Amplification by Frequency Transposition with a Low Critical Temperature DC SQUID

(Received 3 November 1994, accepted 5 October 1995)

Abstract
The performances of a heterodyne amplifier involving a DC SQUID have been modelised and measured. This type of amplification is derived from frequency translation methods using flux modulation. The pump frequency used depends on the signal to be analysed and the translation to a fixed frequency f₀ is carried out by the SQUID which is a nonlinear device. Such a system simplifies the impedance matching between the SQUID and the semiconductor preamplifier and can be operated as a spectrum analyser for signals of relatively high and variable frequency. Flux sensitivities of 5 to 10 have been measured in this configuration for frequencies ranging from 200 kHz to 2 MHz. In order to validate our model, the flux sensitivity of the SQUID in the heterodyne configuration has been compared to that given by direct measurements at the intermediate frequency f₀. Our measurements are in good agreement with a theoretical model based on a simplified voltage to flux characteristic of the DC SQUID.