# Upgrading of the L-P Band Cryogenic Receiver of the Sardinia Radio Telescope: A Feasibility Study
## 4 Results and Discussion
### New Filters Requirements for the P-Band Channel  
Starting from the analysis of the P-band (see Figures 8 and 9) and considering, in particular, the larger bandwidth that the receiver can offer, the most important need is rejecting the TETRA signals (385-395 MHz), which turn out to be the highest unwanted signals in this receiver bandwidth. This depends on the fact that one of the TETRA stations is installed on Monte kai, at about 1 km as the crow flies from SRT. The possible solution to this problem is the design and development of an ad hoc band-stop filter (Notch filter). In this way, it is possible to attenuate the frequencies within 385-395 MHz range (stopband) while passing all other frequencies unaltered. A feasibility study to determine the best features of the Notch filter is conducted using the Matlab software and the ideal frequency response of this kind of filter is shown in Figure 14 (blue curve). In detail, this filter attenuates of about \(-70\) dB at 390 MHz, \(-89\) dB at 392 MHz and \(-55\) dB at 395 MHz. At these frequencies, the TETRA signals measured by SRT, using the old BPF (model 5B340-357.5/T120-O/O from K&L), have levels of amplitude of about \(-40\), \(-8\) and \(-44\) dBm, respectively. Because of the installation of the new Notch filter, these TETRA signals are almost completely cut down, thus allowing a cleaner band of observation. Figure 14: Comparison between the S21 parameter of the old P-band BPF (orange curve), centered at 357.5 MHz with a bandwidth of about 120 MHz (model 5B340-357.5/T120-O/O from K&L) and the new ideal filters (Notch filter and new BPF). The Notch filter (blue curve) rejects the bandwidth 385-395 MHz (TETRA signals of Figure 9) and the new BPF (red curve) has a \(-3\) dB frequency response between 280 and 420 MHz. The Notch filter can be cascaded with the BPF. Considering that the old BPF (model 5B340-357.5/T120-O/O from K&L) is a five-pole filter, it has a S21 curve (orange curve of Figure 14) with an edge band with very soft drop, several out-of-band signals (such as the signals at 225 and 468 MHz of Figure 8) are detected by SRT and turn out a trouble for the scientific research. In fact, for radio astronomy applications, it is necessary to consider at least the \(-30\) dB filter frequency response, in contrast to engineering projects that take into account the \(-3\) dB response. This is due to the fact that the highly sensitive radio astronomy tools integrate received signals in the observation bandwidth and they can detect RFIs (although they are outside the \(-3\) dB bandwidth, with an engineering perspective, of the microwave filter) can assume comparable amplitude with real signals. For this purpose, a new BPF can be designed and developed with an ideal frequency response (i.e., S21 parameter) similar to the features calculated with the Matlab software and shown in Figure 14 (red curve). In detail, the new BPF can have a \(-3\) dB bandwidth slightly larger than the old BPF bandwidth (280-420 MHz vs. 300-420 MHz), but with an edge band with a very step drop (such as \(-101\) dBm vs. \(-32\) dBm at 225 MHz). On the basis of the receiver usage experienced during the years, consideration of the P-band high-temperature superconducting (HTS) BPF [36] (see Figure 12) can be given. Typically, a filter is not used before the LNA because its insertion loss worsens the temperature of the whole system. However, since the P-band is populated by several RFIs also slightly outside the receiver bandwidth, the HTS BPF was used to reject them. Further details of this filter are proposed in [29; 36]. Even though it features high performances in terms of band rejection, the HTS filter has the disadvantage of working only at cryogenic temperature. Consequently, when the receiver is at a temperature of 300 K, the filter behaves like a 50 Ohm load. This aspect represents a limit for the P-band receiver, which can be used only at system temperature of about 20 K, excluding the possibility to perform application at ambient temperature (i.e., radar observation at 300 K, etc.).