Arcetri designs innovative superconductor microwave filters for the Sardinia Ratio Telescope

When observing in radio frequency bands, astronomers have to deal with the signals from commercial technologies that can interfere with the extremely sensitive radio astronomical receivers. Several techniques to mitigate the impact of radio frequency interference (RFI) have been developed. A particular solution consists of microwave filters based on high temperature superconductor (HTS) technology in front of the first stages of amplification avoiding the gain compression and preserving at the same time the low receiver noise temperature.

A new microwave component based on HTS technology has been presented in a recent issue of the Journal of Astronomical Instrumentation, produced by World Scientific Publishing Company, in the article òÀÜA high temperature superconductor microwave filter working in C-band for the Sardinia Radio TelescopeòÀÝ by Pietro Bolli and collaborators (Fig.1). This paper is the result of a national project conducted at the INAF òÀÓ Astrophysical Observatory of Arcetri together with the INAF òÀÓ Institute of Radio Astronomy and an external collaborator from the University of Birmingham (UK).

Fig. 1 òÀÓ Cover page of the volume 3, number 1 (March 2014) of the Journal of Astronomical Instrumentation with a picture of the microwave band-pass filters fabricated from High Temperature Superconductor (YBaCuO). A selection of filters are shown, with carriers, fabricated from copper, silver-plated titanium, and gold-plated titanium (left to right).

 The project was aimed at designing, fabricating and characterizing a planar HTS band-pass filter for the cryogenic front-end of the C-band receiver (center frequency 6.7 GHz with 30% bandwidth) for the Sardinia Radio Telescope (SRT). The SRT represents a new radio astronomical facility of INAF, recently inaugurated (see Fig. 2) consisting of a general purpose, fully steerable 64-m diameter parabolic radio telescope capable of high efficiency in the 0.3òÀÓ116 GHz frequency range.

The HTS technology allows outstanding noise temperature performance improvements as compared to normal metal conductors due to the rapid decrease in the resistance at low temperatures. The HTS are also characterized by the fact that the phenomenon of superconductivity appears at relatively high temperature (significantly above the liquid nitrogen boiling point, 77 K).

The research activity was also addressed to test alternative technical solutions to produce the filters, like for example using different materials for the carrier and for the plating, and also different connector types with respect to the more standard procedures. It turned out that the SMA connectors and the silver plating appear to be valid options to keep the fabrication of the HTS filter simpler and more cost competitive.

The cryogenic scattering parameter measurements show a good agreement with numerical results: the average of the transmission losses turns out to be in the range 0.15òÀÓ0.25 dB depending on the prototypes, whereas the reflection coefficient is below òÈÒ16 dB. Multiple cool-down measurements have been performed successfully proving the data repeatability both in short- and medium-term.

Fig. 2 òÀÓ On 30 September 2013, the inaugural ceremony of SRT took place in the presence of the highest local and national authorities. Several scientists from different European radio astronomical institutes participated to the inauguration on behalf of the international radio astronomical community. Additionally, 2000 people from the neighboring municipalities attended the inauguration.

Edited by P. Bolli and A. Gallazzi