## 4 New Electronic Control System of the Receiver for the Installation on SRT
### Raspberry Pi 2 (Model B): The Core of the Electronic Control System  
Before astronomy observations can be performed with the ex-PdBI W-band receiver, some settings need to be executed to prepare the instrument properly. These settings must be executed remotely from the SRT control room by the astronomers that use the telescope. In particular, tuning the ALMA local oscillator and locking it to the desired frequency is required for each observation. To do this, the backshort of the SIS mixer is placed at the proper position for the chosen frequency, and the junction bias voltage is set. The local oscillator power supply is adjusted to achieve a junction direct current of the order of 20 uA [39]. Typically, this set of setting operations takes a few minutes and it should be completely transparent to the astronomers that use SRT to collect data for scientific research. This is possible because of the single-board computer Raspberry Pi 2 (Model B), which is programmed with ad hoc Python scripts [31], which represents the core of the new control system and allows the remote control of the receiver. The Raspberry Pi 2 is chosen for its quad-core processor, which runs at 900 MHz with 1 gigabyte of random-access memory (RAM) for its USB and Ethernet ports (i.e., four and one, respectively) and because it made it possible to use an operative system based on Linux open-source software [42], as well as its small size. Apart from the technical features of the device that are perfect for the requirements to be met, the self-generation of radio frequency interference (RFI) represents a critical issue for a system that will be installed on a radio telescope. In this respect, an accurate measurement campaign was conducted with the aim to obtain knowledge of all self-produced signals and validate the logistical solutions for their mitigation. Since SRT was designed to operate from 300 MHz to 116 GHz, and considering that some receivers up to 26.5 GHz are installed on it, the RFI measurement campaign is performed in the frequency range of 0.1-6 GHz. The measurement campaign is performed in free space using a log-periodic antenna, which operates in the frequency range between 250 MHz and 7 GHz, and the Rohde and Schwarz \begin{table}
\begin{tabular}{c c c} \hline
**Module Name** & **Technical Features** & **Functioning** \\ \hline \multirow{4}{*}{Raspberry Pi 2 (Model B)} & - 900 MHz quad-core ARM Cortex-A7 CPU; & \\  & - 1 GB RAM; & \\  & - Ethernet port; & \\  & - 4 USB ports. & \\ \hline \multirow{4}{*}{Arduino UNO REV3} & - 16 MHz ATmega328P; & \multirow{4}{*}{Each microcontroller manages one block of the new control system.} \\  & - UART, I2C, SPI; & \\  & - USB port. & \\ \hline \multirow{2}{*}{Mixer Biasing Block} & - 1/O Voltage: 5 V, 12 V, 15 V; & \multirow{2}{*}{Manage the IRAM Junction Bias board for the SIS mixer biasing.} \\  & - 16-bit resolution ADC ADS1115. & \\ \hline \multirow{4}{*}{Mixer Tuning and Receiver Calibration Block} & - 1/O Voltage: 5 V, 12 V, 15 V; & \multirow{2}{*}{\
\begin{tabular}{c} Composed of the SIS mixer backshort motor control board for the mixer tuning, and the receiver calibration motors \\ control board for the receiver calibration. \\ \end{tabular} } \\  & - 16-bit resolution ADC ADS1115. & \\ \hline \multirow{4}{*}{Monitoring and Control Boards for the ALMA Local Oscillator Block} & - 1/O Voltage: +6 V @ 1.5 A, \(\pm\)15 V @ 500 mA; & \multirow{4}{*}{
\begin{tabular}{c} Composed of two modules: one for managing the AMC and the other for controlling the PA of the ALMA local oscillator. \\ \end{tabular} } \\  & - 16-bit resolution ADC ADS1115; & \\ \cline{1-1}  & - SPI bus. & \\ \hline \end{tabular}
\end{table} 
Table 1: Summary of the technical features and functioning of each subsystem of the new electronic control system. FSV40 Spectrum Analyzer as the back end [31]. Three different types of measurement are carried out:  
* Raspberry powered off, to highlight all signals coming from known external sources (i.e., frequency modulation radio signals, mobile communications signals, etc.)