Digital Front-End Development for ALMA

Improvement of ALMA observatory requires increase of bandwidth, sensitivity and number of antennas. All these improvements will meet a bottleneck if ALMA digital system is not addressed. Currently a 2 bit digitizer supporting <8 GHz bandwidth per polarization is employed limiting correlation efficiency at 88%. Implementation of ALMA downconverter substantially limits line searches compared to sensitivity limit and it's bandwidth is not extendable. We have initiated study on possible implementation of ALMA digital front-end system which includes modular digitizer, digital processing at antenna and TCP/IP optical fiber data transfer. We report on requirements, trade-offs and possible implementation of modular digital front-end and analyze its impact on sensitivity of current ALMA system as well as future one. We review technology and performance available in near future. Significant performance improvement can be realized at moderate cost

Digital Front-End Development for ALMA

Improvement of ALMA observatory requires increase of bandwidth, sensitivity and number of antennas. All these improvements will meet a bottleneck if ALMA digital system is not addressed. Currently a 2 bit digitizer supporting <8 GHz bandwidth per polarization is employed limiting correlation efficiency at 88%. Implementation of ALMA downconverter substantially limits line searches compared to sensitivity limit and it's bandwidth is not extendable. We have initiated study on possible implementation of ALMA digital front-end system which includes modular digitizer, digital processing at antenna and TCP/IP optical fiber data transfer. We report on requirements, trade-offs and possible implementation of modular digital front-end and analyze its impact on sensitivity of current ALMA system as well as future one. We review technology and performance available in near future. Significant performance improvement can be realized at moderate cost

Wide band instantaneous coverage receiver concept for ALMA

We propose concept of frequency array receiver. Receiver input beam will be quasi-optically frequency multiplexed on several mixer pixels. All individual mixers will be operated simultaneously creating very large input bandwidth covering several atmospheric windows in high frequency resolution. This system allows to combine large bandwidth advantage of direct detecting with high frequency resolution of heterodyne system. We will discuss initial system concept, and report on mixer designs and their performance for building elements. In particularly we report DSB noise temperature below hf/k level at 260GHz mixer prototype. Sensitivity of such system in presents of atmospheric background will be evaluated

Efficiency of the Image Band Suppression in Sideband Separating SIS Receivers for Radio Astronomy

The sideband separating SIS receivers are known as the most sensitive instruments for millimeter and submillimeter ground based radio astronomy. Sideband Ratio or Sideband Rejection Ration (SRR), is one of the key parameters of heterodyne receivers, because it is strongly influencing the system sensitivity. This effect takes place due to sufficient signal losses in the atmosphere or in the instrument optics, which is reducing the signal to noise ratio. To develop high performance receiver SIS sideband separating receiver with have performed a comprehensive analysis of the signal transformation in both high frequency (RF) and low frequency (IF) parts of the receiver. As result, the entire IRR pattern was simulated. It was found that SRR performance is very much restricted by reflections in RF and IF parts of the receiver. Minimization of these reflections is curtail for achieving SRR levels of 15 dB or higher. This knowledge is used in development of sideband separating SIS receiver for ground based telescope APEX and for Millimetron space mission

Advanced tuning algorithms for high-frequency SIS mixers

Three tuning parameters are key to SIS performance: SIS bias voltage, LO pumping level and Josephson current suppression. The former two are relatively easy to perform automatically, although there are a couple of pitfalls. The suppression of the Josephson current, however, is much more complicated, especially in the high-current-density AlN-barrier junctions used in ALMA Band 9. The tuning parameter sets that were supplied with the receivers were always biased towards safer and more repeatable regimes for operational reasons. Here we present results of the ALMA Band 9 Advanced Tuning Study commissioned by ESO. The objective of this study is to investigate more "intelligent" tuning algorithms which should enable the receivers to operate in more critical regimes that were previously avoided, but with possibly a considerable increase in performance

The line-of-sight analysis of spatial distribution of galaxies in the COSMOS2015 catalogue

New observations of high-redshift objects are crucial for the improvement of the standard $\Lambda$CDM cosmological model and our understanding of the Universe. One of the main directions of modern observational cosmology is the analysis of the large-scale structure of Universe, in particular, in deep fields. We study the large-scale structure of the Universe along the line of sight using the latest version of the COSMOS2015 catalogue, which contains 518,404 high quality photometric redshifts of galaxies selected in the optical range of the COSMOS field ($2\times 2$ deg$^2$), with depth up to the redshift $z \sim 6$. We analyze large-scale fluctuations in the number of galaxies along the line of sight and provide an estimate of the average linear sizes of the self-correlating fluctuations (structures) in independent redshift bins of $\Delta z = 0.1$ along with the estimate of the standard deviation from homogeneity (the observed cosmic variance). We suggest a new method of the line-of-sight analysis based on previous works and formulate further prospects of method development. For the case of the theoretical form of approximation of homogeneity in the $\Lambda$CDM framework, the average standard deviation of detected structures from homogeneity is $\sigma_\text{mean}^{\Lambda \text{CDM}} = 0.09 \pm 0.02$, and the average characteristic size of structures is $R_\text{mean}^{\Lambda \text{CDM}} = 790 \pm 150$ Mpc. For the case of the empirical approximation of homogeneity, the average standard deviation of detected structures from homogeneity is $\sigma_\text{mean}^\text{empiric} = 0.08 \pm 0.01$, and the average characteristic size of structures is $R_\text{mean}^\text{empiric} = 640 \pm 140$ Mpc.Comment: 21 pages, 18 figures, Universe accepted 2020.11.1

High-Performance Smooth-Walled Horn Antennas for THz Frequency Range:Design and Evaluation

Traditionally, corrugated conical horn antennas have been the main choice for use in astronomical receivers in the range of millimeter and submillimeter waves. They present low cross-polar level and high coupling efficiency into the fundamental Gaussian mode. However, this type of antenna is difficult to manufacture, inevitably increasing its price and extending the production process. In this article, we present two kinds of feed horn antennas, aimed for use in a frequency range equivalent to atacama large millimeter/submillimeter array (ALMA) Band 6 (211-275GHz), which can be fabricated in a much simpler way with the conventional machining tools. Specifically, we present the design and performance comparison of smooth-walled spline-profile horns in two geometries, diagonal, and conical. Optimization of the designs has been made by means of an algorithm that allowed us to obtain models whose electrical and mechanical characteristics make them competitive when compared with corrugated horns. In particular, they are $\text{40}\%$ shorter than the conventional corrugated horns suited for this band, representing an advantage given the stringent space constraints of most astronomical receivers. We also demonstrate that they can be coupled efficiently to an astronomical-grade optical system, using ALMA Band-6 receiver as an example. Furthermore, we have constructed the diagonal horn and characterized it thoroughly. Experimental results of the radiation pattern at room temperature show a good cross-polar performance with levels below -20dB and Gaussicity above $\text{96}\%$. Our calculations show a good antenna-efficiency level with losses less than $\text{1}\%$. All these properties demonstrate the feasibility of this type of horns to become the main option at the time of choosing a feed system for cutting-edge astronomical applications

Terahertz Spectroscopy of Gas Absorption Using the Superconducting Flux-Flow Oscillator as an Active Source and the Superconducting Integrated Receiver

We report on the first implementation of a terahertz (THz) source based on a Josephson flux-flow oscillator (FFO) that radiates to open space. The excellent performance of this source and its maturity for practical applications has been demonstrated by the spectroscopy of gas absorption. To study the radiated power, we used a bolometric detection method and additionally calibrated the power by means of pumping the superconductor–insulator–superconductor (SIS) junction, integrated on a single chip with the FFO. For calibration, we developed a program using the SIS-detected power calculations in accordance with the Tien and Gordon model. The power emitted to open space is estimated to be from fractions of µW to several µW in the wide region from 0.25 THz up to 0.75 THz for different designs, with a maximum power of 3.3 µW at 0.34 THz. Next, we used a gas cell and a heterodyne superconducting integrated receiver to trace the absorption lines of water and ammonia with a spectral resolution better than 100 kHz. Our experiment for gas absorption is the first demonstration of the applicability of the FFO as an external active source for different tasks, such as THz spectroscopy, near-field THz imaging and microscopy

The Influence of LO Power Heating of the Tunnel Junction on the Performance of THz SIS Mixers

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Heterodyne Receiver for Origins

The Heterodyne Receiver for Origins (HERO) is the first detailed study of a heterodyne focal plane array receiver for space applications. HERO gives the Origins Space Telescope the capability to observe at very high spectral resolution (R = 107) over an unprecedentedly large far-infrared (FIR) wavelengths range (111 to 617 μm) with high sensitivity, with simultaneous dual polarization and dual-frequency band operation. The design is based on prior successful heterodyne receivers, such as Heterodyne Instrument for the Far-Infrared /Herschel, but surpasses it by one to two orders of magnitude by exploiting the latest technological developments. Innovative components are used to keep the required satellite resources low and thus allowing for the first time a convincing design of a large format heterodyne array receiver for space. HERO on Origins is a unique tool to explore the FIR universe and extends the enormous potential of submillimeter astronomical spectroscopy into new areas of astronomical research