Design of microwave subsystems for radio astronomical receivers

The radio astronomical receivers are devices that they measure the radio emissions coming from celestial sources. Therefore, the aim of these devices is to convert the weak electromagnetic energy from space into a measurable electrical signal. The nature of the signal emitted by celestial sources imposes that such receivers must be broadband, very sensitive and designed to measure noise. In fact, the electromagnetic signal that the receivers must capture and measure, in addition to being very weak, is a random signal with components uncorrelated with each other, with different frequencies and with zero mean value (i.e. it is like the thermal noise produced by a resistor subject to a temperature). Therefore, the weakness and the peculiarity of these signals make the radio astronomy receivers other than those used in the telecommunications. It is in this context that the work of my PhD thesis has been developed. Three different projects, that regard to the microwave subsystems of the radio astronomical receiver, have been treated. The first project is focus on the development of a new configuration of broadband polarizer with very flat phase response in microtrip technology. The second project is a feasibility study on the optics of a 3mm SIS receiver in order to install it in the Gregorian focus of the Sardinia Radio Telescope. The third project is fully dedicated on the development of a high performance wideband feed-horn for a state-of-the-art multi-beam S-band (2.3 - 4.3 GHz) receiver to install in the primary focus of the Sardinia Radio Telescope

A NEW BROADBAND MICROSTRIP QUADRATURE HYBRID WITH VERY FLAT PHASE RESPONSE

A new broadband microstrip branch-line quadrature hybrid with very flat phase response is presented. The device is made by cascading four branch-line couplers with arbitrary power division. The novel design is based on the microstrip transposition of a broadband waveguide polariser [4]. Across a 32% bandwidth centred at 9.3 GHz, the RL and the IL are respectively -15 dB and -3 dB/-4 dB; the phase difference is very flat, i.e. 90°±1.5°

Optical design of S-band multifeed for the Sardinia Radio Telescope primary focus

We present the optical design of an S-band seven feed cryogenic radio astronomy receiver for illuminating the 64-m diameter Sardinia Radio Telescope (SRT) dish from the primary focus. The feeds are arranged in a compact hexagonal configuration with a central one and are cryogenically cooled at 20 K inside a cryostat. Each feed accepts two linear polarizations and use a circular waveguide with a single outer corrugated section to achieve a nearly constant beamwidth and low cross polarization across the 3.0-4.5 GHz band. The simulated radiation pattern of the SRT telescope is obtained by coupling the array of feed-horn beam patterns (optimized with the electromagnetic software CST) with the 64-m parabolic dish (through a physical optics analysis carried out with GRASP). We compare the simulated beam pattern of an isolated feed with those of the same feed embedded in the dense array and analyze the effects of an absorber located inside the cryostat around the cryogenic feeds. We found that the absorber improves the overall system performance by decreasing the cross-coupling effects between the feeds while adding negligible noise to the system

Sardinia aperture array demonstrator: measurement campaigns of radio frequency interferences

Measurement campaigns of radio frequency interferences (RFIs) represent a fundamental aspect to optimize data collected by radio astronomical observations. In order to monitor the state of unwanted man-made signals, every radio telescope needs to have a radio frequency map in the frequencies range of its receivers. The Sardinia Aperture Array Demonstrator (SAD) is an Italian aperture array demonstrator composed of prototypical Vivaldi antennas designed to work at frequencies below 500 MHz. The antennas are located at the Sardinia Radio Telescope (SRT) site and they are arranged with a versatile approach that is able to provide different array configurations. In this paper, we present the results of measurement campaigns conducted with the SAD antennas at the SRT observing site with the aim to monitor the evolution of RFI scenario from 2016 to date. The signal acquisition chain and the software tool used for RFI detection are, also, presented

Upgrading of the L-P band cryogenic receiver of the Sardinia Radio Telescope: A feasibility study

The Sardinia Radio Telescope is a quasi-Gregorian system with a shaped 64 m diameter primary reflector and a 7.9 m diameter secondary reflector. It was designed to operate with high efficiency across the 0.3–116 GHz frequency range. The telescope is equipped with a cryogenic coaxial dual-frequency L-P band receiver, which covers a portion of the P-band (305–410 MHz) and the L-band (1300–1800 MHz). Although this receiver has been used for years in its original design, with satisfactory results, it presents some parts that could be upgraded in order to improve the performances of the system. With the passing of time and with technology advances, the presence of unwanted human-made signals in the area around the telescope, known as radio frequency interferences, has grown exponentially. In addition, the technology of the receiver electronic control system became obsolete and it could be replaced with next-generation electronic boards, which offer better performances both service reliability and low generation of unwanted radio frequency signals. In this paper, a feasibility study for improving the L-P band receiver is discussed, taking into account the mitigation of the main radio frequency interferences. With this study, it is possible to have a sensitive instrument that can be used for scientific research at low frequencies (P- and L-bands), which are usually populated by signals from civil and military mobile communications, TV broadcasting and remote sensing applications

Adaptation of an IRAM W-Band SIS Receiver to the INAF Sardinia Radio Telescope: A Feasibility Study and Preliminary Tests

Radio telescopes are used by astronomers to observe the naturally occurring radio waves generated by planets, interstellar molecular clouds, galaxies, and other cosmic objects. These telescopes are equipped with radio receivers that cover a portion of the radio frequency (RF) and millimetre-wave spectra. The Sardinia Radio Telescope (SRT) is an Italian instrument designed to operate between 300 MHz and 116 GHz. Currently, the SRT maximum observational frequency is 26.5 GHz. A feasibility study and preliminary tests were performed with the goal of equipping the SRT with a W-band (84–116 GHz) mono-feed radio receiver, whose results are presented in this paper. In particular, we describe the adaptation to the SRT of an 84–116 GHz cryogenic receiver developed by the Institute de Radio Astronomie Millimétrique (IRAM) for the Plateau de Bure Interferometer (PdBI) antennas. The receiver was upgraded by INAF with a new electronic control system for the remote control from the SRT control room, with a new local oscillator (LO), and with a new refrigeration system. Our feasibility study includes the design of new receiver optics. The single side band (SSB) receiver noise temperature measured in the laboratory, Trec ≈ 66 K at 86 GHz, is considered sufficiently low to carry out the characterisation of the SRT active surface and metrology system in the 3 mm band

Progettazione, realizzazione e caratterizzazione della catena ricevente per il sistema SADino precursore del Sardinia Aperture Array Demonstrator (SAAD)

Il presente rapporto tecnico riassume la progettazione, realizzazione e caratterizzazione della catena di componenti a microonde e a radio frequenza per l’acquisizione del segnale del sistema SADino, precursore del Sardinia Aperture Array Demonstrator (SAAD). Il progetto SAAD prevede la realizzazione di un aperture array composto da 128 antenne Vivaldi a doppia polarizzazione lineare [1-7], installate al sito del Sardinia Radio Telescope (SRT), ciascuna delle quali verrà collegata alla sua catena dedicata di componenti a microonde per l’acquisizione del segnale, che permette di trasportare il segnale analogico rilevato in radio frequenza dall’antenna fino al back-end digitale. Con l’obiettivo di eseguire velocemente i primi test, inizialmente si è deciso di implementare parzialmente l’array SAAD. L’implementazione parziale del sistema SAAD prende appunto il nome di SADino, che prevede la realizzazione di un mini-array di 16 elementi a doppia polarizzazione, con il quale è possibile effettuare le prime osservazioni e i primi test di beam-forming con il back-end digitale dedicato basato sulle schede Italian Tile Processing Module (iTPM) [8]. Con SADino sono state scelte solo 16 antenne (a doppia polarizzazione) dell’intero array SAAD da 128 elementi, disposte in maniera casuale, poiché il back-end digitale iTPM è dotato di soli 32 ingressi. La catena ricevente (una per ogni canale di polarizzazione di ciascuna antenna Vivaldi) è stata progettata basandosi sui risultati di una campagna di misure, effettuata nell’estate del 2020, utile a valutare la presenza in banda di segnali interferenti generati dall’uomo e indesiderati per le attività di ricerca radioastronomica, noti come radio frequency interference (RFI) [6]. Per l’esecuzione di tale campagna di misure, si è utilizzata una delle antenne Vivaldi del SAAD, equipaggiandola con una catena di acquisizione del segnale che ha fatto da precursore (almeno per quanto riguarda la valutazione degli stadi di amplificazione) alla versione finale di catena ricevente da utilizzare sul sistema SADino, precursore dell’intero SAAD. L’obiettivo di questa campagna di misure RFI è stato quello di selezionare una banda di frequenze il più possibile libera da segnali indesiderati e contenuta ovviamente all’interno della banda di lavoro delle antenne che costituiscono l’array. Le antenne Vivaldi del SAAD sono state progettate per lavorare con buona efficienza in un range di frequenze che va da 50 MHz a 500 MHz [4], mentre i componenti a microonde che costituiscono la catena di acquisizione del segnale sono ottimizzati per lavorare nel range di frequenze selezionato in base ai risultati delle misure RFI e all’interno del quale poi opererà il telescopio. In questo rapporto interno vengono presentati i risultati della campagna di misure RFI preliminare, illustrando la catena ricevente utilizzata per queste misure (vedi Sezione 2). Nella Sezione 3 viene descritta la progettazione, realizzazione e caratterizzazione della catena di componenti a microonde per ciascuna antenna del sistema SADino. Nella Sezione 4 viene descritto il sistema di alimentazione che permette di alimentare i componenti attivi inseriti all’interno della catena ricevente e, infine, nella Sezione 5 si riportano le considerazioni conclusive sul lavoro svolto

Preliminary tests to design an ad hoc signal acquisition chain for the Sardinia Aperture Array Demonstrator

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

The control system of the 3 mm band SIS receiver for the Sardinia Radio Telescope

We present the control system of the 84-116 GHz (3 mm band) Superconductor-Insulator-Superconductor (SIS) heterodyne receiver to be installed at the Gregorian focus of the Sardinia Radio Telescope (SRT). The control system is based on a single-board computer from Raspberry, on microcontrollers from Arduino, and on a Python program for communication between the receiver and the SRT antenna control software, which remotely controls the backshorttuned SIS mixer, the receiver calibration system and the Local Oscillator (LO) system