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IEEE Transactions on Applied Superconductivity

Inconstant Output Noise of SIS Mixers and Its Implication in Input Noise Measurement

Wenlei Shan

Published Dec 11, 2023• 0 citations
Publisher

Abstract

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.

DOI
10.1109/tasc.2023.3341064

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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.
We present measurements on two high frequency S(uperconductor)I(nsulator) S(uperconductor) heterodyne receivers at 660 GHz and 810 GHz. Their noise temperatures are 163 K and 860 K, respectively. The contribution of the high frequency (RF) input loss to this number is measured with the commonly used method of intersecting lines. The result is compared to a more rigorous analysis using the full quantum theory of mixing. We will show that in this frequency range the method of intersecting lines in general leads to an overestimation of the RF input loss. Even at 660 GHz, with a well matched SIS junction, the input noise contribution is overestimated by about 25 percent. This is due to the summation of the contributions due to the vacuum fluctuations, which is the highest contribution, the mismatch at the intermediate frequency (IF), and the fact that the mixer output noise is not completely independent of local oscillator (LO) power. At 810 GHz, due to a much stronger loss-induced mismatch of the SIS junction, the mixer output noise varies considerably with LO-power. The large loss of approximately 8 dB directly in front of the mixer enhances this contribution to the measured RF input loss. The contribution of the IF-mismatch is likewise enhanced. Together with the contribution of the vacuum fluctuations, this can lead to an overestimation of the RF input noise by more than 100 percent, depending on the actual mismatch of the mixers. We therefore conclude that, for the severe mismatch that occurs in Nb based SIS mixers above 700 GHz, the method of intersecting lines gives incorrect results and the full analysis should be applied.
The authors present numerical calculations of the gain and noise of a two-LO (local oscillator) mixer based on an ideal SIS (superconductor-insulator-superconductor) IV curve. They demonstrate the phase-sensitive response of the mixer with two-LO pumping and present calculations of the noise for various DC bias conditions and embedding impedances. Preliminary results indicate that for certain DC bias conditions and signal terminations, the noise added to a signal by the two-LO mixer operated in the single-sideband mode can be a factor of 25 less than that added by a conventional single-sideband single-LO mixer.>
Accurate knowledge of radio-frequency (RF) losses in front of superconductor--insulator--superconductor (SIS) mixers is essential for optimizing millimeter- and terahertz heterodyne receivers. However, such losses are difficult to extract from conventional hot/cold-load measurements because the measured receiver noise combines contributions from the input optics, the mixer, and the intermediate-frequency chain. In this work, we present a numerical implementation of the straight-line, or intersecting-lines, method for estimating input-referred RF noise and optical loss from standard Y-factor measurements. Hot/cold IF spectra are acquired while sweeping the local oscillator (LO) pumping level from under-pumped to over-pumped operation, so that each IF frequency bin gives a family of linear relations between receiver output power and input load temperature. We evaluate all pairwise intersections between these lines and analyze the crossing cloud to obtain the effective RF input noise. We repeat the analysis across the IF band and multiple LO tunings which provides a frequency-resolved characterisation of the input losses over the full band. Our implementation turns the straight-line technique into a reproducible framework for receiver characterisation and relative optical-loss comparison in low-loss front-end configurations.
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.
A conventional superconductor-insulator-superconductor (SIS) mixer, that is pumped by a single-frequency local oscillator (LO) functions as a phase-insensitive linear amplifier and may achieve a noise level near the quantum limit of one-half photon added noise per unit bandwidth. A phase-sensitive linear amplifier may have less noise than this quantum limit. To overcome the quantum noise limit with an SIS mixer, a two-LO technique which makes the mixer's gain dependent upon the phase of the incoming signal was used. The phase sensitive gain of this two-LO mixer is experimentally demonstrated. The gain variation from maximum to minimum is more than 20 dB. Theoretical predictions of the noise of the two-LO mixer are presented.
The output noise temperature, rather than the standard input noise temperature, is shown to be the fundamental measure of the shot noise of the superconducting (SIS) quasiparticle mixer. This conclusion is based on extensive numerical computations of the properties of optimized SIS (superconductor-insulator-superconductor) receivers and also on analytic calculations with separate consideration of correlated and uncorrelated noise sources. The quantum theory of mixing is used to derive a simple analytic expression for the mixer output noise which is independent of frequency, and qualitatively explains the dependence of the output temperature on each of the receiver's operating parameters. The derivation leads to a novel understanding of the SIS mixer which, distinguishes between its correlated and uncorrelated noise sources. The authors predict T(receiver)=2*T(mixer) for many SIS receivers. Numerous experiments substantiate the analysis.>