# Upgrading of the L-P Band Cryogenic Receiver of the Sardinia Radio Telescope: A Feasibility Study
## 3 Study of the Receiver Criticalities and Workflow for the Upgrading Feasibility Study
### Mitigation of the RFIs Derived from the Surrounding Area
In case of detection of new unknown signals, the RFI mobile laboratory can be used in order to investigate the source of them. As regards the P-band, an explicative spectrum of data collected by the RFI system based on one of the Vivaldi antennas of the SAD telescope [45; 46] is reported in Figure 8. In this case, the bandwidth 50-550 MHz is analyzed with the scope to identify the highest level unwanted signals. In this spectrum, some impulsive signals are not visible for reason of graphical representation, but they are equally detected by the system. The measurement campaign performed with this system can be considered a preliminary analysis for studying the situation in the neighborhood of the P-band receiver bandwidth (305-410 MHz squared in red in Figure 8). In the bandwidth 50-550 MHz considered by this system, the significant unwanted man-made signals are imputable to the following:  
* FM radio band (88-108 MHz);
* Security services (160-185 MHz);
* Digital Video Broadcasting (DVB) services (203.5 MHz);
* Terrestrial Trunked Radio (TETRA) in use by the Italian Ministry of Defense (385-395 MHz); unfortunately, one of the TETRA stations is installed on Monte lxi, at about 1 km as the crow flies from SRT;
* Weather balloons (402-405 MHz, but it is also possible to detect them in the radio astronomical service (RAS) band 406.1-410 MHz);
* Sardinia emergency department (460 MHz);
* Radio TV broadcasting signals (470-828 MHz). Figure 8: Spectra with data collected by the RFI detection system based on Sardinia Aperture Array Demonstrator (SAD). According to this preliminary investigation, it is clear that the signal with the highest level of amplitude comes from the TETRA station and it represents a serious limitation for the receiver performances. Furthermore, despite the BPF of the receiver (model 5B340-357.5/T120-O/O from K&L) has a \(-3\) dB bandwidth from 295 MHz to 420 MHz, several signals (i.e., signal at 225 MHz) enter in the signal acquisition chain (smoothed in accordance with the BPF frequency response, but quite high for a highly sensitive system such as SRT) and represent unwanted signals for back-ends. For these reasons, in this feasibility study, we investigate to upgrade the system with a more performant BPF in order to mitigate these painful signals. The features of these new filters, obtained with the upgrading feasibility study, are described in Section 4 (results and discussion). The spectrum of data collected by SRT using the P-band receiver, with the selection of the BPF centered at 357.5 MHz with a bandwidth of about 120 MHz (model 5B340-357.5/T120-O/O from K&L), and using the Rohde and Schwarz FSV40 spectrum analyzer as back-end, is shown in Figure 9.  
This spectrum presents a better resolution bandwidth respect to Figure 8 and further RFIs are visible. In particular, in the frequency range between 310 and 330 MHz, there are signals from Italian Air Force. TETRA signals are clearly visible also in this plot, with its high level of amplitude that in certain moments saturates the whole receiver system. Moreover, in the spectrum of Figure 9, are visible several signals with a considerable peak level, such as at 355, 360, 400-401 and 410 MHz, which are self-produced by the electronic control system of the receiver. The mitigation of these RFIs is addressed in the next Section 3.2.