## 5 Results and Discussion
### Measurements of the Noise Temperature of the System  
When a radio telescope detects a signal, the noise from the receiver is added to it. The most important requirement to be met in a receiver design is the minimisation of the self-produced thermal noise, known as the equivalent noise temperature of the system. A low noise temperature implies the high sensitivity of the receiver [48]. As described in Section 2, a two-load calibration subsystem is included in the original version of the ex-PdBI receiver, which is useful for the calibration when it is installed on the telescope, allowing the measurement of the receiver noise. In our laboratory, we performed noise temperature measurements at several frequencies using the cold and ambient load (Y factor) method [49], without using the internal cryogenic calibration load. Some photos of the experiment are shown in Figure 13. In particular, the absorbing panel (i.e., Emerson & Cuming AN72 material) of the calibration system was used as the room temperature load (see Figure 13a). The cold load was an absorbing panel immersed in a container filled with liquid nitrogen in such a way that the load temperature was lowered to the one of the liquid nitrogen temperature, 77 K (see Figure 13b). The room temperature and cold loads were alternately placed in front of the HDPE vacuum window (and, consequently, in front of the feed of the receiver, as shown in Figure 13c). A power meter was used as back end and the noise temperature was calculated using the following formulas [49]:  
\[Y=\frac{P_{hot}}{P_{cold}} \tag{2}\]  
\[T_{rec}=\frac{T_{hot}-Y\cdot T_{cold}}{Y-1} \tag{3}\]  
where \(Y\) represents the ratio between the measured power level \(P_{hot}\) when the hot load is positioned in front of the receiver, and the measured power level \(P_{cold}\) when the cold load is considered. The noise temperature of the receiver \(T_{rec}\) is calculated using formula (3), where for the radiated temperature \(T_{hot}\) we assumed the room temperature (approximately 293 K) and for \(T_{cold}\) the nitrogen temperature of the cold load (approximately 77 K). Table 2 summarises the results of the experiment in the frequency range between 84 and 115 GHz. The measured receiver noise temperature varies considerably as a function of the LO frequency, as shown in the plot of Figure 14. The noise has a minimum of approximately 66 K around 86 GHz RF and increases to approximately 290 K at 105 GHz RF. The measured noise is considerably higher than the one delivered by the original PdBI  
Figure 13: (**a**) Room temperature load placed in front of the HDPE system and the feed of the receiver. (**b**) Absorbing panel immersed in a container filled with liquid nitrogen. (**c**) Cold load placed in front of the HDPE system and the feed of the receiver. receiver (where noise of order \(\mathrm{Trec}\approx 30\) K was reported), which was based on a third cryogenic stage cryogenically cooled by liquid helium at 4.2 K, allowing the thermal cooling of the SIS mixer to less than half of the critical temperature of the niobium superconductor (with \(\mathrm{Tc}\approx 9.2\) K). The discrepancy between the receiver noise temperatures measured by IRAM in the original receiver configuration and the one measured at INAF with the modified receiver are mainly due to the different physical temperatures achieved for the third stage (4 K stage), where the mixer is thermalised. The receiver noise is expected to degrade considerably if the physical temperature of the SIS mixer is increased beyond \(\approx\)4.5 K. A cryogenic temperature of approximately 6.6 K (much greater than the required \(\approx\)4 K value) was achieved during the laboratory tests, in part due to the limited thermal cooling power delivered by the ARS cold head, and in part due to the non-optimal thermal isolation of the coldest cryogenic stage, which requires improvement. This aspect caused a degradation of the receiver noise performance.