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IEEE Transactions on Applied Superconductivity

An Nb-based waveguide SIS distributed mixer employing coplanar inductor loaded microstrip transformer for the 800 GHz frequency band

Hiroyuki Maezawa, Edward Tong, Takashi Noguchi, T. Matsunaga, R. Blundell, Shing-Kuo Pan

Published Jun 1, 2003• 0 citations
Publisher

Abstract

We present a novel superconducting waveguide mixer for 800 GHz. In this mixer, a half-wave Nb/Al-AlO/sub x//Nb superconducting-insulator-superconductor (SIS) resonator is integrated with a microstrip superconducting transmission line loaded with coplanar inductors. This hybrid coplanar/microstrip transmission line acts as an impedance matching section between the distributed junction and the waveguide feed point of the mixer mount. We have designed mixers with a simple model which predicts that the insertion loss of the matching network is less than 10 percent up to 840 GHz. Preliminary experimental results show that the resonator can work up to 800 GHz but the parameters of the coplanar inductor loaded microstrip line need to be refined to improve power coupling to the resonator.

DOI
10.1109/tasc.2003.813991

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We present a novel superconducting waveguide mixer for 800 GHz. In this mixer, a half-wave Nb/Al-AlO/sub x//Nb superconducting-insulator-superconductor (SIS) resonator is integrated with a microstrip superconducting transmission line loaded with coplanar inductors. This hybrid coplanar/microstrip transmission line acts as an impedance matching section between the distributed junction and the waveguide feed point of the mixer mount. We have designed mixers with a simple model which predicts that the insertion loss of the matching network is less than 10 percent up to 840 GHz. Preliminary experimental results show that the resonator can work up to 800 GHz but the parameters of the coplanar inductor loaded microstrip line need to be refined to improve power coupling to the resonator.
A Superconductor-Insulator-Superconductor (SIS) distributed mixer with no integrated tuning circuit has been designed for use in a fixed-tuned waveguide mixer block. This mixer employs two half-wave SIS resonators connected in series across the feed point of the waveguide circuit. By selecting appropriate dimensions for the resonators, we can obtain proper impedance matching over a reasonable input bandwidth. Since there is no lossy thin film microstrip line in front of the SIS junction, efficient power transfer from the waveguide feed to the mixer element can be achieved. A receiver noise temperature of 185 K at 650 GHz has been measured with a dual 0.33/spl times/16.8 /spl mu/m SIS resonator. The noise temperature remains below 300 K over an input bandwidth of about 10%. This design can be used at much higher frequency where the loss of tuning circuit becomes significant.
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The performance of a broadband SIS (superconductor-insulator-superconductor) receiver with no mechanical tuning elements was modeled and tested. The mixer mount had a broadband-waveguide-to-microstripline transition consisting of a four-step Chebyshev single-ridge transformer. The last step of the ridge connected the waveguide to a microstripline circuit. The on-chip circuit consisted of a microstripline that transmitted the RF and the local oscillator signals to the SIS mixer. Both the SIS element and the inductor had RF grounds provided by 90 degrees radial stubs. The inductor tuned out the junction capacitance to allow operation over the full frequency band. The SIS element can be a single junction or series array using Nb-AlO/sub x/-Nb trilayer tunnel junctions with areas as small as 0.5 mu m/sup 2/ and V/sub m/(2 mV)=39 mV. Preliminary results indicated a DSB receiver noise temperature of 65-80 K across the band measured at 4.4 K with an internal cryogenic RF hot/cold source and with T/sub if/=21 K. With another device, a mixer noise temperature of 35 K at 100 GHz was achieved, increasing to 45 K at 79.5 and 110 GHz. Coupled mixer gain of up to +3 dB and negative dynamic resistance on the first photon step were observed. The lowest noise temperature was obtained for an untuned single-junction mixer at 80 GHz: T/sub R/=41 K and T/sub M/=20 K were measured.
The mixer mount for a 75 -110 GHz SIS (superconductor-insulator-superconductor) receiver was scale modeled at 2 - 8 GHz . The mixer mount had a 4-step Chebychev single ridge transformer to launch the rf signal from the waveguide to a microstripline circuit. The microstripline circuit coupled the rf signal into the on-chip SIS element. A wire inductor in parallel with the mixer was used to tune out the capacitance of the SIS junction. Both the junction and the inductor had rf grounds provided by 90° radial stubs. In the scale model, a 50 Ω chip resistor in parallel with a 1 pF chip capacitor was used in place of the SIS junction to measure the rf matching properties. The inductor was modeled with a thin copper wire. The VSWR of the entire mount was less than 2.1 : 1 over the frequency band corresponding to 75 - 107 GHz. Receiver measurements with the actual mixer mount employing trilayer SIS tunnel junctions gave similar trends across the band.
In this paper, we present the experimental results of a waveguide superconductor-insulator-superconductor (SIS) mixer with an inhomogeneous distributed junction array. The mixer consists of five Nb-AlO/sub x/-Nb SIS junctions with different dimensions distributed on a superconducting microstrip. The critical current density of the fabricated junctions was 3100 A/cm/sup 2/. Detailed measurements of the receiver noise temperature have been made in the frequency range from 190 to 284 GHz at 4.2 K. A double-sideband (DSB) receiver noise temperature of 5 hw/k was obtained in the frequency range from 202 to 284 GHz and the lowest DSB receiver noise temperature of 46 K was obtained at 210 GHz.
We have successfully demonstrated a low-noise 0.78–0.95-THz waveguide mixer based on Nb/AlO x /Nb superconductor–insulator–superconductor (SIS) junctions integrated with an NbTiN/SiO 2 /Al superconducting inverted microstrip. The mixer employs a twin-junction tuning circuit and an impedance transformer which are designed to achieve both low noise and wideband operation. Corrected for optical input losses in front of the mixer, the double-sideband (DSB) noise temperature is below 120 K (<3 hf / k B ) around the local oscillator (LO) frequency of 0.88 THz and at an operating temperature of 2 K. The results represent state-of-the-art performance for mixers in the terahertz frequency range.