Low-noise S-band DC SQUID based amplifier

A low-noise rf amplifier based on a de SQUID (SQA) is tested in the frequency range 3.3-4.1 GHz, A new signal launching system for the SQA rf coupling has been developed and successfully implemented. The following parameters have been measured at 3.65 GHz using a band-pass filter at the input of a single-stage SQA: gain (11.0 +/-1.0) dB, 3 dB bandwidth of 300 MHz and noise temperature (4.0 +/-1.0) K, This figure corresponds to a flux noise S-Phi(1/2) approximate to 0.6 mu Phi (0)/Hz(1/2) and an energy sensitivity epsilon (i)approximate to 75 h. The input saturation power, P-S, (1 dB gain compression) is measured for different bandwidths of the input band-pass filter. A corresponding input signal saturation temperature (normalized for a 1 GHz bandwidth) T-SAT(1GHZ) = P-SAT/k(B) is estimated to be 11.5 K GHz at an SQA bias voltage 27 muV (condition for minimum noise temperature). The dependencies of the SQA gain, noise temperature and saturation level on the operation point are studied, A reason of the SQA saturation is discussed.