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Journal of Applied Physics

A millimeter wave Josephson mixer employing a high-T c GdBaCuO point contact

H. Olsson, William R. McGrath, T. Claeson, S. Eriksson, Lars Gunnar Johansson

Published Dec 15, 1987• 18 citations
Publisher

Abstract

We have investigated a Josephson effect heterodyne mixer for the millimeter wave band employing high-Tc GdBaCuO point contacts. Mixer performance was in qualitative agreement with theory. A mixing response was observed up to 55 K, the highest operating temperature achieved for such a device to date. The voltage separation of rf-induced steps gave a value of h/2e=2.08×10−15 V s which is in excellent agreement with the value expected for Cooper pairs. In addition, the temperature dependence of the I0R product was found to agree with Bardeen–Cooper–Schrieffer theory in the weak coupling limit.

DOI
10.1063/1.339003

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A millimeter band mixer employing Nb--Nb superconducting point contacts at 4.2/sup 0/K with an external heterodyne was investigated experimentally. It is shown that the mixing model needs to be somewhat more complex. The distortions of the IF signal, the effect of the IF currents on the I-V characteristic of the contacts, the dynamic characteristics, and cases of deviation from the mixing model are examined. The best contacts have an NEP approximately 0.5 x 10/sup -15/ W/Hz and conversion losses of approximately 30 db.

•Apr 1, 1976•0 citations
We have investigated a Josephson effect heterodyne mixer for the millimeter wave band employing high-Tc GdBaCuO point contacts. Mixer performance was in qualitative agreement with theory. A mixing response was observed up to 55 K, the highest operating temperature achieved for such a device to date. The voltage separation of rf-induced steps gave a value of h/2e=2.08×10−15 V s which is in excellent agreement with the value expected for Cooper pairs. In addition, the temperature dependence of the I0R product was found to agree with Bardeen–Cooper–Schrieffer theory in the weak coupling limit.
Adjustable niobium Josephson point contacts, mounted in fundamental-mode waveguide cavities, have been used as mixers in an experimental heterodyne receiver with a 450 GHz laser as the local oscillator. It was possible to estimate separately the various factors, such as intrinsic mixer conversion efficiency, mixer noise, and input coupling efficiency that contribute to the overall receiver sensitivity. Mixer performance agreed reasonably well with qualitative predictions of resistively-shunted-junction theoretical models, and there was broad quantitative agreement for some aspects of performance. Spurious, and potentially very misleading non-heterodyne responses to thermal signals were observed under some regimes of applied DC bias and local oscillator power. The lowest overall receiver noise temperature measured was 2100K (double side band), although reduction to 1000K should be possible with quite simple improvements. The intrinsic mixer noise temperatures were usually in the range 500-1000K (single side band), and local oscillator power required by the receiver was usually less than one microwatt. This performance makes these mixers competitive as the basis of very sensitive near-millimetre-wave receivers.
Experiments are reported in which two millimeter-wave signals incident on point-contact Josephson junctions produced changes in the junction dc voltage versus current characteristic and an intermediate frequency output whose amplitude depended sensitively on both junction bias and applied power. Equations are derived, based on Josephson's phenomenological equations, for the Josephson current in a junction exposed to two applied rf signals. When the applied signals differ appreciably in frequency, additional constant-voltage steps in the $V\ensuremath{-}I$ curve are predicted which are spaced at the difference frequency. These steps have been observed in experiments employing sources at 64 and 72 Gc/sec. Results of mixing experiments utilizing two sources nearly equal in frequency are reported at 23 and at 72 Gc/sec. In this case the two waves beat together and are equivalent in their effect to a single signal amplitude modulated at the difference frequency. Also explained on the same basis are experiments in which the third harmonic of a signal at 24 Gc/sec mixed with a signal at 72 Gc/sec. These results demonstrate the existence of the Josephson mixing mechanism as opposed to classical nonlinear mixing, and they show that it is operative at microwave and millimeter-wave frequencies over a wide range of power.
We have recently shown that point contact Josephson junctions can function as millimeter wave heterodyne mixers with conversion gain. The best results achieved thus far show a single sideband conversion gain of 1.3 and a mixer contribution to the system noise temperature of 54K. Both of these results are ∼ 5 times better than the best published figures for cooled Schottky barrier diode mixers operated at the same frequency. The measured noise for a variety of junctions can be expressed as a universal function of the normalized rf frequency \Omega = \hbar \omega/2eI_{c}R . It is about a factor 2 larger than the calculated noise arising from the thermal noise in the junction shunt resistance R. The noise calculation was done for the resistively shunted junction model using an analog junction simulator.
Heterodyne detection with a point-contact Josephson junction has been investigated both experimentally and theoretically. The measured performance of the device at 36 GHz is in good agreement with the theory. By operating vanadium point contacts at 1.4 K, we have achieved a single-sideband (SSB) mixer noise temperature of 54K with a conversion gain of 1.35 and a signal bandwidth on the order of 1 GHz. From our results we can extrapolate a potentially impressive performance for these devices at submillimeter wavelengths.
We have recently shown that point contact Josephson junctions can function as millimeter wave heterodyne mixers with conversion gain. The best results achieved thus far show a single sideband conversion gain of 1.3 and a mixer contribution to the system noise temperature of 54K. Both of these results are ∼ 5 times better than the best published figures for cooled Schottky barrier diode mixers operated at the same frequency. The measured noise for a variety of junctions can be expressed as a universal function of the normalized rf frequency\Omega = \hbar \omega/2eI_{c}R. It is about a factor 2 larger than the calculated noise arising from the thermal noise in the junction shunt resistance R. The noise calculation was done for the resistively shunted junction model using an analog junction simulator.
A millimeter band mixer employing Nb--Nb superconducting point contacts at 4.2/sup 0/K with an external heterodyne was investigated experimentally. It is shown that the mixing model needs to be somewhat more complex. The distortions of the IF signal, the effect of the IF currents on the I-V characteristic of the contacts, the dynamic characteristics, and cases of deviation from the mixing model are examined. The best contacts have an NEP approximately 0.5 x 10/sup -15/ W/Hz and conversion losses of approximately 30 db.