ABSTRACT. This work reports the preliminary development of 4-20 GHz cryogenic low noise amplifiers based on Diramics InP MMIC technology and designed to improve the sensitivity of the future ALMA sub-millimeter receivers. Two three-stage architectures were implemented. The prototype with detached input circuitry demonstrates state-of-the art average noise temperature around 4 K while simultaneously achieving input return loss above 15 dB in most of the band, enabling operation without cryogenic isolators.

ABSTRACT. The production and qualification of the cryogenic 1st stage low noise amplifiers (LNAs) for the ALMA Band 2 receivers is completed. 145 fully qualified LNA have been delivered to ESO. The full data set of RF performance measurements allows us to have a closer look into statistics and the reproducibility of the production. Most importantly the state-of-the-art noise performance over the extended W-band frequency range (67 - 116 GHz) is confirmed. The 145 LNA module exhibit an average noise temperature of 22.1 K.

ABSTRACT. waveguide loss of the components in front of the first active low-noise devices (mixers or low-noise amplifiers) would degrade the receiver noise performance. In this work, we have investigated the conductor loss of rectangular waveguide components in W- and G-band, the factors investigated include the effect of gold-plating, the surface roughness, and the conductivity of the aluminum alloy in cryogenic environment. In summary, the W-band waveguide components could be qualified in usual milling process, but the G-band waveguide components should be carefully treating the process of fabrication.

ABSTRACT. Abstract— We present details of a beam measurement setup built in the labs of Group for Advanced Receiver Development (GARD) and a new front-end GUI software for acquiring the receiver’s beam pattern and delivering processed data to users. Keywords—Measurement setup, front-end GUI, beam pattern. I. INTRODUCTION Achieving optimal performance for millimetre wave and terahertz radio astronomy observatories including ALMA, APEX and possibly AtLast, requires not only rigorous characterization of individual components but also comprehensive tests of full receiver assemblies under cryogenic conditions, employing a variety of measurement techniques. A critical aspect of this process is the investigation of receiver optics, which ensures precise alignment when installed in the telescopes and enables the high sensitivity in scientific observations. In this report, we present a beam measurement system, derived from an earlier ALMA-specific setup [1] but significantly enhanced with upgraded hardware to improve measurement accuracy, stability and reliability when testing cartridge type THz receivers. II. BEAM MEASUREMENTS SETUP The primary objective of this upgrade was to support measurements within the frequency range of 275–373 GHz, while also extending testing capability to additional frequency bands. The system incorporates a cryogenically cooled test dewar, with its atmospheric optical path inside a dry nitrogen flushed enclosure to keep water vapour out and prevent any condensation and ice buildup at liquid nitrogen container surfaces where the cold load is placed. The enclosure also houses components such as the test source/pilot signal, xyz linear stages for pilot signal beam scanning, and hot/cold loads for noise characterisation. The hot load has been designed with a compact structure and excellent temperature stabilization for receiver saturation measurements, whereas the cold load is made of new foam materials’ container and internal supports to enhance mechanical stability. The cryogenically cooled dewar supports one cartridge (ALMA compatible) receiver system [2]. The frequency dependent optical components in the dewar i.e. infra-red (IR) filters and optical window must match the quasi-optical scheme and frequency as the intended receiver cartridge under test. In addition, for setup validation, the beam measurement system was built up with receiver and transmitter for pilot signal operating at warm temperature, including a separate quasi-optical system, replicating the final cold Rx beam as close as possible for verification of the data collection and data analysis. Both measurement modes, i.e. the warm and the cryogenically cooled receiver, make use of a Vector Network Analyzer (VNA) for the amplitude and phase data collection. In its warm temperature mode, the setup uses two VNA frequency extenders, one as pilot signal and the other as a receiver within any wanted designated waveguide band. The local oscillator for the Rx in cryogenic measurement mode, features an ALMA Warm Cartridge Assembly (WCA) with a NRAO Amplifier Multiplication Chain (AMC), and an identical AMC is used as pilot signal. III. FRONT-END GRAPHICAL USER INTERFACE For streamlining and automating beam measurement routines, a dedicated front-end graphical user interface (GUI) has been developed. This data collection software has a survey mode, where speed is prioritized and data is reused to assist in finding the beam centre around where the actual measurement should be done. Several different strategies are available for the actual data collection to better suite the post processing. Strategies may differ in coordinate pattern, if they need to be in discrete steps or not, circular or square border, how phase should be tracked and/or corrected. The GUI uses the GARD Measurement System (GMS) [3], a hardware abstraction layer implemented in Qt/C++, the back-end software enables efficient communication between multiple GUIs and the measurement instruments, ensuring reliable data acquisition and.

At the Conference, we will present the design of the updated beam measurement system, the architecture of the front end GUI software, and representative results obtained with the new setup. These developments substantially advance GARD’s characterization capabilities, supporting the construction of state of the art next generation receivers for radio astronomy.

REFERENCES [1] O. Nystrom, et. al., “Integrated Setup for THz Receiver Characterization”, Proceedings of the 21st International Symposium on Space Terahertz Technology, Oxford, UK, March 23–25, 2010, pp 374-378. [2] Yutaro Sekimoto, et. al., “Cartridge Test Cryostats for ALMA Front End”, ALMA Memo #455, [Online]. Available: https://library.nrao.edu/public/memos/alma/memo455.pdf. [3] M. Strandberg et al., "Setup for Measurement and Characterization of Cryogenic Low-Noise Receivers," 2024 4th URSI Atlantic Radio Science Meeting (AT-RASC), Meloneras, Spain, 2024, pp. 1-4.

ABSTRACT. Blackbody on-board calibration targets are key components of spaceborne terahertz radiometers. These must have very low reflectivity, which can be achieved by applying a THz absorbing coating to a surface with a complex reflection geometry. However, microwave-absorbing media are poorly characterized in the 0.5–5.0 THz. We use a quantum-cascade laser self-mixing interferometry technique to measure the reflectance of a range of black coatings. Surrey Nanosystems Vantablack S-VIS and Nextel Velvet-Coating 811-21 provide the highest normal-incidence return losses of ≥26 dB at 3.4 THz.

ABSTRACT. We demonstrate terahertz (THz) gas spectroscopy using difference-frequency generation with two mid-IR quantum cascade lasers mixed on a nonlinear THz metasurface. The THz emission is tuned from 4.650 to 4.695 THz (45 GHz tuning bandwidth) to observe the products of a reaction between heavy water (D2O) and water (H2O) at a gas cell pressure of 5 Torr. The narrow-band emission from the metasurface enables us to resolve HDO as a product of deuterium exchange between D2O and H2O.

ABSTRACT. For high angular resolution observation in terahertz frequencies, intensity interferometry has been proposed. Due to Bose-Einstein statistics of thermal radiation, photons are bunched in terahertz frequencies. Photon bunches can be a measure of delay time between telescopes to obtain a complex visibility. Using our laboratory setups of terahertz intensity interferometry, we have started fast intensity measurements of a blackbody furnace to measure photon bunches. We will be discussing updated experimental status and prospects for Antarctic and space-borne intensity interferometry.

ABSTRACT. Optical losses in cryogenic heterodyne receivers are hard to quantify because they are small, distributed across multiple elements, and temperature dependent, yet they can dominate the effective receiver noise. We present a numerical straight-line/intersecting-lines framework that extracts optical loss and mixer/IF parameters from standard hot/cold (Y-factor) measurements by sweeping LO drive below the noise optimum. In this regime, the family of P(T) lines obtained at different LO drives exhibits a common crossing whose intercept yields an input-referred temperature linked to front-end losses and IF contribution. As whe scale the analysis across the full IF band, dense LO-drive sampling, and multiple LO frequencies, we obtain frequency-resolved loss estimates with confidence intervals from crossing statistics and improved repeatability compared to sparse manual analyses.

ABSTRACT. I present Erik L. Kollberg’s final theoretical work, a crucial study on the impedance of superconducting hot-electron bolometer (HEB) mixers at terahertz frequencies, which was completed after his retirement and published posthumously. The work addresses a fundamental yet long-standing question in HEB mixer physics: the nature of current distribution in ultrathin superconducting films at terahertz frequencies and its consequences for HEB impedance.

ABSTRACT. We demonstrate the operation of the NbN HEB mixer with negative differential resistance (NDR). This mode of operation has led to increased conversion gain, as shown experimentally. This was made possible by designing a conditioning circuit around the HEB device that reduces the risk of parasitic oscillations in the bias circuit.

ABSTRACT. Although terahertz instrumentation technologies are rapidly advancing, the upper edge of the terahertz band is still missing well-studied detector solutions for high-resolution heterodyne receivers. In this work, we present results of our studies on the performance of a quasioptical hot electron bolometer (HEB) mixer at 10.7 THz. We have measured output power of the mixer at intermediate frequency, responding to the hot/cold load change, as a function of bias voltage and local oscillator (LO) power. At a bath temperature of 4 K, the estimated optimum absorbed LO power and double sideband (DSB) noise temperature of the mixer are equal to 250 nW and 9500 K, respectively.

ABSTRACT. Abstract— We present the latest results in the development of a wideband 2SB SIS mixer that the covers RF band 210-375 GHz with an IF band 4-18.5(20) GHz. This mixer and receiver based on this mixer have a wide range of applications starting from single- dish observations, e.g. APEX SEPIA and potentially in ngEHT project and space VLBI. The SIS mixer features an integrated IF circuitry implemented on alumina substrate with Nb superconducting transmission lines and comprising IF matching components, IF/DC bias-T and IF 90-degree 3 dB hybrid. The mixer employs SIS junctions produced in-house with tunnel barrier made of AlN and having junction area about 2 µm2 in the twin configuration with RnA =14 Ω•µm2. The tested noise performance of the SIS receiver with this mixer in combination with OMT, corrugated feed and optics optimized for ALMA B6+7 RF band demonstrates the noise temperature about x5 quantum noise hf/k SSB with sideband rejection above 10 dB.

ABSTRACT. We have developed and tested Sideband-separating (2SB) receiver for 211-275 GHz band based on superconductor-insulator-superconductor (SIS) mixers. The 2SB mixer is a prototype for the Millimetron space observatory VLBI instrument. The 2SB mixer demonstrates good performance in 4-8 GHz range of intermediate frequencies with a prospective to be improved to 4-12 GHz band. The uncorrected noise temperature is on average below 100 K. Here we present the performance of the developed 2SB SIS mixer and will report on current status of the Millimetron space observatory.

ABSTRACT. We report on the design and measurement of SIS mixers for the ALMA Band 6v2 receiver upgrade. There have been three design iterations. The first two iterations utilized junctions similar to those of the original Band 6v1 production SIS mixers, with a target normal resistance of 60 ohms. These mixers tended to have modest conversion loss, low noise, and low input reflection, but high IF output impedance. To achieve wider IF bands up 4-20 GHz with low noise and low ripple, wideband low-loss IF isolators directly following the mixer are almost indispensable, but add loss. The third design iteration deviates significantly from the first two in order to meet the challenging Band 6v2 receiver noise specifications. The driving goal is to have lower IF output impedance, while maintaining low conversion loss and noise, which is realized by going to lower normal resistance junctions with higher critical current density. This enabled designs with lower conversion loss, and even a small conversion gain. Such designs helped overcome the added noise contribution of the isolator, leading to acceptable receiver noise temperatures. Design details and measurements will be presented.