additional positive feedback, bias current feedback, and noise cancellation are categorized and discussed. This chapter introduces two special DC SQUID readout schemes, the two-stage scheme and the double relaxation oscillation (DROS) scheme.