Performance of NoiseTech’s Cryogenic Impedance Generators

Cryogenic low-noise amplifiers (Cryo-LNAs) are key receiver components in ultra-sensitive systems such as radio telescopes, satellites, and quantum computing. These LNAs are capable of achieving exceptionally low noise temperatures, in the order of just 1 or 2 K. With noise temperature this low, measurement of Cryo-LNA noise performance is difficult and subject to systematic errors such as those due to noise source uncertainties in the range of 7 to 20 K (0.1-to-0.3dB uncertainty in noise source ENR). Clearly such large uncertainties make accurate characterization of Cryo-LNAs difficult. To overcome this problem, cold-attenuator method is commonly used. With this method, an attenuator is placed in the cryogenic dewar at the input to the LNA. The attenuator loss reduces the amount of noise source uncertainty and improves the Cryo-LNA noise-figure measurement accuracy. The recent trend is to equip radio telescopes with phased-array feeds to achieve wider field of view by creating multiple simultaneous beam and improve the telescope field of view and survey speed. A great example of such a system is the Green Bank Telescope cryogenic phased array feed [4]. Developments of phased-array feeds however create a new Cryo-LNA characterization problem. For proper noise matching Cryo-LNAs to phased-array-feed elements, the Cryo-LNAs have to be designed such that their optimum reflection coefficients for minimum noise, Γopt, equaled the active reflection coefficient of the antenna array, Γact. The theory on determining Γact has been well developed, however, the measurement of Cryo-LNA noise parameters and in particular Γopt is problematic.