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A mixer employing superconducting point contacts

A. I. Akimenko, В. С. Соловьев, I. K. Ianson

Published Apr 1, 1976• 0 citations
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Abstract

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.

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Journal of Applied Physics•Dec 15, 1987•18 citations
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.
The Josephson frequency conversion in the millimeter and submillimeter wave region is studied with superconducting Nb-Nb point contact junctions at 4.2 K, using the contact both as the local oscillator and the mixer with intermediate frequencies of 0.1 and 9.2 GHz. Frequency conversion signals are only detected in the latter case with a microwave receiver at 9.2 GHz in a range of Josephson frequencies between 145 and 1 600 GHz. The experiments with intermediate frequency fi of 0.1 GHz indicate for this small value of f i that the observed signals are not related to frequency conversion but noise effects. An analysis of the frequency conversion mechanism in a Josephson junction operated as an oscillator-mixer is proposed which includes the noise effects and also the feedback effects induced in a junction driven by a constant current source. We have then investigated the noise properties up to 450 GHz of superconducting point-contacts using the junction as an oscillator-mixer where the Josephson current is mixed with harmonics of an externally applied signal. The emitted intermediate frequency is detected at 9.2 GHz and its linewidth is measured. In the very high frequency range the dc quasiparticle current dominates in a Josephson weak-link and we compare the corresponding theoretical linewidth calculated by Scalapino with our experimental results.
Properties of a niobium-niobium Josephson superconducting point contact (SPC) used as a mixer-multiplier have been investigated experimentally. Conversion of a microwave signal with frequency fs= 127 GHz into a signal with the intermediate frequency fif= 8.6 GHz has been obtained using heterodyne generators with frequencies fh= 136.68, 45, and 6.8 GHz, so that fif= nfh1,2,3 ....20- fswhere n = 1.2, 3.20 are heterodyne harmonics produced in the SPC. Dependences of the conversion efficiency (ηn) on the bias voltage, the heterodyne power and the heterodyne harmonics have been measured. In the case of the second and the twentieth harmonics, the best value of η20/η2was 0.25. This value is essentially higher than that obtained for the usual (conventional) semiconducting mixer. Performed experiments have demonstrated the perspective of utilization of SPC as a mixer-multiplier in millimeter and submillimeter wavebands.
Microwave superconducting mixers operating at 10–13 GHz utilizing Josephson junctions have been investigated analytically and experimentally. Measurements on mixer sensitivity were carried out at 13 GHz using waveguide-mounted experimental point-contact Josephson junctions at 4.2 K. Results showed sensitivities of −90 dBm/MHz (10−18 W/Hz) which included mismatch losses encountered in the waveguide structure. The Josephson junction mixing mechanism was analyzed on the basis of small-signal modulation of the staircase like dc I-V characteristics. It was shown that a small-signal microwave excitation results in an ac lateral shift of the regions between steps, such that: (a) Optimum Josephson mixing occurs halfway between steps. (b) Mixing occurs between all steps with maximum conversion efficiency between the zero and first step. (c) Predicted low conversion loss and possibly internal gain for a matched mixer. The analytical model was simulated by a digital computer and the behavior of the Josephson mixer predicted for various conditions of excitation. The model chosen for the analysis and the computer simulation is based on the simplest assumptions concerning bulk-formed Josephson junctions. In the laboratory an external local oscillator set 60 MHz below the signal frequency was used to induce the well-known step structure in the dc I-V characteristic of the point-contact Josephson junction. Mixing of the Lo and signal was found to occur between constant voltage steps in agreement with both analysis and computer simulation. The correctness of the model is demonstrated by the agreement between the theory and experiment.
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.