Harmonic Analysis of High-Tc Rf SQUID to Determine the Optimum Working Condition for Its Automatic Application

High-Tc radio-frequency (rf) SQUIDs are well researched and have been utilized in a range of applications. The rf SQUID operating in a nonhysteretic mode has been elaborated theoretically in a few of literatures, among which an effective method of harmonic analysis was presented to find its optimum working point. A suitable flux-to-voltage curve of the rf SQUID should be acquired before applying the harmonic analysis method, but the approach of getting the suitable curve is unclear. This article carried out an analysis with respect to the three harmonics, which primarily affect the waveform of flux-to-voltage curves and came to the condition, which need be satisfied to acquire the desirable kind of curves. In which, the one with the largest value of its maximum transfer function could be considered as working in the optimum condition. Three rf SQUIDs were employed in experiment to demonstrate the detailed procedure of determining their optimum working conditions, which were the rf frequencies and the attenuator voltages of their tank circuits. Judging from the experimental results, the method proposed in this article is very effective, the optimum working conditions of the three rf SQUIDs can be accurately obtained. The findings would be instructional for automatically setting the working conditions of high-Tc rf SQUIDs in practical applications.