# 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
Regarding the P-band channel (see Figures 3 and 5a), this section of the receiver front-end includes, besides a microwave isolator as a first component, a coaxial switch (model 87104A from Agilent, Santa Clara, CA, USA [41]) that allows us to select the desired filter and, as last component, a commercial LNA (model ZRL-700 from Mini-circuits, New York, NY, USA) that works at ambient temperature with a gain of about 30 dB [42]. The installed commercial filters are:  
* A band pass filter (BPF) centered at 357.5 MHz with a bandwidth of about 120 MHz (model 5B340-357.5/T120-O/O from K&L, Salisbury, MD, USA [43]). It is a five-pole tubular filter with a \(-3\) dB bandwidth from 295 MHz to 420 MHz and an insertion loss less than 1 dB;
* A BPF centered at 330 MHz with a bandwidth of 50 MHz (model 5B340-330/T5O-O/O from K&L, Salisbury, MD, USA [43]). In particular, it is a five-pole tubular filter with a \(-3\) dB frequency response in the frequency range between 300 MHz and 360 MHz, and an insertion loss less than 1 dB;
* A BPF centered at 410 MHz with a bandwidth of 16 MHz (model 3B110-410/T15-O/O from K&L, Salisbury, MD, USA [43]). In detail, it is a three-pole tubular filter with a \(-3\) dB bandwidth from 402 MHz to 418 MHz and an insertion loss of about 1 dB. All these filters are used for both polarization channels (i.e., P-band H-pol and V-pol), respectively. As concerns the L-band filter selector block (see Figures 3 and 5b), it includes a microwave isolator as a first component, a coaxial switch (model 87104A from Agilent,  
Figure 5: (**a**) P-band filter selector block; (**b**) L-band filter selector block. Santa Clara, California [41]) that allows us to select the desired filter and, as last component, a commercial LNA (model ZRL-2150 from Mini-circuits, New York, NY, USA) that works at ambient temperature with a gain of about 25 dB [44]. The available commercial filters, for each polarization channel, are:  
* A BPF centered at 1540 MHz with a bandwidth of 520 MHz (model 5B120-1540/T520-O/O from K&L, Salisbury, MD, USA [43]). In particular, it is a five-pole tubular filter with a \(-3\) dB frequency response in the frequency range between 1250 MHz and 1820 MHz, and an insertion loss less than 1.5 dB;
* A BPF centered at 1400 MHz with a bandwidth of 120 MHz (model 5B120-1400/T120-O/O from K&L, Salisbury, MD, USA [43]). In detail, it is a five-pole tubular filter with a \(-3\) dB bandwidth from 1340 MHz to 1460 MHz and an insertion loss less than 1.5 dB;
* A BPF centered at 1655 MHz with a bandwidth of 120 MHz (model 5B120-1655/T120-O/O from K&L, Salisbury, MD, USA [43]). In particular, it is a five-pole tubular filter with a \(-3\) dB bandwidth from 1570 MHz to 1730 MHz and an insertion loss less than 1 dB;
* The combination of three cascaded filters in order to obtain an observation bandwidth in the range between 1300 MHz and 1800 MHz, cutting the main RFIs at 1310-1340 MHz and 1790-1960 MHz. These three filters are: a band rejection (Notch) filter (model 6N45-1320/E62.7-O/O from K&L, Salisbury, MD, USA [43]), a BPF centered at 1540 MHz with a bandwidth of 520 MHz (model 5B120-1540/T520-O/O from K&L, Salisbury, MD, USA [43]) and another Notch filter (model 6 NS11-1880/E138-O/O from K&L, Salisbury, MD, USA [43]). Further details about the architecture of the original L-P band receiver are reported in [29].