# Upgrading of the L-P Band Cryogenic Receiver of the Sardinia Radio Telescope: A Feasibility Study
## 2 Architecture of the Original L-P Band Cryogenic Receiver of SRT  
As mentioned in the Introduction section, the L-P receiver consists of in a cryogenic coaxial dual-frequency system, which permits concurrent observations in the P- and L-bands, along with single band observations, in either linear or circular polarization. In particular, it is composed of two coaxial feeds that are designed to work optimally in the range 305-410 MHz of the P-band and 1.3-1.8 GHz of the L-band, respectively [29]. Since SRT is a super-heterodyne system that works at a baseband frequency of 0.1-2.1 GHz [30], a down-conversion system for the radio frequency (RF) signals detected by the L-P receiver is unnecessary. A block diagram of the receiver is reported in Figure 2 and a schematic representation of the original receiver front-end, focusing on the microwave components, is shown in Figure 3. All RF and electronic components of the original receiver architecture are inte  
Figure 1: (**a**) Sketch of the Sardinia Radio Telescope (SRT), where the position of the L-P band receiver (primary focus of the telescope), the APEX room and the ladder for the APEX room, are highlighted; (**b**) Photo of the L-P band receiver installed on the primary focus of SRT. grated into a single box, installed directly on the primary focus positioner of SRT, with a volume of about 3 cubic meters and a weight of about 700 kg [29]. The features of these blocks are summarized below. Analyzing in detail the block diagram of the receiver architecture (Figure 2), it is composed of:  
* The L-P band coaxial feeds, working at ambient temperature (i.e., 300 K);
* The Dewar (or cryostat) block that works at cryogenic temperature of about 20 K;
* The cooling system, which enables the Dewar refrigeration;
* The noise calibration unit, useful for the receiver calibration for both P-band and L-band, thanks to the injection of a noise source (using the broadband microwave coaxial noise source NST26-B model from Micronetics [31]) in the RF signal acquisition chains;
* The P-band and L-band polarizer blocks, which permit us to also obtain the circular polarization;
* The P-band and L-band filter selector blocks, which allow us to select the suitable microwave filter for radio astronomy observations. All architecture blocks can be managed remotely thanks to a dedicated electronic control system (see Figure 4b), composed of some electronic boards described in detail in [32]. The main electronic board is composed of several digital input/output (I/O) electronic components, in order to create an interface that adds the ability to input and output digital signals in parallel to a computer. The used I/O electronic components are based on the main I/O communication technologies, such as transistor-transistor logic (TTL), inter-integrated circuit (IIC) serial bus, serial communications interface (SCI) bus, controller area network (CAN) bus and serial peripheral interface (SPI) bus. The microcontroller used for the board implementation is the FR50 32-bit RISC Core model from Fujitsu [33]. In particular, the board, interfaced with all receiver components, was programmed with an ad hoc firmware and software in order to manage the cooling system, with the aim to achieve the cryogenic temperature of about 20 K, and supplying the necessary voltage to all active microwave components of the system. The board permits us only to switch on the microwave components, without any control to the voltage levels and the possibility to temporarily switch off the components. However, the board enables us to collect data about the state of the Dewar (the temperature and the pressure) and the state of the active components (i.e., the voltages of the low noise amplifier stages). In this way, it  
Figure 2: The block diagram of the L-P band receiver of SRT. is possible to conduct statistical analysis and have valuable information about the correct functioning of the receiver. Further details about this electronic board are presented in [32]. Figure 3: (**a–d**) Schematic of the L-P band receiver [29].