Abstract: A liquid cryogen dewar is constructed in which two or more liquid cryogen chambers are suspended within a case wherein the innermost cryogens have a higher boiling, or sublimation, temperature than the cryogen in the next outermost chamber. The chambers may contain liquid helium, liquid nitrogen and solid CO.sub.2 proceeding from the innermost to the outermost chamber. Superconducting electronic circuitry such as SQUID devices may be connected to the outer case of the liquid helium cryogen chamber. An isothermal shield is also suspended outside the helium chamber. The elements of the dewar are supported by glass epoxy rods and polyester cords of low thermal conductivity. A fill and exhaust port is provided which communicates with fill and exhaust tubes that are heat sunk to various locations of the rods and the Dacron cords.

Abstract: A two-junction DC SQUID is supplied a single square-wave frequency signal that both biases the SQUID and provides modulation with a square-wave train that is 90.degree. phase displaced with respect to the bias square-wave train in order to eliminate low frequency noise with cross-modulation frequency products.