Some recent developments in the design of SIS mixers

P. L. Richards

A summary is presented of recent SIS quasiparticle heterodyne mixer work done at Berkeley. Developments related to millimeter wavelength waveguide mixers include techniques for accurate measurements of noise and gain, integrated RF matching elements for large instantaneous bandwidths, large scaled modeling experiments, new W-band mixers with wide tuning bandwidth and only one mechanical adjustment, observation of very large coupled gain, systematic studies of the properties of array mixers, analysis of the effect of saturation on mixer measurements, very low noise in mixers with high quality junctions, and computer modeling to test the quantum theory of mixing. Developments related to planar lithographed mixers with quasi-optical antennas include construction of test apparatus, large scale modeling, and the design and evaluation of mixers which use RF matching structures integrated with bow tie antennas.

T. C. L. G. Sollner

S. C. Shi, Junji Inatani, Takashi Noguchi +1 more

J. Mees

Since several years waveguide and planar antenna SIS-mixers for space applications are developed at SRON in orderof ESA. Recent results show excellent mixer performance up to frequencies above the superconducting gap of Nb.Goal is the development of a 1THz-mixer. Best results show a receiver noise temperature of Taw=400K at 720GHz incase of the waveguide mixer and T„„=200K at 490GHz for the double dipole planar antenna mixer. We will presentthe status in the development of SIS-junctions, integrated tuning network, mixers and mounts.

U. Pisani, Franco Rosso, L. Brunetti

Jonas Stasys Zmuidzinas, Henry George LeDuc, Jeff A. Stern

We are developing improved versions of a slot antenna SIS mixer which we have previously described. The initial work demonstrated a double sideband noise temperature of 420 K for a 500 GHz quasi-optical SIS mixer employing a twin-slot antenna on a quartz dielectric substrate. A quartz hyperhemispherical lens is used to focus the incoming radiation onto the twin-slot antenna. The advantages of a twin-slot antenna include a low impedance (35 omega) and a clean, symmetric beam pattern into the dielectric with a 70 percent efficiency. In our original mixer, the radiation was coupled from the two slot antennas onto superconducting microstrip lines which fed the SIS junction. By performing an impedance transformation using tapered lines and by feeding the radiation from the two slots to the junction in parallel, the effective (real) impedance seen by the junction was reduced to just 4 omega. This very low impedance allowed a junction area of 2.3 sq micron to be used at 500 GHz, which was manufactured using optical lithography. However, no attempt was made to tune out the junction capacitance. We estimate that this capacitance reduces the impedance coupling efficiency to eta(sub Z) approx. equals 0.23, for our junction with omega R(sub N) C = 5.3 at 500 GHz. The recent development of techniques using electron-beam lithography to manufacture junctions with very small areas (approx. equals 0.1 sq microns) now allows considerably more flexibility in the design of SIS mixer circuits. We have redesigned the slot-antenna mixer to take advantage of this possibility. In particular, we have included a novel circuit which allows the junction capacitance to be tune out over a broad bandwidth. For instance, mixers designed for 800 GHz using NbN/MgO/NbN junctions with realistic parameters achieve a 3 dB impedance bandwidth of nearly 400 GHz. Furthermore, our circuit uses only short lengths of microstrip and should be less sensitive to RF losses than other designs. The improved impedance match should give a large reduction in noise temperature as compared to our previous mixer. The new devices are currently under fabrication. Further details of the design and any available experimental results are discussed.