# 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 Self-Produced RFIs
#### 3.2.1 Mitigation of the RFIs Generated by the Electronic Control System  
The RFIs generated by the original electronic control system are mostly in P-band, such as the signals at 355, 360, 400-401, and 410 MHz, which can be viewed in Figure 9. These electronic boards provide the exact supply voltage for all active components installed on the receiver (i.e., switch, amplifiers, etc. ), as well as the monitoring of the system. Unfortunately, in addition to the unwanted signals generation, the original electronic control board does not permit us to temporarily switch off the LNAs, which is an important aspect for isolation in case of high self-produced RFIs [29]. However, as discussed in Section 2, these devices are of paramount importance for the receiver monitoring and they cannot be removed from the system. The issue of RFIs generation can probably be improved with an RF shielding of the original control electronic boards. However, this does not resolve the criticality regarding the LNAs management. For these reasons, in this feasibility study, the replacement of the outdated technology, i.e., the original electronic boards [32], is proposed, considering the latest cutting-edge electronic engineering, with a focus on low RFIs generation components. The new electronic board has to be backward compatible with earlier boards with the aim to maintain the original receiver architecture. In the new receivers developed for the PON project [25; 26; 27], a new electronic board, named Gain Attenuation Intelligent Amplifier (GAIA), is used for the remote control [26]. This board can be considered for the receiver upgrade feasibility study, in order to align  
Figure 10: Spectra with data collected by SRT using the L-band receiver, with the selection of the BPF centered at 1540 MHz with a bandwidth of 520 MHz (model 5B120-1540/T520-O/O from K&L), and using the Rodhe and Schwarz FSV40 spectrum analyzer as back-end. the L-P band receiver technology with the PON new receivers. The board is based on a microcontroller (model ATMega2560 from Microchip Atmel [50; 51]) and on digital potentiometers (model AD5231 from Analog Devices [52]) designed for biasing, remote monitoring and control of the gate and the drain voltages (V\({}_{\text{g}}\) and V\({}_{\text{d}}\), respectively) of cryogenic LNAs [26]. The GAIA board [26; 53] offers ultra-stable biasing of the P- and L-band cryogenic LNAs. GAIA is equipped with a network module based on chip model W5100 from Wiznet [54] for the Ethernet communication. The board allows SPI, IIC and universal asynchronous receiver-transmitter (UART) communication protocols. Further details of the GAIA board are reported in [26; 53]. A dedicated RFI measurement campaign, with the goal to study the signal emission of the GAIA board, is presented in [53]. The measurements are performed in an outdoor environment, in open field. The measurement system is composed of an electric field probe connected to a spectrum analyzer (model FSV40 from Rohde and Schwarz). For detecting impulsive signals, the instrument is setup in clear/write mode with resolution bandwidth of 1 MHz and sweep time of 100 microseconds [53]. This analysis did not reveal the presence of impulsive signals generated by the board. Furthermore, the spectrum analyzer is setup in average mode with span equal to 1 GHz, resolution bandwidth of 200 kHz and 5000 resolution points, for detecting continuous signals [53]. The results of the measurements from 0 to 1000 MHz (including the interesting P-band for our application) are shown in Figure 11a and the results for the bandwidth 1000-2000 MHz (L-band) are plotted in Figure 11b [53]. Analyzing the plot of Figure 11a, it is visible that the GAIA board generates narrowband signals at 4, 12, 32, 36, 192, and 875 MHz with regard to the P-band [53]. This last RFI represents the highest of signals gave off by GAIA in P-band, but it does not represent an issue for the receiver, since it is far from the P-band receiver bandwidth (i.e., the bandwidth of the P-band feed is 305-410 MHz).