# Upgrading of the L-P Band Cryogenic Receiver of the Sardinia Radio Telescope: A Feasibility Study
SRT also operates in the Very Long Baseline Interferometry (VLBI) network, a technique that performs correlation between data collected by several antennas that observe the same radio source, simultaneously [16; 17; 18]. Recently, the antenna has also been employed for Space Situational Awareness (SSA) operations [8; 19; 20; 21; 22; 23; 24]. In this kind of applications, the cryogenic coaxial dual-frequency L-P band receiver has been of fundamental importance to cover low frequencies with satisfactory results. With the aim of maximizing the scientific research results and extending them to new high frequencies (up to 116 GHz) not yet covered by SRT, INAF has recently expressed an interest to upgrade the telescope. For these reasons, a National Operational Program (PON) funding [25] has been allocated to INAF by the Italian Ministry of University and Research, with the aim to install on the telescope four new receivers that operate in Q-band (33-50 GHz) and W-band (70-116 GHz) [25; 26; 27]. The works relating to the PON project have imposed a stop on the SRT operations and, consequently, the temporary dismantling of the old receivers (P-band, L-band, C-band and K-band [9], respectively). By taking advantage to this, it is possible to study, upgrade, maintaining, testing and refurbishing them in laboratory to verify their actual performances. Focusing on the L-P band receiver of the SRT, it is installed on the primary focus of the telescope (see Figure 1a,b) and it permits simultaneous observations in both the P-band and L-band, as well as classical observation at a single frequency [28; 29]. Although the receiver has been used in its original design with good results for years, it presents some aspects that could be upgraded in order to improve the performances of the system. With advancing years, the technology used for developing the control system of the receiver became obsolete and it could be substituted with next-generation electronic boards. These new electronic systems offer better performances both service reliability and low generation of unwanted radio frequency signals, better known as radio frequency interferences (RFIs). They represent spurious signals and harmonics from others frequency bands, self-produced signals (generated by the old electronic control system), undesired signals overlapping the receiver band (i.e., the signals of civil and military mobile communications, the TV broadcasting and the remote sensing applications) and out-of-band signals not properly rejected by the microwave filters of the system. Unfortunately, the RFI scenario of the surrounding environment evolves over time and it is worsened with respect to the time when the L-P band receiver was developed and installed on the SRT. For these reasons,a feasibility study is necessary to establish what changes to make to the receiver. These changes regard the replacement of original electronic control system with a next-generation board, which offer better performances both service reliability and low RFIs generation, and the study of the requirements of new microwave filters for high RFIs rejection. These new components will replace the old filters of the receiver, allowing us to cut inoperable frequency sub-bands affected by the RFIs. Furthermore, considering that the RFI scenario could deteriorate further in the next future, it could be necessary to modify the operative frequency of the system with the installation of other filters. Consequently, it is necessary to have the filters block of the receiver in a most comfortable position respect to the primary focus of the telescope, which can be only reached using a lifting platform. For this reason, in the feasibility study presented in this paper, the authors consider to move the filters block in the APEX room of SRT, adequately prepared to accommodate them and accessible by a ladder installed on one of the four quadruped legs (see Figure 1a).