# THz Range Low-Noise SIS Receivers for Space and Ground-Based Radio Astronomy +
## 2 Materials and Methods
### Mixer and Mixer Block Designs  
In the design of the mixer block for the frequency range 211-275 GHz, we follow a successful modular construction approach developed for the ALMA Band 9 receiver [24]. The mixer block (see Figure 4) consists of a few separate elements: (1) an input horn; (2) a central part with the waveguide; (3) a back piece (BP) unit where the mixer chip is installed; and (4) a magnet block unit with two magnet pins to suppress the Josephson critical current of the SIS mixer. The receiving chip (width 150 \(\upmu\)m), is located in a rectangular 1000 \(\times\) 500 \(\upmu\)m waveguide at a distance of 230 \(\upmu\)m from the back short in the waveguide, orthogonal to the propagation direction (Figure 5). The Nb/AlO\({}_{\rm x}\)/Nb SIS junction is placed into a planar Nb/SiO\({}_{2}\)/Nb tuning structure made on a 125 \(\upmu\)m thick quartz substrate. To prevent leakage of the high frequency input signal through this dielectrically filled waveguide, blocking low-pass filters were used. Figure 4: A 3D model of the mixer block for the frequency range 211–275 GHz; all main parts are shown: **(1)** input horn, **(2)** central part with the waveguide, **(3)** back piece with mixer chip, **(4)** magnet block unit. Figure 3: Dependence of the tunnel barrier parameters on resistivity \(R_{n}A\): **(a)** barrier height \(\varphi\); **(b)** barrier thickness \(d\). Data for the Nb/AlO\({}_{\rm x}\)/Nb junctions are shown by squares and for the Nb/AlN/Nb junctions by asterisks; the dotted and dashed lines are linear approximations for appropriate data. To achieve a good SIS mixer performance, the tunnel junction should be matched properly with the input waveguide impedance at high frequencies and with a 50 Ohm amplifier impedance in the whole IF frequency band. To match the waveguide impedance of approximately 400 Ohm and the high frequency SIS impedance, a waveguide probe and a tuning structure were used. A combination of the coplanar (CPW) and microstrip (MSL) lines was used to tune out the intrinsic SIS capacitance and to provide the matching of the resulting SIS impedance to the waveguide at high frequencies (see Figure 6). This approach allowed us to make the tuning structures more compact. As the wavelengths in the tuning lines in the range of IF are much larger than the dimensions of the structure, the tuning microstrip lines add some capacitance to the sample impedance on the IF. To adjust the output impedance of the receiver, a matching circuit should be implemented at the mixer output. In this work, we used a PCB with a 50 Ohm line without any additional tuning elements (see Figure 7). Figure 5: A 3D model for a double-side mixer placed inside a waveguide channel. The input horn is placed in the front side of the waveguide mixer block (Waveguide in), and the back short position is marked in a back view. Figure 6: Photo of the SIS mixer structure for the frequency range 211–275 GHz. The SIS element is inserted in the planar structure formed by segments of coplanar and microstrip Nb-SiO\({}_{2}\)-Nb lines. The waveguide probe is shown on the right. Figure 7: A 3D model of the back piece. A PCB plate, the SIS mixer element and waveguide are depicted. To make a wideband receiver for the 790-950 GHz frequency range for CHAMP+, we used twin SIS junctions [46; 47] (each with an area of 0.5 \(\upmu\)m\({}^{2}\), see Figure 8), coupled by a waveguide probe to the E-field of a rectangular waveguide of 300 \(\times\) 75 \(\upmu\)m. The SIS mixer [48; 49] was based on high critical current density Nb/AlN/NbN tunnel junctions included in the NbTiN-Al microstrip line [39]. The microstrip electrodes (320 nm thick bottom NbTiN electrode and a 500 nm thick top electrode made of Al) were separated by a 250 nm SiO\({}_{2}\) insulator. The detailed procedure for making the circuit is described in the previous subsection.