J. Barkhof

The ALMA Band 9 receiver - Design, construction, characterization, and first light

Aims. We describe the design, construction, and characterization of the Band 9 heterodyne receivers (600–720 GHz) for the Atacama Large Millimeter/submillimeter Array (ALMA). First-light Band 9 data, obtained during ALMA commissioning and science verification phases, are presented as well. Methods. The ALMA Band 9 receiver units (so-called “cartridges”), which are installed in the telescope’s front end, have been designed to detect and down-convert two orthogonal linear polarization components of the light collected by the ALMA antennas. The light entering the front end is refocused with a compact arrangement of mirrors, which is fully contained within the cartridge. The arrangement contains a grid to separate the polarizations and two beam splitters to combine each resulting beam with a local oscillator signal. The combined beams are fed into independent double-sideband mixers, each with a corrugated feedhorn coupling the radiation by way of a waveguide with backshort cavity into an impedance-tuned superconductor-insulator-superconductor (SIS) junction that performs the heterodyne down-conversion. Finally, the generated intermediate frequency (IF) signals are amplified by cryogenic and room-temperature HEMT amplifiers and exported to the telescope’s IF back end for further processing and, finally, correlation. Results. The receivers have been constructed and tested in the laboratory and they show an excellent performance, complying with ALMA requirements. Performance statistics on all 73 Band 9 receivers are reported. Importantly, two di_erent tunnel-barrier technologies (necessitating di_erent tuning circuits) for the SIS junctions have been used, namely conventional AlOx barriers and the more recent high-current-density AlN barriers. On-sky characterization and tests of the performance of the Band 9 cartridges are presented using commissioning data. Continuum and line images of the low-mass protobinary IRAS 16293-2422 are presented which were obtained as part of the ALMA science verification program. An 8 GHz wide Band 9 spectrum extracted over a 0:300 _0:300 region near source B, containing more than 100 emission lines, illustrates the quality of the data.

Design and performance of the 600-720 GHz ALMA band 9 cartridge

The ALMA band 9 cartridge is a compact receiver unit for the Atacama large millimeter array (ALMA) containing the core of a 600-720 GHz heterodyne front end, including a solid-state local oscillator; broadband SIS mixers; and a 4-12 GHz IF amplifier chain. This paper describes the design and performance of the first band 9 cartridge. The experience gained with this unit is being used to optimize the design in preparation for the production of sufficient cartridges to fully populate the ALMA and ALMA compact arrays (ultimately requiring roughly 80 cartridges).

Sideband separating mixer for 600-720 GHz for ALMA band 9 upgrade

For high-frequency observational bands like ALMA (Atacama Large Millimeter Array) Band 9 (600—720 GHz), which tend to be dominated by atmospheric noise, implementation of sideband-separating mixers can reduce, up to a factor of two, the integration time needed to reach a certain signal-to-noise ratio for spectral line observations. Because of very high oversubscription factor for observation in ALMA Band 9, an upgrade of the current Double Sideband (DSB) mixer to a Two Sideband (2SB) configuration is a promising option for future ALMA development. Here we present a developed 2SB mixer and a modified cartridge design. The 2SB mixer includes a waveguide RF hybrid block, which have been produced on a micro-milling machine and equipped with standard Band 9 SIS mixer devices. These two SIS mixers have been separately tested in DSB mode. The SSB noise temperature is within the ALMA requirements of 336 K over 80% of the band, and 500 K over the entire band. The 2SB mixer has the sideband rejection ratio better than 12 dB over the full RF band, which is also well within the ALMA specifications of 10 dB.

SEPIA - a new single pixel receiver at the APEX Telescope

Context: We describe the new SEPIA (Swedish-ESO PI Instrument for APEX) receiver, which was designed and built by the Group for Advanced Receiver Development (GARD), at Onsala Space Observatory (OSO) in collaboration with ESO. It was installed and commissioned at the APEX telescope during 2015 with an ALMA Band 5 receiver channel and updated with a new frequency channel (ALMA Band 9) in February 2016. Aims: This manuscript aims to provide, for observers who use the SEPIA receiver, a reference in terms of the hardware description, optics and performance as well as the commissioning results. Methods: Out of three available receiver cartridge positions in SEPIA, the two current frequency channels, corresponding to ALMA Band 5, the RF band 158--211 GHz, and Band 9, the RF band 600--722 GHz, provide state-of-the-art dual polarization receivers. The Band 5 frequency channel uses 2SB SIS mixers with an average SSB noise temperature around 45K with IF (intermediate frequency) band 4--8 GHz for each sideband providing total 4x4 GHz IF band. The Band 9 frequency channel uses DSB SIS mixers with a noise temperature of 75--125K with IF band 4--12 GHz for each polarization. Results: Both current SEPIA receiver channels are available to all APEX observers.

Modular 2SB SIS Receiver for 600–720 GHz: Performance and Characterization Methods

A modular sideband-separating (2SB) receiver for 600-720 GHz has been built and tested. The used modular design allows to characterize all the parts separately, including testing of the superconductor-insulator-superconductor (SIS) junctions individually in a double-sideband mode before building them in to the 2SB assembly. The developed 2SB mixer has a single sideband noise temperature below 380 K in the entire operating band and reaches a level of 200 K in the best point. At the same time, the image rejection ratio (IRR) was demonstrated to be better than 11.5 dB over the entire band. However, we have found a discrepancy between the observed and expected performance. To investigate this problem, we have developed and applied methods to characterize RF and intermediate frequency imbalances of a fully assembled 2SB mixer using the SIS junction properties. As result, the IRR of our mixer was found to be limited by the RF imbalance, which is caused by complex standing waves created by reflections from the SIS junctions, the RF hybrid and the RF absorption load.

ALMA Band 5 receiver cartridge - Design, performance, and commissioning

We describe the design, performance, and commissioning results for the new ALMA Band 5 receiver channel, 163–211 GHz, which is in the final stage of full deployment and expected to be available for observations in 2018. This manuscript provides the description of the new ALMA Band 5 receiver cartridge and serves as a reference for observers using the ALMA Band 5 receiver for observations. At the time of writing this paper, the ALMA Band 5 Production Consortium consisting of NOVA Instrumentation group, based in Groningen, NL, and GARD in Sweden have produced and delivered to ALMA Observatory over 60 receiver cartridges. All 60 cartridges fulfil the new more stringent specifications for Band 5 and demonstrate excellent noise temperatures, typically below 45 K single sideband (SSB) at 4 K detector physical temperature and below 35 K SSB at 3.5 K (typical for operation at the ALMA Frontend), providing the average sideband rejection better than 15 dB, and the integrated cross-polarization level better than –25 dB. The 70 warm cartridge assemblies, hosting Band 5 local oscillator and DC bias electronics, have been produced and delivered to ALMA by NRAO. The commissioning results confirm the excellent performance of the receivers.

Ultra-pure digital sideband separation at sub-millimeter wavelengths

Context. Deep spectral-line surveys in the mm and sub-mm range can detect thousands of lines per band uncovering the rich chemistry of molecular clouds, star forming regions and circumstellar envelopes, among others objects. The ability to study the faintest features of spectroscopic observation is, nevertheless, limited by a number of factors. The most important are the source complexity (line density), limited spectral resolution and insufficient sideband (image) rejection (SRR). Dual sideband (2SB) millimeter receivers separate upper and lower sideband rejecting the unwanted image by about 15 dB, but they are difficult to build and, until now, only feasible up to about 500 GHz (equivalent to ALMA Band 8). For example ALMA Bands 9 (602-720 GHz) and 10 (787-950 GHz) are currently double sideband (DSB) receivers. Aims: This article reports the implementation of an ALMA Band 9 2SB prototype receiver that makes use of a new technique called calibrated digital sideband separation. The new method promises to ease the manufacturing of 2SB receivers, dramatically increase sideband rejection and allow 2SB instruments at the high frequencies currently covered only by DSB or bolometric detectors. Methods: We made use of a Field Programmable Gate Array (FPGA) and fast analog-to-digital converters (ADCs) to measure and calibrate the receivers front end phase and amplitude imbalances to achieve sideband separation beyond the possibilities of purely analog receivers. The technique could in principle allow the operation of 2SB receivers even when only imbalanced front ends can be built, particularly at very high frequencies. Results: This digital 2SB receiver shows an average sideband rejection of 45.9 dB while small portions of the band drop below 40 dB. The performance is 27 dB (a factor of 500) better than the average performance of the proof-of-concept Band 9 purely-analog 2SB prototype receiver developed by SRON. Conclusions: We demonstrate that this technique has the potential of implementing 2SB receivers at frequencies where no such instruments exists, as well as improving the image rejection of current millimeter 2SB receivers to a level where sideband contamination is so low that would become negligible for any known astronomical source.

A new high-performance sideband-separating mixer for 650GHz

In the modular sideband-separating mixers that we built over the last years, we observe a clear anti-correlation between the image rejection ratio obtained with a certain block and its noise performance, as well as strong correlations between the image rejection and imbalances in the pumping of the mixer devices. We report on the mechanisms responsible for these effects, and conclude that the reduction of the image rejection is largely explained by the presence of standing waves. We demonstrate the rejection ratio to be very sensitive to those. In principle, all potential round-trip paths should be terminated in matched loads, so no standing waves can develop. In practice, the typical high reflections from the SIS mixers combined with imperfect loads and non-negligible input/output reflections of the other components give many opportunities for standing waves. Since most of the loss of image rejection can be attributed to standing waves, the anti-correlation with the noise temperature can be understood by considering any excess loss in the structure, as the waveguides start acting as distribured loads. This reduces the standing waves, and thereby improves the rejection ratio, at the expense of noise temperature. Based on these experiences, we designed a new waveguide structure, with a basic waveguide size of 400×200 μm and improved loads. Strong emphasis was placed on low input and output reflections of the waveguide components, in some places at the cost of phase or amplitude imbalance. For the latter there is ample margin not to impair the performance, however. Apart from further details of the design, we present the first results of the new mixers, tested in a modified production-level ALMA Band 9 receiver, and show that even in an unfinished state, it simultaneously meets requirements for image rejection and noise temperature.