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Journal of Physics Conference Series

SIS junction as a microwave noise source

Hirofumi Inoue, Takashi Noguchi, Kotaro Kohno

Published Jun 1, 2010• 3 citations

Abstract

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.

DOI: 10.1088/1742-6596/234/4/042014

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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.

Cryogenic noise measurements are typically carried out using a variable-temperature load or a commercial noise diode in conjunction with a cryogenically-cooled attenuator. In both of these cases, thermal gradients can present a significant source of uncertainty. In the astronomy community, SIS mixers biased beyond their gap voltage are often used as shot-noise sources and employed to determine the noise of IF amplifiers. In this work, a tunnel junction with metal electrodes is used as noise source for characterization of cryogenic amplifiers. It is shown that this method is easily calibrated and can provide results that agree well with those obtained using the cold-attenuator method.

This paper describes a low-noise microwave amplifier based on up- and down-frequency-conversion processes in quasiparticle superconductor–insulator–superconductor (SIS) tunnel junctions. The SIS amplifier was configured with two SIS frequency-converter modules and a cryogenic millimeter-wave isolator inserted between them. Moreover, a local oscillator (LO) using millimeter-wave attenuators and a phase shifter was considered. This setup allowed the control of individual LO power and differential phase in these SIS frequency converters to optimize the amplifier performance. The SIS amplifier showed noise temperatures as low as 11 K and a 6–8 dB gain from nearly DC to 5 GHz. The attained microwave performance is promising for obtaining large-format arrays, such as multibeam heterodyne receivers. Moreover, this two-frequency-converter concept based on SIS junctions might enable microwave applications, such as wideband non-reciprocal circuits in isolators, gyrators, and circulators, which are essential devices in the quantum computing and radio astronomy fields.

We are investigating a possible microwave amplifier with low noise and low power consumption at cryogenic temperature for large scale multi-pixel heterodyne superconductor-insulator-superconductor (SIS) receivers at millimeter and submillimeter wavelengths. We propose the use of SIS junctions as amplifier elements based on quasi-particle mixing. By connecting an SIS up-converter and an SIS down-converter in series with gain in both converters, a lownoise and low-power-consumption high-frequency amplifier can be obtained in principle. A proof-of-concept study has been made by configuring an amplifier with two Nb/Al-AlOx/Nb mixers in the 150-GHz band in a standard noise and gain measurement setup at 4 K with a microwave noise source as an input signal. We observed a maximum gain of more than 10 dB and a minimum noise temperature of less than 10 K, which suggests that our proposed SIS amplifier is capable for multi-pixel SIS receivers. On the other hand, we also observed a periodical behavior in frequency dependence of the measured noise temperature and gain due to a standing-wave effect between the two SIS mixers, which is a problem to be solved.

In the assessment of a superconductor-insulator-superconductor (SIS) mixer, the breakdown of the overall receiver noise into intrinsic mixer noise and noises from other components is indispensable. The input noise due to losses in front of the mixer is conventionally measured by assessing the receiver noise as a linear function of mixer conversion loss at low local oscillator power and extrapolating the line to an infinite gain. However, it has been observed that this method tends to overestimate the input noise. To address this, the underlying theory of the input noise measurement method was revisited. It was found that the product of mixer gain and noise, i.e., the output noise of the mixer, is not a constant with respect to LO power. Therefore, extrapolating the linear relation to a large gain, where the input noise is inferred, becomes problematic. Numerical simulations were conducted to validate this analysis. Finally, a two-step measurement approach is introduced, in which an attenuator is inserted in front of the SIS mixer at the 4-K cold stage, to obtain the actual RF input noise.

With sensitivity approaching the quantum limit, superconductor-insulator-superconductor (SIS) mixers play an important role in radio astronomy and atmospheric science at millimeter and submillimeter wavelengths. As one of the intrinsic noise sources in superconducting tunneling junctions, shot noise is still not well understood, particularly for those of relatively high energy gap (e.g., NbN/AlN/NbN). In this paper, we mainly study the multiple Andreev reflection (MAR) enhanced shot noise of two different NbN SIS junctions (i.e., junction array and long junction) as well as their temperature dependence. Barrier strength Z is taken into consideration for theoretical analysis of effective charge of the MAR effect based on the Blonder-Tinkham-Klapwijk (BTK) theory with the Andreev clusters method. It has been found that the effective charge of the MAR effect is inversely proportional to temperature, the barrier strength Z and transparency T are proportional to temperature. Detailed measurement results and analysis will be presented.