REVIEW OF SCIENTIFIC INSTRUMENTS 86, 024702 (2015) The thirty gigahertz instrument receiver for the Q-U-I Joint Tenerife experiment: Concept and experimental results Enrique Villa,1,a) Juan L. Cano,1 Jaime Cagigas,1 David Ortiz,2 Francisco J. Casas,2 Ana R. Pérez,1 Beatriz Aja,1 J. Vicente Terán,1 Luisa de la Fuente,1 Eduardo Artal,1 Roger Hoyland,3 and Ángel Mediavilla1 1Departamento Ingeniería de Comunicaciones, Universidad de Cantabria, Plaza de la Ciencia s/n, Santander 39005, Spain 2Instituto de Física de Cantabria, Avda. Los Castros s/n, Santander 39005, Spain 3Instituto de Astrofísica de Canarias, Vía Láctea s/n, La Laguna 38205, Spain (Received 12 December 2014; accepted 19 January 2015; published online 4 February 2015) This paper presents the analysis, design, and characterization of the thirty gigahertz instrument receiver developed for the Q-U-I Joint Tenerife experiment. The receiver is aimed to obtain polarization data of the cosmic microwave background radiation from the sky, obtaining the Q, U, and I Stokes parameters of the incoming signal simultaneously. A comprehensive analysis of the theory behind the proposed receiver is presented for a linearly polarized input signal, and the functionality tests have demonstrated adequate results in terms of Stokes parameters, which validate the concept of the receiver based on electronic phase switching. C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4907015] I. INTRODUCTION the moment right after the Big Bang.6 Among ground-based projects, QUIET7 and FARADAY8 have developed sensitive The Cosmic Microwave Background (CMB) is the ther- radiometers based on the schemes of previous receivers per- mal radiation from the Big Bang explosion, which fills the forming high-quality sky maps. A new ground-based polar- whole Universe.1 The existence of the CMB radiation2 was ization experiment, called Q-U-I Joint Tenerife (QUIJOTE) postulated by Gamow, Alpher, and Herman in the late 1940s experiment, is under development with the aim of charac- when they were investigating the primordial nucleosynthesis terizing the polarization of the CMB and other galactic and of light elements. However, it was first detected in 1964, when extragalactic signals at medium and large angular scales in the Penzias and Wilson accidentally measured an excess of noise- frequency range from 10 to 47 GHz.9,10 The configuration of like signal which could not be removed.3 The characterization the receiver enables to obtain Q, U, and I Stokes parameters of the CMB radiation is, nowadays, one of the most important simultaneously, which describe the polarization states of an observational probes in cosmology, as it allows to set con- electromagnetic radiation in terms of the total intensity, the straints on the main cosmological parameters that describe fractional degree of polarization, and the shape parameters of our Universe. Hence, a great effort has been invested over the polarization ellipse.11 the last decades in developing the necessary technology, in This paper presents the analysis, design, and characteriza- particular radio astronomy receivers, to collect high-sensitivity tion of the receiver for the QUIJOTE Thirty-GHz Instrument CMB data. A first space mission dedicated to the analysis of (TGI), verifying the functionality of the receiver according to the CMB was the Cosmic Background Explorer (COBE)4 in the theory. A representative polarimeter is analyzed obtaining the late 1980s, which measured the CMB anisotropies and the detected voltages in each output which depend on the whose results were improved by the Wilkinson Microwave polarization of the incoming signal. Furthermore, the measure- Anisotropy Probe (WMAP)4 in the early 2000s. More recently, ments of the receiver response validate the instrument concept the PLANCK mission5 was launched with a wider frequency comparing measured values with the theoretical ones. The range, more sensitive receivers, and a higher angular resolution document is divided into five sections. The first one gives an than the previous missions. While the focus of the previous introduction and, then, the QUIJOTE experiment is described space missions, as well as of many other ground-based or and analyzed in Sec.II. Section III is focused in the subsystems balloon-borne experiments, has been the characterization of which make up the receiver, giving an overview of their perfor- the intensity properties of the CMB, nowadays, the interest of mances. The functionality test is presented and discussed in the community has shifted to the study of the polarization of Sec.IV, and, finally, Sec.V draws general conclusions. the CMB. The reason for this is the search for the primordial B-mode signal that might be encoded in the CMB polarization pattern. This signal may be a proof of inflation, a theoret- II. TGI QUIJOTE EXPERIMENT ical framework that explains the evolution of the Universe in The QUIJOTE project is a ground-based experiment in- stalled and being operated at Teide observatory (Canary Is- a)Author to whom correspondence should be addressed. Electronic mail: lands, Spain) and it is divided into two different stages. In the [email protected] first one, called Phase I, a multi-frequency instrument (MFI), 0034-6748/2015/86(2)/024702/9/$30.00 86, 024702-1 © 2015 AIP Publishing LLC This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 193.144.201.248 On: Wed, 04 Feb 2015 15:05:07 024702-2 Villa et al. Rev. Sci. Instrum. 86, 024702 (2015) consisting of 4 pixels with 8 channels per horn, characterizes providing the power of the signal through the I parameter and the CMB covering different bands within the frequency range the linear polarization from the Q and U parameters. from 10 to 20 GHz. In the Phase II, two instruments are under The calculation of the Stokes parameters is achieved from development for a second telescope, a 30-GHz (TGI—Thirty- the combination of measurable signals in the receiver. A polar- GHz Instrument) and a 40-GHz (FGI—Forty-GHz Instru- izer, placed in front of an orthomode transducer (OMT), pro- ment). The TGI and the FGI instruments are composed of 31 vides left- and right-hand circular polarization output signals, pixels each one working in the 26–36 GHz and the 35–47 GHz which detected and properly combined enable to obtain the frequency band, respectively. parameters. The TGI polarimeter block diagram is shown in Fig.1. A. Scientific goals Each pixel of the TGI is composed of a cold stage module (20 K) and a room temperature (298 K) module. The cryogenic The main scientific goals of QUIJOTE are to detect the part is made up of a feedhorn, a polarizer, an OMT, and two primordial B-mode signal down to r = 0:05 and to characterize low-noise amplifiers (LNAs). Outside the cryostat, two gain the polarization of low-frequency Galactic foregrounds, in and filtering modules, the phase switches module, andthe particular, the synchrotron emission and the anomalous micro- correlation and detection module operate at room temperature, 10 wave emission. QUIJOTE data will be a valuable comple- in which the microwave signal is amplified, filtered, correlated ment of Planck polarization data at higher frequency, which by 180◦ microstrip hybrids and, finally, converted into DC volt- will provide information about the polarization properties of ages using square-law detectors. These signals are collected by the thermal dust, a foreground that shows up at frequencies a data acquisition system (DAS). The phase switches modules above 100 GHz. comprise 0◦/180◦ and 0◦/90◦ phase switches generating four 12 From previous experiments, most CMB information has polarization states and their performance is crucial in order to been obtained only from intensity measurements. Therefore, obtain the Stokes parameters, minimizing the leakage among the analysis of its polarization signal provides helpful data in them and, at the same time, overcoming the 1/f noise and order to completely characterize the CMB radiation. different systematic errors in the receiver. The standard theory states that the CMB is linearly polar- ized, so its polarization state can be described using Q and C. TGI receiver analysis U Stokes parameters, defined by complex spin spherical har- monics. However, scientific polarization maps are usually The sketch in Fig.1 shows four outputs ( Vd1–Vd4) provid- defined in terms of E- and B-field components, which corre- ing the detected DC voltages, which are combined in the DAS. spond to a combination of Q and U coefficients. These param- Considering a circular coordinate system, the Stokes eters enable the calculation of the angular power spectra in parameters are defined by terms of temperature, E- and B-modes, which define the way 2 2 in which the CMB anisotropies are originated from scalar or I = El + Er ; (1) gravitational waves perturbations, respectively. | | |∗ | Q = 2·Re El ·Er ; (2) The QUIJOTE experiment is defined with two polariza- U = −2·Im E∗·E ; (3) tion surveys. This strategy enables the receiver to scan sky l r 2 2 areas and to obtain significant information of inflationary state V = El − Er ; (4) of the CMB. The first one is a deep survey intended to analyze | | | | a sky area of around 3000 square degrees, whereas the sec- where El and Er are the electrical field components in a cir- ond one is a shallow survey scanning around 18 000 square cular coordinate system. The parameter V is assumed to be degrees. Both surveys will reach low sensitivities in order to V = 0, since it is the sign of circular polarization and the obtain sky maps and synchrotron information. CMB is considered as linearly polarized, and therefore it is not measured.