Infinite available gain in a 115 GHz SIS mixer

Dag Winkler, A. Hodge Worsham, N. G. Ugras +3 more

Aldert van der Ziel

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.

Michael J. Wengler, Noshir Dubash, G. Pance +1 more

Superconducting tunnel diode (SIS) mixers are used for radio astronomy from 100 to 500 GHz. They are being considered for NASA spacebome astronomy at frequencies near 1,000 GHz. We report measurements of gain and noise in SIS mixers at 230 and 492 GHz. We measure high gain and noise associated with Josephson currents which have not been previously reported. These measurements show that Josephson currents are increasingly important as operating frequencies are raised. We discuss the techniques we use to make these measurements. Measurements made with hot and cold blackbodies are shown to be inaccurate at high frequencies. The problem is that SIS 's do not always respond linearly to the signal power incident on them. This is particularly important when 1) very broad band mixers are used and 2) Josephson effect currents are important. Both of these circumstances are present in the quasioptical SIS mixers favored for 500 GHz and higher. We use monochromatic signals to measure gain and noise to get around these problems.

Yuto Kozuki, H Ishida, Yohei Hasegawa +14 more

William R. McGrath, P. L. Richards, A. D. Smith +2 more

We have measured the properties of a superconductor-insulator-superconductor (SIS) quasiparticle mixer which is operated in the quantum limit. Single sideband conversion gain larger than +4 dB was observed at 36 GHz with a mixer noise temperature T=9 + or - 6K, which is to be compared with the (Planck) quantum limit h-bar omega/(kln2) approx. 2.5K. Complete 3-port mixer calculations are presented which are in good agreement with the gain measurements. Negative resistance was observed which implies that arbitrarily large gain is available.