RoF technology has proven to be fundamental in enabling the construction of SKA-Low, a huge array of more than 130K antennas operating at low frequency in a particularly hostile region, from an environmental point of view, such as the outback of Western Australia. The motivations that led to its adoption and how it has been implemented in the receiving system of the radio telescope are here described.

In the context of Radioastronomic applications where the Analog Radio-over-Fiber technology is used for the antenna downlink, detrimental nonlinearity effects arise because of the interference between the forward signal generated by the laser and the Rayleigh backscattered one which is re-forwarded by the laser itself toward the photodetector. The adoption of the so called dithering technique, which involves the direct modulation of the laser with a sinusoidal tone and takes advantage of the laser chirping phenomenon, has been proved to reduce such Rayleigh Back Scattering - induced nonlinearities. The frequency and the amplitude of the dithering tone should both be as low as possible, in order to avoid undesired collateral effects on the received spectrum as well as keep at low levels the global energy consumption. Through a comprehensive analysis of dithered Radio over Fiber systems, it is demonstrated that a progressive reduction of the dithering tone frequency affects in a peculiar fashion both the chirping characteristics of the field emitted by the laser and the spectrum pattern of the received signal at the fiber end. Accounting for the concurrent effects caused by such phenomena, optimal operating conditions are identified for the implementation of the dithering tone technique in radioastronomic systems.

Abstract. The signal reception chain is an essential element for achieving the square kilometer array-low (SKA-low) system requirements in terms of high sensitivity and dynamic range. The balance between gain, linearity, and low power consumption, as well as the cost, are fundamental parameters that influence the selection of the most suitable technology for SKA-low. Further factors, such as low self-generated radio frequency (RF) interference, high reliability, robustness under extreme environment, and last but not least, the distance between the antennas and the acquisition systems, have impacts on the selection for both architecture and receiver system design. The selected technology for the SKA-low RF signal transportation is RF-over-fiber systems, where the preamplified RF signal picked up by the antennas is carried via analogue modulation over optical fiber. The rationales behind the selection are reported, along with descriptions on the development of the receiver prototypes. The prototypes were deployed and installed on the demonstrator arrays at the selected SKA-low site in Western Australian. Particular attention has been put on the thermal characterization of the receiver system under the actual operating temperature on site, especially when both transmitting part and the optical medium are subjected to external ambient temperature variations. Performance issues encountered in the demonstrator arrays are also discussed along with some proposals for future activities.

The design and development process for the Square Kilometre Array (SKA) radio telescope, the Low Frequency Aperture Array component, was progressed during the SKA pre-construction phase by an international consortium, with the goal of meeting requirements for a critical design review. As part of the development process a full-sized prototype SKA Low station was deployed, the Aperture Array Verification System 1 (AAVS1). We provide a system overview and describe the commissioning results of AAVS1, which is a low frequency radio telescope with 256 dual-polarisation log-periodic dipole antennas working as a phased array. A detailed system description is provided, including an in-depth overview of relevant sub-systems, ranging from hardware, firmware, software, calibration,and control sub-systems. Early commissioning results cover initial bootstrapping, array calibration, stability testing, beam-forming,and on-sky sensitivity validation. Lessons learned are presented, along with future developments.

In the framework of radio astronomic systems, and in particular in the realization of the Aperture Array Verification Systems of the low frequency part of the Square Kilometre Array project, a monitoring of the delay introduced by the fiber optic cable of the antenna downlink is necessary in order to properly calibrate the receiver chain. A simple and relatively low-cost system is here proposed, which can directly provide such measurement without the use of complex post-processing systems, with precision estimated in 60ps for 6Km of fiber length.

In the context of the Radio over Fiber downlinks belonging to the Aperture Array Verification System (AAVS) of the low frequency Square Kilometre Array (SKA-LOW) radio telescope, the combination of modulation of the optical carrier by Radio Frequency Interfering Tones (RFIs) and fiber-induced Rayleigh Backscattering (RBS) determines the presence of undesired spurious frequency terms. A successful countermeasure to the phenomenon has been proposed, based on the additional modulation with a low frequency tone (called dithering tone) which mitigates such distortions exploiting the laser chirp effect. In the present work, through a rigorous theoretical and experimental study, the optimal design parameters of such solution are put into evidence, showing that an appropriate choice of the dithering tone frequency allows to reduce to acceptable levels the RBS induced nonlinearities, while minimizing the power consumption of the corresponding introduced devices.

The Sardinia Aperture Array Demonstrator (SAD) is an Italian facility, which is composed of 128 prototypical Vivaldi antennas, specifically designed to operate across the 50–500 MHz frequency range. As well known, one of the major burden at low frequency are the radio frequency interferences, thus after several accurate measurement campaigns we realized that a specific signal conditioning is needed in order to feed the digital beamformer with the proper signal level. In this paper, we report the results of the preliminary tests that we carried out in order to design an ad hoc receiving chain for the SAD array.

Optical links based on the Radio over Fiber (RoF) Technology are, and are going to be, utilized within the antenna downlinks of important Radioastronomic facilities. In this context, the presence of spurious frequency terms has been observed despite the relatively low power levels of the transmitted of RF signals. The present work gives the theoretical explanation of the onset of these nonlinearities, and rigorously demonstrates an effective countermeasure to the undesired phenomenon.

We describe the beamforming strategy and the preliminary laboratory characterization results of the beam pattern synthesized by the PHAROS2 Phased Array Feed (PAF), a 4-8 GHz PAF with digital beamformer for radio astronomy application. The PAF is based on an array of 10×11 dual-polarization Vivaldi antennas cryogenically cooled at 20 K along with low noise amplification modules (LNAs) cascaded with a multi-channel Warm Section (WS) receiver. We present the beamforming and test procedures used to, respectively digitally synthesize and characterize the PHAROS2 antenna array beam pattern at 6 GHz. The tests of the array were carried out at room temperature by directly connecting 24 antenna elements to the WS and iTPM digital beamformer in a laboratory measurement setup.

An active dual-polarized Log-Periodic antenna has been designed to meet the requirements of the low-frequency (50 - 350 MHz) radio telescope of the Square Kilometre Array (SKA). The integration of antenna and low noise amplifier has been conceived in order to achieve a high degree of testability. This aspect has been found to be crucial to obtain a smooth frequency response compatible with the SKA science cases. The design has also been driven by other factors such as the large-volume production (more than 130 000 antennas will be built) and the environmental conditions of the harsh Australian desert. A specific verification approach based on both wideband radiometric spectral and spatial measurements in relevant laboratory and in-situ conditions has been developed. Electromagnetic analyses and experimental results exhibit a very good agreement. In December 2019, this antenna was part of the reference solution for the System Critical Design Review of the SKA.

A novel version of digital hardware Italian Tile Processing Module (ITPM) 1.6 has been released for the Low-Frequency Aperture Array (LFAA) component of the Square Kilometre Array (SKA). This back-end includes two plugged-in main blocks, as an analog device , the Pre-ADU board, and an Analog to Digital Unit (ADU), a 6U board containing sixteen dual-inputs Analog to Digital Converters and two Field Programmable Gate Array (FPGA) devices, capable of digitizing and processing 32 RF input signals (50-650 MHz). We present the main features of the upgrade of the board compared to previous versions: there are new and high performance components improving processing capability, mechanical changes matching the design of the housing sub-rack and finally a general reduction of the overall power consumption. The ITPM ADU 1.6 version, now in engineering phase together with its sub-rack system, is currently the last prototype before the design of the industrial line for mass production, necessary for the LFAA deployment. Results of system performances will be presented.

Radio over Fiber (RoF) Systems exploiting a direct modulation of the laser source are presently utilized within important Radioastronomic scenarios. Due to the particular operating conditions of some of these realizations, the phenomena which typically generate nonlinearities in RoF links for telecommunications applications can be here regarded as substantially harmless. However, these same operating conditions can make the RoF systems vulnerable to different kinds of nonlinear effects, related to the influence of the Rayleigh Backscattered signal on the transmitted one. A rigorous description of the phenomenon is performed, and an effective countermeasure to the problem is proposed and demonstrated, both theoretically and experimentally.

We describe the design, fabrication and test results of a multi-channel heterodyne receiver operating at room temperature across the 2.3-8.2 GHz Radio Frequency (RF) band. Such a "Warm Section" (WS) receiver is part of a Phased Array Feed (PAF) demonstrator that is being built for radio astronomy application. The WS receiver is cascaded to the PAF cryogenic section that incorporates an antenna array with low noise pre-amplification stages. The WS receiver consists of four rack-mountable modules, each of which can process eight RF inputs. Four modules are arranged in a standard 19" rack to allow handling a total of 32 RF signals. The modules perform filtering (through four-way switch filter bank) and down-conversion (to the 375-650 MHz IF band). The IF signals are converted to optical through analogue Wavelength Division Multiplexing IFoF (IF over fiber) transmitters incorporated into the WS receiver. The signals are sent through optical fibers to a backend, where they are converted back to IF before digitization by an Analog-to-Digital Unit.

We describe the development of a multi-channel “warm receiver section” (WS) and of a digital beamformer for the PHAROS2 Phased Array Feed (PAF), a PAF demonstrator for radio astronomy application across the 4-8 GHz radio frequency (RF) band. The PAF is based on an array of 10×11 dual-polarization Vivaldi antennas cryogenically cooled at 20 K along with low noise amplification modules (LNAs). The WS receiver can process the signals from a subset of 24 antenna elements of the array by downconverting them to an intermediate frequency (IF) range, 375-650 MHz, suitable for digitization by the digital beamformer. The latter is based on the iTPM (Italian Tile Processing Module), developed for the Square Kilometer Array (SKA) Low Frequency Aperture Array (LFAA). We modified the iTPM firmware to synthesize four independent beams across the 275 MHz instantaneous IF bandwidth in the iTPM FPGAs (Field Programmable Gate Arrays). The 24 signals are sent from the WS to the iTPM through analogue IFoF (IF over fiber) optical links. In this paper we present the design and performance of the WS and of the digital bemaformer for PHAROS2.

This paper presents the theoretical and experimental study of unexpected distortions induced by the Rayleigh backscattering phenomenon in Radio-over-Fiber (RoF) links within the Square Kilometer Array (SKA) project. For this system, the radio frequency (RF) signals coming from the sky and celestial sources are received by more than 130k antennas individually and then transmitted to the signal processing room through an analogue RoF link. The low frequency band considered (from 50MHz to 350MHz) and the relatively low levels of the power of such RF signals are identified as the cause of these distortion effects which can decrease drastically the receiver dynamic range. An analysis of the phenomenon is presented in this work, together with an efficient solution optimized for the considered applicative case.

In the realization of the Low Frequency Aperture Array (LFAA) within the Square Kilometre Array (SKA) project, the radiofrequency (RF) signals coming from sky and celestial sources, and received by each of the 130k+ antennas, are transmitted to the signal processing unit through analogue Radio over Fibre links. Within this low frequency band, the relatively low levels of the power of such RF signals may give rise to unexpected distortion effects. The present works highlights the physical origin of such undesired spurious frequencies generation, which can be also found in other applicative telecommunications scenarios. The details of a possible solution able to keep the above mentioned distortion terms at acceptably low levels are finally illustrated.

This paper describes the design, fabrication, and test results of a room temperature multi-channel heterodyne receiver operating across the 2.3–8.2 GHz radio frequency (RF) band. Such a “Warm Section” (WS) receiver is part of phased arrays for reflector observing systems 2 (PHAROS2), a C-band phased array feed (PAF) demonstrator with digital beamformer for radio astronomy application. The WS receiver is cascaded to the PHAROS2 cryostat, which includes an array of Vivaldi antennas with low noise pre-amplification stages. The WS can handle up to 32 RF signals and, for each of them, realizes the operations of filtering, RF amplification and down-conversion from the RF to the 375–650 MHz intermediate frequency (IF). Also, the WS incorporates an IF-to-optical signal conversion through analogue wavelength division multiplexing IF over fiber (IFoF) and fiber-optic transmitters (OTXs). The 32-channel WS receiver consists of four eight-channel WS RF/IF modules, one local oscillator (LO) splitter module and one monitoring and control module, all hosted in a standard 6U × 19-inch rack.

The Square Kilometre Array (SKA) [1] telescope consists in its first phase of two arrays, a dish array to be constructed in South Africa and a low frequency aperture array to be constructed in Western Australia. The aperture array, SKAI-Low, will consist of 512 stations, each with 256 wide bandwidth log periodic antennas. The frequency range of SKAI-Low is 50 to 350 MHz. The Low Frequency Aperture Array (LFAA) consortium is tasked to design the station, the infrastructure around them and the station signal processing.

In view of the realization of short- and medium-range Mobile Front-Haul connections for present (LTE) and future (5G) cellular networks, a cost effective, low-consumption radio over fiber system is proposed, based on 850-nm Single Mode Vertical Cavity Surface Emitting Lasers and Standard Single Mode Fibers (SSMFs). An efficient countermeasure to possible impairments due to the bimodal behavior of SSMFs at 850 nm allows even in critical cases to maintain at high level the quality of the received signal. The performances are evaluated with reference to the Physical Downlink Shared Channel of an entire LTE frame with 20-MHz bandwidth centered in band 20 of the standard. In terms of error vector magnitude and outage probability under temperature stress, the system is able to transmit 256-QAM signals in compliance with the LTE standard, which corresponds to a raw data rate transmission of 134.4 Mbit/s, up to distances of 1.5 km.