# THz Range Low-Noise SIS Receivers for Space and Ground-Based Radio Astronomy +
## 3 Experimental Results
### High Band (790-950 GHz) Mixers  
The SIS mixers for the waveguide receiver with an operation frequency up to 950 GHz were developed and tested [49; 50; 51]; these mixers are based on Nb/AlN/NbN twin tunnel junctions incorporated in a NbTiN/Al microstrip line and are intended for the Chinese radio observatory at Dome A, Antarctic, for Champ II+ (APEX observatory at Atacama site in Chile) and Brazilian LLAMA located in the Andes. The above technology makes it possible to achieve a junction current density of up to 30 kA/cm\({}^{2}\); providing a figure of merit (the ratio of the resistance of the sub-gap to normal) above 20. Using the twin SIS junction design, we have achieved a wide receiver operating range (675 to 950 GHz) and quite good noise temperature in this frequency range, as shown in Figure 11.  
The original CHAMP + high-frequency mixers used Nb/AlO\({}_{x}\)/Nb SIS technology; the high-frequency mixers were based on twin SIS junctions, as opposed to the single-junction design for the low frequency range; the twin junction circuit was inserted into a microstrip line with an NbTiN bottom and an aluminum top wire [22], similar to HIFI band 3 mixers [36]. The photos of the high band (790-950 GHz) array of 7-pixels, placed in a hexagonal configuration, are presented in Figure 12. Since the one-off design was created without further iteration, the performance of previous mixers was not optimized, leaving room for improvement. The noise temperature of the original mixers, measured before being installed in the receiver cryostat [46], is shown in Figure 13 by blue curves. This is corrected for noise caused by input window loss and beam splitter for LO injection. The data does not cover the entire range due to the limited range of the laboratory LO (based on BWO) available at the time. These characteristics were confirmed by recent laboratory measurements of the original center pixel mixer after it was removed from the telescope (green curve in Figure 13) [51]. Generally speaking, the noise temperature of the original mixers ranges from 300-400 K at low frequencies to double the value at high frequencies. Figure 11: DSB noise temperature for three SIS mixers as a function of frequency; noise temperature was corrected for losses in the input window and beam splitter. The dotted line shows the noise temperature, which corresponds to \(3\)_Jif_/\(k_{B}\). To upgrade high frequency receivers, we have developed mixers based on twin Nb/AlN/NbN SIS junctions embedded in the NbTiN/Al line [38; 49; 50; 51]. This technology allowed us to significantly improve the performance of the 7-pixel array for the CHAMP + instrument, mounted on the APEX (Atacama Pathfinder EXperiment) telescope, input  
Figure 12: 7-pixel array for high band (790–950 GHz) receiver CHAMP+ at radio telescope APEX (Atacama Pathfinder EXperiment): (**a**) Photo of the pixel mixer block elements, (**b**) 7-pixel mixer array with open horns, (**c**) Lens array at the top of the cartridge body. Figure 13: DSB mixer noise temperature for the entire 4–12 GHz IF band vs. LO frequency. The noise temperature is corrected for approximately 10% insertion loss of a beam splitter and the hot LO (300 K) contribution. The old mixers’ performances (blue curves), including the additional recent verification for the central pixel (green curve), are compared here with the results for the new mixers (red curves) [51]. frequency range 790-950 GHz, IF range 4-12 GHz. All upgraded mixers used the second minimum critical current when suppressed by the magnetic field; the DSB noise temperature for upgraded mixers is shown in Figure 13 with red curves [51]. From the data presented in Figure 13, we can conclude that the modernization of the mixers should improve the sensitivity of the CHAMP + high frequency range by 1.3-1.5 times [51]. In the case of good weather conditions, given an atmosphere transparency of approximately 50% [52], the mapping speed will be improved by approximately 40%.