We present the results of a digital calibration technique applied to an Atacama Large Millimeter/submillimeter Array sideband separating wideband astronomical receiver of 275 to 500 GHz radio frequency (RF) and 3 to 22 GHz intermediate frequency bandwidth. The calibration technique consists of computing the magnitude ratio and the phase difference of the receiver output, and then applying correction constants to the digitized signals. Two analog-digital converters are used to digitize the signals and an field-programmable gate array for the processing. No modification in the analog receiver is required to apply the calibration, as it works directly on upper sideband/lower sideband signals. The technique improved the receiver temperature compared with the double sideband case by increasing the sideband rejection ratio by around 30 dB on average. It is shown that even more rejection can be obtained with more careful control of the RF calibration input power.
KEYWORDS: Polarization, Field programmable gate arrays, Receivers, Calibration, Analog electronics, Signal processing, Spectroscopy, Digital signal processing, Radio astronomy, Telescopes
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.
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