Low-noise S-band DC SQUID based amplifier

A low-noise rf amplifier based on a dc 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_Φ^½=0.6 μΦ₀/Hz^½ and an energy sensitivity ε_i≈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 μV (condition for minimum noise temperature). The dependencies of the SQA gain, noise temperature and saturation level on the operation point are studied. The reason for the SQA saturation is discussed