Measurement of Gain and Noise in SIS Quasiparticle Mixers.

Alexandre Karpov, B. Plathner, K. H. Gundlach +2 more

William R. McGrath, Antti V. Räisänen, P. L. Richards +2 more

We have constructed a 30-40 GHz test apparatus which allows us to measure the noise temperatures of SIS mixers with an accuracy of better than ±1K. This is a factor of six improvement over earlier measurements. The most accurate measurement made thus far of a mixer which uses a single Pb-alloy junction yielded Tm= 9.2 ± 0.9 K; and mixer gain: Gm= 0.240 ± 0.005. In addition, SIS mixers employing arrays of N = 1, 5, 10, 25, and 50 tunnel junctions in series have been tested. The input power required to saturate the array mixers was found to increase as N2and the gain and noise temperature of the array mixers were independent of N.

A. D. Smith, William R. McGrath, P. L. Richards +4 more

Quasiparticle mixers have shown strong quantum effects, conversion gain, and noise levels approaching the quantum limit, but only in tunnel junctions with very low sub-gap "leakage" conductance. It has been suggested that SIS' tunnel junctions, made from two different conductors with unequal gaps, will function as high gain mixers since the dynamic conductance below the gap is negative. We report results of the first SIS' mixing and direct detection experiments. At 36 GHz, a conversion efficiency of -4 dB with a noise temperature of 33 K (SSB) has been obtained. A novel gain mechanism for SIS' direct detectors is predicted. A direct detector responsivity of 250 A/W was measured. We compare our results to quantum theory models. In addition, we demonstrate quasiparticle harmonic mixing of a 36 GHz signal with an 18 GHz local oscillator in tin SIS junctions. Harmonic mixers can have important advantages at high microwave frequencies where insufficient local oscillator power is available.

Aldert van der Ziel

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.

U. Pisani, Franco Rosso, L. Brunetti