The Microstrip DC Squid Amplifier

The fabrication, operation and performance of the dc Superconducting Quantum Interference Device (SQUID) are briefly described. The SQUID is used as a radiofrequency amplifier in a new mode in which the input signal is coupled between the square washer that forms the body of the SQUID and one end of the spiral coil overlaying the washer. The coil and the groundplane, together with the intervening dielectric layer, form a microstrip which resonates when its length is equal to one-half the signal wavelength. The resonance can be tuned from about 0.1 to 0.8 GHz by varying the length of the coil The magnetic flux generated in the microstrip is coupled into the SQUID. Positive feedback from the output of the SQUID to the microstrip enhances the gain and the quality factor. At 1.8 K and with a cooled semiconductor post-amplifier, a gain of 24.5 ± 0.5 dB and an intrinsic noise temperature of 0.19 ± 0.06 K have been achieved at 365 MHz. Cooled to 0.48 K in a 3He refrigerator, a microstrip SQUID amplifier achieved a gain of 20 ± 0.5 dB and a noise temperature of 0.12 ± 0.10 K at 438 MHz. The resonant frequency can be reduced by about 40% by means of a varactor diode connected across the open end of the microstrip. Two microstrip SQUIDs have been operated in cascade to achieve an overall gain of 33.5 ± 1 dB at 386 MHz. The microstrip SQUID amplifier is intended for a second-generation axion detector.