Field Programmable Gate Arrays (FPGAs) capacity and Analog to Digital Converters (ADCs) speed have largely increased in the last decade. Nowadays we can find one million or more logic blocks (slices) as well as several thousand arithmetic units (ALUs/DSP) available on a single FPGA chip. We can also commercially procure ADC chips reaching 10 GSPS, with 8 bits resolution or more. This unprecedented power of computing hardware has allowed the digitalization of signal processes traditionally performed by analog components. In radio astronomy, the clearest example has been the development of digital sideband separating receivers which, by replacing the IF hybrid and calibrating the system imbalances, have exhibited a sideband rejection above 40dB; this is 20 to 30dB higher than traditional analog sideband separating (2SB) receivers. In Rodriguez et al.,1 and Finger et al.,2 we have demonstrated very high digital sideband separation at 3mm and 1mm wavelengths, using laboratory setups. We here show the first implementation of such technique with a 3mm receiver integrated into a telescope, where the calibration was performed by quasi-optical injection of the test tone in front of the Cassegrain antenna. We also reported progress in digital polarization synthesis, particularly in the implementation of a calibrated Digital Ortho-Mode Transducer (DOMT) based on the Morgan et al. proof of concept.3 They showed off- line synthesis of polarization with isolation higher than 40dB. We plan to implement a digital polarimeter in a real-time FPGA-based (ROACH-2) platform, to show ultra-pure polarization isolation in a non-stop integrating spectrometer.
Here we describe the status of the upgrade program that is being performed to modernize the Southern 1.2m
Wave Telescope. The Telescope was built during early ´80 to complete the first Galactic survey of Molecular
Clouds in the CO(1-0) line. After a fruitful operation in CTIO the telescope was relocated to the Universidad
de Chile, Cerro Calán Observatory. The new site has an altitude of 850m and allows observations in the
millimeter range throughout the year. The telescope was upgraded, including a new building to house
operations, new control system, and new receiver and back-end technologies. The new front end is a
sideband-separating receiver based on a HEMT amplifier and sub-harmonic mixers. It is cooled with Liquid
Nitrogen to diminish its noise temperature. The back-end is a digital spectrometer, based on the
Reconfigurable Open Architecture Computing Hardware (ROACH). The new spectrometer includes IF
hybridization capabilities to avoid analog hybrids and, therefore, improve the sideband rejection ratio of the
Sideband-separating receivers are usually preferred in the presence of high atmospheric noise. However, one of the most important figures of merit for this kind of receiver, the sideband ratio, is still low and typically around 10 dB. This is because keeping low amplitude and phase imbalances over large RF and IF frequencies is extremely difficult. However, by introducing a digital back-end that mimics the performance of an IF-hybrid, such imbalances can be calibrated out. We have recently presented a digital sideband-separating receiver, working at the W band, that can achieve sideband ratios well above 35 dB. Here we extend this work by demonstrating that it can also be applied to receivers that incorporate a second down-conversion stage with the same performance.
In this paper we describe the status of an upgrading program that it is being carried out to modernize the front
and back ends of the Southern 1.2-m mm-Wave Telescope to cover eciently the band from 86 to 115 GHz. On
the one hand, the new front end will have a sideband-separating conguration to suppress the image band noise.
This front end will use one low noise amplier and two Schottky diodes for down conversion. On the other hand,
we are developing a modern digital spectrometer, based on the Recongurable Open Architecture Computing
Hardware (ROACH), to ease the operation and improve the spectral resolution up to a factor of 4. Moreover,
the spectrometer will include IF hybridization capabilities to avoid analog hybrids and, therefore, improve the
sideband rejection ratio of the receiver.