Experimental determination of HTS dc-SQUID amplifier inductance and noise performance

The inductance L_sq, transimpedance Z, transfer function V_Φ and flux noise S_Φ have been experimentally determined, in open loop mode, for a series of simple dc-SQUID amplifiers which varied only in the length of the loop. The SQUID design had no input flux transformer but included a current path for directly injecting flux into the loop. Correlations between these parameters were obtained which compare favorably with theoretical estimates. We confirm that the best device performance (small S_Φ) is obtained if L_sq is kept small whilst V_Φ is maximized; the latter was achieved in one device by a novel "ion-beam trimming" technique.