## 6 Conclusions and Future Work  
We presented the feasibility study and preliminary tests for the installation on the SRT of a W-band radio receiver based on a SSB SIS mixer. The instrument was expected to provide very low noise receiver performance across the frequency range 84-116 GHz if the SIS mixer is cryogenically cooled at a physical temperature of approximately 4 K. It was developed by IRAM for the PdBI antennas. A new dedicated electronic control system was designed and developed by our INAF team for its remote control. The feasibility study of the receiver optics, the upgrade of the cooling system, and the evaluation of using a new dedicated local oscillator were presented. The local oscillator system is the same type  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**RF Bandwidth (GHz)** & **Local Oscillator Frequency (GHz)** & **Voltage and Current Biasing Values** & **Noise Temperature (K)** \\ \hline
83.7–84.3 & 85.5 & V = 4.07 mV; I = 28.0 \(\mu\)A & 73 \\
85.7–86.3 & 87.5 & V = 4.08 mV; I = 30.0 \(\mu\)A & 65.8 \\
89.7–90.3 & 91.5 & V = 4.06 mV; I = 34.5 \(\mu\)A & 78 \\
94.7–95.3 & 96.5 & V = 4.05 mV; I = 31.7 \(\mu\)A & 143 \\
98.2–98.8 & 100 & V = 4.03 mV; I = 28.7 \(\mu\)A & 158 \\
104.7–105.3 & 106.5 & V = 3.93 mV; I = 28.5 \(\mu\)A & 287 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Results of the noise temperature measurements for SIS mixer tuned in LSB at several LO frequencies.  
Figure 14: Noise temperature of the upgraded ex-PdBI receiver to the frequency variation.  
adopted for the ALMA Band 3 project. The local oscillator was successfully tested and evaluated in the laboratory. The SSB receiver noise temperature measured in the laboratory, at the frequency bandwidth of 84-95 GHz, was less than 150 K, although the temperature of the third cryogenic stage to which the SIS mixer was thermalised was far from optimum, and of approximately 6 K (the recommended temperature of the SIS mixer should be less than approximately 4.5 K). In particular, the noise temperature at 86 GHz was less than 70 K, which would be acceptable for preliminary testing of the SRT active surface, metrology system and evaluating the atmospheric noise contribution and system noise at the 600 m altitude of the SRT site. The receiver could also be used for millimetre VLBI experiments at 86 GHz.  
In future work, the receiver could be modified for improved thermal isolation of the third cryogenic stage of the cold head. The receiver noise would considerably improve (decrease) by reducing the physical noise of the third stage.  
The experience gained in this work proved to be useful for the recent project to upgrade SRT to allow observations at high radio frequencies. The specific details of this project are described in [27].  
Conceptualisation, A.L., A.N. and T.P.; methodology, A.L., P.O., A.S., M.P., A.N. and P.M.; software, A.S. and F.G.; validation, A.L., P.O., A.S., A.N. and F.G.; formal analysis, A.L., P.O., A.S. and A.N.; investigation, A.L., P.O., A.S. and A.N.; resources, M.P., P.M. and T.P.; data curation, A.L., L.S. and A.N.; writing--original draft preparation, L.S.; writing--review and editing, A.L., L.S., A.S., A.N. and T.P.; visualisation, A.L., L.S. and A.N.; supervision, A.L., A.N. and T.P.; project administration, A.N.; funding acquisition, T.P. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Conflicts of Interest: The authors declare no conflict of interest.