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IEEE Transactions on Magnetics

Capability of a MM-wave mixer with Nb/Al-AlOx/Nb SIS junction

Takashi Kasuga, Masato Tsuboi, Junji Inatani, Hiroyuki Iwashita, Akira Sakamoto

Published Mar 1, 1987• 2 citations
Publisher

Abstract

Characteristics of a millimeter-wave mixer with Nb/Al-AlOx/Nb SIS junction were calculated along the 3-port mixer treatment. Signal and intermediate frequency dependences are obtained. The results show that the mixer has conversion gain between signal frequencies of 30 and 250 GHz. Intermediate frequency up to 10 GHz can be used in the USB mode.

DOI
10.1109/tmag.1987.1065123

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Oct 13, 1995•3 citations
Characteristics of a millimeter-wave mixer with Nb/Al-AlOx/Nb SIS junction were calculated along the 3-port mixer treatment. Signal and intermediate frequency dependences are obtained. The results show that the mixer has conversion gain between signal frequencies of 30 and 250 GHz. Intermediate frequency up to 10 GHz can be used in the USB mode.
We have made an all-refractory SIS mixer with four Nb/Al-AlOx/Nb junctions connected in series. The mixer is designed to operate at 80-120 GHz. The area of each junction is 2.5 × 2.5 μm2. Both these SIS junctions and a mm-wave choke filter are fabricated on a crystalline quartz substrate. These Nb/Al-AlOx/Nb junctions have shown ideal dc-characteristics and clear photon-assisted tunneling steps. RF measurements were made with a mixer mount which had two mechanical tuners. The conversion loss was less than 9 dB for 75-95 GHz. The minimum value was 8 dB. This value will be drastically improved by optimizing the normal resistance of the SIS junctions.
We study noise and conversion gain in the SIS mixers with Nb/MgO/NbN junctions at the millimeter and submillimeter wavelengths in order to estimate the possibility to extent the low noise operation towards the theoretical limit of 4/spl Delta//h=2.4 THz. We present as preliminary result the lowest achieved receiver noise with NbN SIS junctions of 65 K with conversion gain of -6 dB at 162 GHz and 230 K receiver noise with conversion gain of -10 dB at 306 GHz. The junction area is 2 /spl mu/m/sup 2/ and Josephson critical current density is 1.4 KA/cm/sup 2/. The optimum R/sub N//spl omega/C of NbN junction for mixers is estimated as 600 GHz/V. In preliminary tests the junctions R/sub N//spl omega/C is 10-20 time superior to the optimum and a significant improvement of NbN SIS mixers may be expected. The noise sources in the NbN SIS junctions are studied and a possible mixer sensitivity improvement is discussed. The thermal properties of the SIS NbN are studied. The frequency limit of the low loss integrated tuning structure in NbN is estimated as at least as 1.5 THz.
Heterodyne mixers based on superconducting SIS (superconductor-insulator-superconductor) tunnel junctions have been demonstrated to be the most sensitive coherent detectors at millimeter and submillimeter wavelengths. In fact, conventional superconducting SIS mixers with Nb/AlOx/Nb junction and Nb/SiO2/Nb tuning circuit have shown good performances with the noise temperature reaching as low as three times the quantum limit below 0.7THz, which is the gap frequency of Nb-based SIS junctions. However, due to the large loss in Nb thin-film superconducting microstrip lines, the noise performance of Nb SIS mixers deteriorates significantly above 0.7THz. With a gap frequency double that of Nb-based SIS junctions, NbN-based SIS junctions are of particular interest for the development of heterodyne mixers in the terahertz region. Considering the bandwidth and output power of local-oscillator (LO) signal sources are quite limited around 1THz, we firstly develop a waveguide NbN-based SIS mixer at 0.5THz. Three types of SIS junctions, i.e., long junction, parallel-connected tunnel junction (PCTJ) and distributed junction array (DJ) are investigated. They are all comprised of NbN-AlN-NbN tri-layer fabricated on an MgO substrate and have the same current density (Jc) of 10kA/cm2. In this paper, we describe their design, fabrication and preliminary experimental results.
An mm-wave mixing integrated circuit with a Josephson junction has been fabricated. An Nb–Te–Nb sandwich-type Josephson junction with no hysteresis on its I-V curve was used as the active element. Both RF and IF circuits were constructed in the form of superconducting microstriplines using an Si wafer as the dielectric layer. The circuit was tested at a signal frequency of 50 GHz, and the conversion efficiency η was measured at -7.44 dB, for a junction with a 0.42 mV I C R N product. The measured efficiency showed good ageement with the theoretical one calculated using the RSJ model and taking the impedance of the RF source into account.
The authors have tested the performance of planar SIS (superconductor-insulator-superconductor) mixers with log-periodic antennas at near millimeter and submillimeter wave frequencies from 90 to 360 GHz. The large omega R/sub N/C product ( approximately 10 at 90 GHz) of the Nb/NbO/sub x//Pb-In-Au junctions requires an integrated inductive tuning element to resonate the junction capacitance at the operating frequencies. Two types of integrated tuning element were used which were designed with the aid of measurements using a Fourier transform spectrometer. Preliminary results indicate that the tuning elements can give very good mixer performance up to at least 200 GHz. An inductive wire in parallel with a 5-junction array gives a minimum mixer noise temperature of 115 K (DSB) at 90 GHz with a FWHM (full width at half maximum) bandwidth of 8 GHz. An open-ended microstrip stub in parallel with a single junction gives minimum mixer noise temperatures at 150 and 200 K (DSB) near 90 and 180 GHz with FWHM bandwidths of 4 and 3 GHz, respectively. The relatively high mixer noise temperatures compared to those of waveguide SIS mixers in a similar frequency range are attributed mainly to the losses in the optical system.>
We present the noise properties of a mixer with the NbN-MgO-NbN quasiparticle tunnel junctions. Our work is based on experiment in the 120-180 GHz range with the NbN tunnel junctions in the mixer. Mixer printed circuit is totally made of NbN. The mixer operates at 5.4 K temperature unacceptable with Nb junctions. The minimum DSB receiver noise temperature is about 65 K at 162 GHz and approaches the Nb SIS mixer performance in mm band. It has been found that the noise sources in the NbN junctions are comparable to the Nb junctions and that the receiver noise with the NbN SIS mixer may be only few times more than the quantum limit of noise in the frequency range below the gap frequency. Output noise of the SIS mixer has been found constant in a wide frequency band and within an important range of the local oscillator amplitudes.