# THz Range Low-Noise SIS Receivers for Space and Ground-Based Radio Astronomy +
## 3 Experimental Results  
### 211-275 GHz SIS-Mixer Measurements  
To evaluate a wideband radiation matching of the SIS mixer at sub-THz frequencies, the Michelson Fourier transform spectrometer (FTS) technique was used [43; 44]. A wideband sub-THz glow bar source (1500 K black body) was matched with the FTS, which was loaded by the SIS mixer as a detector. The mixer was voltage biased at 2.5 mV; a direct current response was measured versus a mirror position. These data were Fourier-transformed into a mixer response on the frequency. The experimental data demonstrate a good agreement with the results of numerical simulations [48; 49].  
The DSB mixer noise temperature was determined by the standard Y-factor measurement method; an absorber at room temperature (295 K) was used as a \"hot\" load, and an absorber cooled by liquid nitrogen (78 K) was used as a \"cold\" load [48; 49]. Figure 9 shows the SIS receiver output versus the bias voltage measured for 241 GHz LO at 6.5 GHz IF (60 MHz IF filter bandwidth). The Y-factor was determined by subtracting the responses measured for hot and cold loads. The value of the Y-factor at the best point exceeds 5.0 dB, which corresponds to a receiver noise temperature of approximately 22 K. The values of the noise temperature \(T_{n}\) were obtained without any corrections for losses in the beam splitter and the cryostat window [49]; they were only twice the value of \(\textit{l}\textit{/}\textit{}\textit{}\textit{}\textit{}\textit{}\textit{} \textit{}\textit{}\textit{}\textit{}\textit{}\textit{}\textit{}\textit{}\)\({}_{B}\) in the frequency range from 240 to 275 GHz (see Figure 10); the obtained values meet the technical requirements  
Figure 8: Layout (**left**) and SEM photo (**right**) of the CHAMP+ high band SIS mixer. The central part of the chip (including SIS twin junction, transformer, and probe) is magnified.  
for the 211-275 GHz receiver of the Millimetron space radio telescope. It should be noted that the receiver linearity was confirmed by direct measurements at most frequencies in the tuning range, except for two points around 260 GHz; these points were excluded from Figure 10.  
A detailed description of mixer noise breakdown was discussed in Ref. [49]; the estimates of the contributions of the device components can be listed as follows: (a) 6 micron mylar beam splitter--0.6 K; (b) receiver input window--0.5 K; (c) IF circuit (which is a combination of a Pamtech 4-12 GHz cryogenic isolator and a cryogenic amplifier)--6 K. Note that the quantum limit for the DSB SIS mixer element is _lf_/2\(k_{B}\)[6]; which corresponds to 5.8 K at 250 GHz. The receiver stability at a 241.2 GHz LO frequency was evaluated by recording the receiver output vs. time at the same operating conditions as for noise  
Figure 10: DSB receiver noise temperature of the SIS receiver on the local oscillator (LO) frequency; the experimental data are presented without any corrections for losses in the input optic elements. Measurement uncertainty is approximately 2 K, shown by error bars. The dash-dotted line is given as a guide for the eyes.  
Figure 9: The output signal of the SIS receiver at intermediate frequency (IF) 6.5 GHz on the SIS bias voltage, measured for the cold and hot input loads (78 K—blue curve and 295 K—red curve, correspondingly) at the local-oscillator frequency 241 GHz.  
temperature measurements. The hot load was presented in the form of the receiver. A 60 MHz bandpass filter was used to limit the signal bandwidth. An Allan time up to 10 s was registered [49] determined by the stability of the set-up components and is a typical value for all SIS mixers evaluated in this setting.