SIS junction as a microwave noise source

Cooled low noise amplifier (LNA) is used as the first or second component of receiver system in radio astronomy. It is developed all over the world to improve its noise. Noise temperature is one of the most important parameters of LNA because it governs the observation time. It is usually measured by inputting two reference signal which have different power each other and measuring the responses. For accuracy, the power of these reference signals is required to be close to the noise of the LNA. In the conventional method output of a Avalanche diode is attenuated to 1/100 by a cold attenuator and used as a reference signal. In this method the error caused by the attenuator and the cable between the Avalanche diode and LNA is too large (a few 10 %) and another accurate noise measurement method is required. In this research we propose a new method using Superconductor-Insulator-Superconductor junction (SIS) as a reference signal. When biased, SIS generates small shot noise that is proportional to the bias voltage so it can be used as a reference signal that make an accurate noise measurement possible (a few % error). First, SIS and SIS housing is designed and fabricated for the shot noise to transmit without any loss and reflection. Then, noise temperature of the LNA is measured by using the conventional method and SIS shot noise method and compared. As a result, there was difference and it was shown that the capacitance of SIS is one of the candidate cause.