The Yebes Observatory has recently completed the design, implementation and installation of a new broadband receiver for its 40-m diameter radio telescope. The receiver is sensitive in the 4.5 to 9 GHz frequency range, and replaces two older, frequency-limited receivers in C-band (5 and 6 GHz) and X-band (8 GHz), allowing simultaneous observation in these two bands and in additional, previously unavailable frequencies.
For the European Very Long Baseline Interferometry Network, the EVN, the contribution of the new receiver from the Yebes Observatory is an important step in adapting the network to the scientific challenges of the next decade. The EVN is currently being upgraded to work in collaboration with the next generation of radio telescopes, such as the Square Kilometre Array (SKA), and other large-scale telescopes such as the Extremely Large Telescope (ELT), which will operate in the optical and infrared range, or the Cherenkov Telescope Array (CTA), which will detect gamma rays.
The use of low-noise receivers with large instantaneous bandwidths, such as the one in question, will make possible to quadruple the sensitivity of the EVN observations, and to carry out complete spectral and polarization studies of the emission from the radio sources. These technical improvements will ensure that the EVN will continue to make valuable contributions in the multi-messenger landscape of the new astrophysics.
The development of these state-of-the-art receivers involves a number of technological challenges. The Increase of the observable bandwidth has to be done without degrading the noise characteristics and thus the sensitivity of the receiver, and in parallel, without compromising the purity of its polarization response.
The new Yebes CX band receiver meets these requirements and becomes one of the most sensitive receivers currently in use in the EVN in this frequency range. It consists of a phase-corrected conical corrugated horn which, due to its large size, cannot be cooled and needs to work at room temperature, followed by a cooled orthomode transducer (OMT) which provides the two linear polarizations. To transform to circular polarization, more appropriate for VLBI observations, a 3dB and 90 degrees cooled hybrid circuit has been used, feeding two low noise InP HEMT type amplifiers, also cooled at cryogenic temperatures. The amplifiers have been optimized in input reflection to reduce ripple effects in the receiver frequency band. All these receiver components, as well as the cryostat for its cooling, have been designed, manufactured and characterized at the Yebes Observatory. Overall, the mean noise temperature of the receiver is about 12 K, about -261 °C, contributing minimally to the total noise during the observations. The receiver is also equipped with gain and phase calibration signals. The associated frequency converter, which receives the receiver signals via fiber optic links, is capable of providing the signals at intermediate frequencies of 500 MHz or 4 GHz, properly conditioned to feed different types of VLBI backends or spectrometers for spectral line studies.
Last December 16, the CX-band receiver was successfully tested in an interferometry observation thanks to the participation of the Effelsberg, Medicina, Westerbork, Noto and Onsala radio telescopes, all members of the EVN, under the coordination of JIVE, the correlation and analysis centre of the network, which obtained and analysed the results during the observation.
This project is part of a more ambitious program that includes the development of an even broader receiver that will cover the 4 to 18 GHz frequency range. The program to update the infrastructures of the radio telescopes at the Yebes Observatory (YNART) is co-funded with ERDF 2014-2020 funds, granted by the Spanish Ministry of Economy and Competitiveness.
Author: Cristina García Miró (Yebes Observatory)