Some New Aspects concerning the X-Band SQUID for the Measurement of RF Attenuation

In this report the performance of a microwave-pumped SQUID with niobium, tantalum, and vanadium point-contact junctions is discussed. These SQUID's were developed for measurements of RF attenuation in the frequency range 0-1 GHz. The equipment described here is controlled by a desktop computer and can be used to measure attenuation up to 60 dB. A simple analytical theory for the operation of the X-band SQUID is also described. In our experiments we have reached the conclusion that the most important limitations on RF measurements at high frequencies and high signal levels are the voltage and the power dependence of the quasi-particle conductance of the Josephson junction. In this paper we have considered this source of error in some detail. We have been able to show that a SQUID can be used successfully for RF measurements at very high signal frequencies if a junction material with a sufficiently low critical temperature is chosen.