The ALMA Front End. Hans Rudolf

Transcription

1 The ALMA Front End Hans Rudolf European Southern Observatory, ALMA, Karl-Schwarzschild-Straße 2, Garching, Germany, , Abstract The Atacama Large Millimeter Array (ALMA) is a 50 antenna array of radio telescopes that is currently being built in Northern Chile. In order to devide the efforts it is based on equal partnership between Europe and North America. The Front End is the analogue part of the receiver. It converts two orthogonally polarised 30 to 960 GHz signals into an IF of 4 to 12 GHz. All Front Ends are fed with a coherent Local Oscillator that is opticaly transmitted over the distance from the centre of the array to the antennae. This distance can reach 14 km. The RF band is split into 10 receiving bands. For each band a receiver, called a cartridge, is designed. The cartridges are housed in a common vacuum chamber and cooled to 4 K. The specifications of the Front End are extremely challenging, for instance the sensitivity is specified between 6 and 10 photons, according to the RF frequency. The first cartridges are finished and not only meet, but exceed these specifications. I. INTRODUCTION The Atacama Large Millimeter Array (ALMA) is a revolutionary instrument in its scientific concept and its engineering design [1]. ALMA will provide scientists with precise images of galaxies in formation seen as they were twelve billion years ago; it will reveal the chemical composition of heretofore unknown stars and planets still in their formative process; and it will provide an accurate census of the size and motion of the icy fragments left over from the formation of our own solar system that are now orbiting beyond the planet Neptune [2]. These science objectives, and many hundreds more, are made possible owing to the design concept of ALMA that combines the imaging clarity of detail provided by a 50-antenna interferometric array together with the brightness sensitivity of a single dish antenna. The challenges of engineering the unique ALMA telescope begin with the need for the telescope to operate in the thin, dry air found only at elevations high in the Earth s atmosphere where the light at millimetre and sub-millimetre wavelengths from cosmic sources penetrates to the ground. ALMA will be sited in the Altiplano of northern Chile at an elevation of 5000 metres (16,500 feet) above sea level. The ALMA site is the highest, permanent, astronomical observing site in the world. On this remote site the meter diameter ALMA antennas will each operate superconducting receivers that are cryogenically cooled to less than 4 degrees above absolute zero. The signals from these receivers are digitized and transmitted to a central processing facility where they are combined and processed at a rate of 1.6 x operations per second. As an engineering project, ALMA is a concert of 50 preciselytuned mechanical structures each weighing more than 50 tons, Fig. 1. An artist view of ALMA superconducting electronics cryogenically cooled, and optical transmission of terabit data rates all operating together, continuously, on a site more than 5 kilometres high in the Andes mountains [3]. ALMA is an international astronomy facility based on an equal partnership between Europe and North America, in cooperation with the Republic of Chile, and is funded in North America by the U.S. National Science Foundation (NSF) in cooperation with the National Research Council of Canada (NRC), and in Europe by the European Southern Observatory (ESO) and Spain. ALMA construction and operations are led on behalf of North America by the National Radio Astronomy Observatory (NRAO), which is managed by Associated Universities, Inc. (AUI), and on behalf of Europe by ESO [1]. In addition, Japan has also entered the ALMA project. A preliminary agreement has been signed by all partners. The Japanese activities are led on behalf of the National Astronomical Observatory of Japan (NAOJ) [4]. II. THE ALMA FRONT END ASSEMBLY A. The Scientific Specifications The required frequency coverage (all atmospheric windows from 31.3 to 950 GHz) is achieved in 10 separate bands. The band edge frequencies were chosen to provide good coverage of the windows while limiting the edge frequency ratio for each band to < 1.35, considered the largest feasible value.

2 Fig. 2. A 3D drawing of the Front End Assembly The exact frequencies follow closely the recommendations of a scientific working group [5]. TABLE I THE ALMA FREQUENCY BANDS Band Manufacturer Frequency Noise Temperature GHz 26 K (SSB) GHz 47 K (SSB) 3 HIA GHz 60 K (SSB) 4 NAOJ GHz 82 K (SSB) 5 Chalmers GHz 105 K (SSB) 6 NRAO GHz 136 K (SSB) 7 IRAM GHz 219 K (SSB) 8 NAOJ GHz 292 K (SSB) 9 SRON/NOVA GHz 261 K (DSB) 10 (NAOJ) GHz 344 K (DSB) The requirements on the noise temperature of these receivers are extremely challenging and range from 6 to 15 photons. The specifications call for a band specific noise temperature of T SSB = A(f) hf k + 4K (1) over the full frequency range. The frequency specific value A(f) follows the following values: below 275 GHz A(f) = GHz GHz A(f) = 12 above 602 GHz A(f) = 15 A value of 2 3 of the indicated value must be met over 80 % of the frequency range. In order to synthesise an image in the correlator, it is extremely important to maintain the coherence between antennas. This coherence must be preserved in both amplitude and phase noise. The amplitude noise, expressed in terms of Allan variance (σ 2 ) must exceed for timescales between 0.1 seconds and 10 seconds. The phase jitter (1 second) must be better than 38fs and the phase drift better (300 seconds) than 12.5fs (corresponding to 4.3 o at 950 GHz).

3 B. The Technical Implementation All of the components described in this section are parts of the Front End Assembly, which includes the vacuum chamber with cryocooler and an attached frame that houses the room temperature electronics. 1) The Cartridges: Cryogenically cooled components of the front end (FE) that are specific to one band are in separate assemblies known as cartridges, but these are housed in a common vacuum chamber and cooled by a common cryocooler. Only four of the bands will be implemented initially, band 3, band 6, band 7, and band 9. Two bands will be provided as contribution from NAOJ, band 4 and band 8. These bands will follow shortly after the four initially implemented bands. For band 5 funding is available for the development and production of eight pre-production cartridges from the sixth framework programme of the European Union. Fig. 3. A picture of the band 7 cartridge For the two lowest-frequency bands, the initial active element is an HFET amplifier. The HFET amplifiers, along with the filters and mixers that follow them, are cooled to a nominal temperature of 15K. All other cartridges employ Superconductor-Insulator-Superconductor (SIS) mixers for frequency translation. The SIS mixers, along with the InP amplifiers and isolators are cooled to a nominal temperature of 4.0K. Each band has its own tertiary optics, consisting of a lens or a pair of ellipsoidal mirrors, to match the wave that arrives at the secondary focus to a corrugated feed horn. The signal is separated into the two polarisations, nominally linear and orthogonal, and delivered in waveguide to the amplifier or mixer. For bands 1 through 6 (to 275 GHz), polarisation splitting is achieved in an orthomode waveguide junction just after the feed horn. For higher frequencies, it is achieved with a wire grid within the tertiary optics, and in these cases each channel has a separate feed horn. In all cases, the signal is converted to an IF band between 4 and 12 GHz, either by the SIS mixer or by a Schottky diode mixer. The mixers for both polarisation channels are driven at the same LO frequency. For the HFET bands, a filter ahead of the mixer produces a single-sideband response (upper sideband for band 1 and lower sideband for band 2). The SIS bands are of two different types: sideband separating (2SB) and double sideband (DSB). The 2SB case supplies two IF outputs simultaneously, carrying opposite sidebands of the LO. The DSB case supplies a single IF output carrying a linear combination (nominally equal) of responses from both sidebands. Bands 3 through 8 use 2SB mixers, and bands 9 and 10 use DSB mixers. In order to utilize fully the signal transmission system and correlator, each polarisation channel delivers 8 GHz of instantaneous bandwidth at IF. Each IF output from a mixer is amplified in a cooled pre-amplifier closely associated with that mixer. The cooled amplifiers and mixers are supported by active bias circuits. Gains are near 35 db in most cases, allowing the subsequent cable loss (especially the transition to room temperature) to have negligible effect on the receiver noise temperature Currently the pre-production of eight Front End assemblies is under way. The amplified IF signals are brought out of the vacuum chamber on coaxial cables amplified with room temperature LNAs and delivered to IF Processing assemblies. 2) The IF Processing Assembly: Each IF Processing assembly accepts one signal from each band and contains additional amplification, switching to select one band s signal for further processing, gain equalization, and variable attenuation to adjust the power to a specified level. There are 4 IF processing assemblies. Two for each polarisation and for each side-band. 3) The 1 st Local Oscillator: Nearly all time-dependent functions in the array must be coherent with a single master oscillator from which reference signals are derived and distributed. This is a hydrogen maser. A mm-wavelength reference is then synthesized for the first LO. This is the only variable-frequency signal that is distributed to the antennas. The process uses a microwave synthesizer to produce GHz in 5 MHz steps, followed by synthesis of

4 Fig. 4. A block diagram of the band 7 cartridge GHz as the difference between two laser-generated optical frequencies. Generation of the mm/sub-mm wavelength first LO is based on phase-locking a local VCO to the highest-feasible reference frequency that can be distributed from the center. The twolaser reference signal is recovered by photomixing. The phaselocked signal is then amplified with GaAs and InP amplifiers and multiplied to higher frequencies, as needed, in diode multiplier assemblies; these multipliers are operated at cryogenic temperatures (110K) to maximize efficiency. Reference frequencies in the range 68.5 to GHz are needed to cover bands 2 through 10 with cold multiplication factors of 1 (no multiplier), 2, 3, 5, or 9. The band 1 reference and VCO are at 27.3 to 33 GHz. The local VCO is implemented as a YIGtuned microwave oscillator and amplifier/multiplier chain. The YTO frequencies and hence the required multiplication factor are chosen for engineering reasons, but the minimum oscillator frequency is kept above 12 GHz to avoid the possibility of spurious sidetones of the LO falling in the RF band that extends +/- 12 GHz from the LO. The locked VCO is offset from the reference by MHz from a direct digital synthesizer through which phase tracking (fringe rotation) and phase switching are implemented. 4) The Cooler & Vacuum System: The vacuum chamber with cryocooler and cartridges is part of the overall FE Assembly, located in the centre of the antenna s receiver cabin, a room behind the focal plane that moves with both axes of antenna pointing. To maintain good optical alignment, the FE Assembly is rigidly attached to the antenna structure very near the focal plane. The overall weight of the front assembly is in the order of one metric ton. A single 3-stage Gifford-McMahon cryocooler is used; the stages are nominally at 110K, 15K, and 4K. The cryocooler is supported by a single-stage helium compressor that is mounted outdoors on a platform that rotates with the antenna in azimuth but not in elevation. 5) The Calibration Device: In general, calibration of the ALMA instrument will rely on observations of known astronomical sources. To supplement this and to compensate for the shortage of appropriate natural sources, a calibration device will be built into each antenna for the purpose of generating calibration signals. The calibration device consists of a robot arm that places a load of a known temperature in front of the observing cartridge. Two loads are available, one at ambient temperature and one at 60 o C. The absolute error of the gain after calibration will be less than 3 % and less than 5 % for frequencies higher than 373 GHz. This procedure will be repeated every 5 minutes. 6) The Water Vapour Radiometer: The water vapour column density is measured by special radiometers (WVRs) built into each antenna. These measure the brightness temperature of the sky at several frequencies around the 183 GHz water line. Assuming that the sky brightness at these frequencies is dominated by emission from the water vapour, the spectral shape helps determine the altitude, temperature, and degree of saturation of the line. In principle, this allows the column density of water vapour to be deduced, and from it the signal delay at the observing frequency.

5 III. RESULTS ALMA is currently in the pre-production phase. On the Chajnantor site antenna foundations and buildings are being built. The first four cryostats are at the Front End integration Centres. The first cartridges have been completed and are currently integrated in the cryostat to form the first Front End. Though the specifications have been beyond the state of the art when they were postulated, not only most of them are met, but some are exceeded with a considerable margin. In figure 5 a plot of the receiver noise performance for the first band 7 cartridge is shown. Fig. 6. The image rejection for the band 7 cartridge Currently the pre-production of eight Front End assemblies is under way. The first ALMA Front End is being integrated in the North American Front End Integration Centre in Charlottesville, VA. REFERENCES Fig. 5. A noise figure plot of the band 7 cartridge It can be seen, that the specification is clearly met. Also the other specifications that are important for system sensitivity, such as image rejection as shown in figure 6 or compression ratio are met and exceed the expectations. The other cartridges meet and exceed the specifications in a similiar way. IV. CONCLUSION The ALMA Front End is a milestone in development of radio astronomy receivers. Compared to all existing systems, not only it brings the frequency limit for ground based radio astronomy to the threshold of 1 THz, but also has it pushed the sensitivity to new frontiers. The specifications that were set beyond the -then- state of the art [5] are not only met but in many cases even exceeded. [1] Atacama Large Millimeter Array. Homepage [2] Science with ALMA. Paper [3] ALMA Construction Project Book. Homepage [4] Status of the Atacama Large Millimeter Array. Paper [5] ALMA Scientific Specifications and Requirements. Alma a- spe. papers/ alma a-spe.pdf, 2004.