# 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
Focusing on the microwave components of the system, a detailed schematic is reported in Figure 3. The L-P band feeds are realized using a coaxial configuration. In particular, the P-band feed and its ortho-mode junction (OMJ) are designed and realized in a coaxial circular waveguide, with an outer diameter that represents the P-band feed aperture equipped with external corrugations for the radiation pattern improvement, and an inner diameter corresponding to the L-band feed, which is a simple truncated circular waveguide (see Figure 4a). This part of the system works at 300 K. Further details of the feed design are explained in [29]. Continuing with the analysis of Figure 3, the feeds are directly connected to the Dewar (or cryostat) system. This block is directly attached to a cooling system that guarantees to achieve the cryogenic temperature of about 20 K, as shown above in Figure 2. The cryostat size is optimized with the goal to not have too many empty spaces between microwave components of the signal acquisition chain. In this way, the cryogenic temperature can be maintained more easily. On the Dewar block, all RF microwave components that compose the signal acquisition chain for the P-band and L-band channels are installed, respectively (see Figure 3). Regarding the P-band channel, the first component of the RF chain is a cryogenic 180\({}^{\circ}\) hybrid with integrated directional coupler [34]. This is a six-port microwave component that is realized using a planar fractal 180\({}^{\circ}\) hybrid configuration and a coupled lines directional coupler in cascade on the same layout, in order to significantly reduce its size on the Dewar [34]. Downstream from this component (see Figure 3), there is a commercial coaxial mechanical switch (model Agilent 8761B [35]), provided for the P-band low noise amplifier (LNA) isolation in case of high self-produced RFIs [29]. The next component of the Dewar signal acquisition chain is a cryogenic high-temperature superconducting (HTS) band pass filter (BPF) [36]. This filter is used before the LNA with the aim to reject high RFIs slightly outside the receiver bandwidth, which would be amplified in an undesirable mode by the LNA. The last component (into the Dewar block)  
Figure 4: (**a**) Photo of the feed; (**b**) Photo of the warm section (which works at the temperature of 300 K) of the receiver, composed of the linear-to-circular polarizer, the filter selector block and the electronic control system. of the P-band signal acquisition chain is the cryogenic LNA, with a gain of about 27 dB at the center frequency [37]. As concerns the L-band channel, on the Dewar block there is the ortho-mode transducer as a first component of the RF chain. It consists of a cylindrical ortho-mode junction (OMJ) and two identical 180\({}^{\circ}\) hybrid power combiners in a double ridged waveguide [38]. After this component, a commercial switch (model Agilent 8761B [35]) was installed for the same reason described for the P-band channel. The next component is a directional coupler that permits us to inject a noise source in the RF path, useful for the receiver calibration [39]. Finally, the last component of the chain is the L-band LNA, with a gain of about 38 dB at the center frequency [40]. Each output signal from the Dewar enters into the P-band/L-band linear-to-circular polarizer block, respectively (see Figure 3). This block maintains both linear polarizations (i.e., H-pol and V-pol) and, using a commercial 90\({}^{\circ}\) hybrid, collects them in right-hand circular polarization (RHCP) and left-hand circular polarization (LHCP). The final block of the signal acquisition chain is the filter selector block.