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A LOW NOISE 410-495 GHZ Nb/AI,OJNb SIS WAVEG

Gert J. de Lange, Junction Fabri

Published Jan 1, 1993• 0 citations
OpenAlex

Abstract

The noise and gain of a heterodyne waveguide mixer employing Nb/A120JNb superconducting tunnel junc- tions is measured in the 400-500 GHz frequency range. Three different arrays of two junctions in series are analyzed. An array with integrated tuning stubs has lowest receiver noise temperatures of T, = 120 K DSB at 480 GHz, which is to our knowledge the lowest value reported in this frequency range. We compare data of the pumped I-V curves with the Wert- hamer-Tucker theory and demonstrate for the first time an excellent agreement at frequencies up to 500 GHz. For an array without integrated tuning stubs, a mixer noise tempe- rature T- = 90 f 30 K and a DSB mixer gain G-= -12.5 f 0.6 dB is measured. A comparison of the measured gain versus bias voltage with the quantum theory of mixing shows a good qualitative agreement, indicating the applicability of this theory, to Nb tunnel junctions up to 500 GHz. The noise temperature of an array with a lower gap voltage, is 220 K at 495 GHz. This frequency is 85 % of the reduced gap frequen- cy, indicating that Nb SIS mixers can be used up to at least the gap frequency of 680 GHz. I. INTRODUCTION

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IEEE Transactions on Applied Superconductivity•Jun 1, 1997•21 citations
The noise and gain of a heterodyne waveguide mixer employing Nb/A120JNb superconducting tunnel junc- tions is measured in the 400-500 GHz frequency range. Three different arrays of two junctions in series are analyzed. An array with integrated tuning stubs has lowest receiver noise temperatures of T, = 120 K DSB at 480 GHz, which is to our knowledge the lowest value reported in this frequency range. We compare data of the pumped I-V curves with the Wert- hamer-Tucker theory and demonstrate for the first time an excellent agreement at frequencies up to 500 GHz. For an array without integrated tuning stubs, a mixer noise tempe- rature T- = 90 f 30 K and a DSB mixer gain G-= -12.5 f 0.6 dB is measured. A comparison of the measured gain versus bias voltage with the quantum theory of mixing shows a good qualitative agreement, indicating the applicability of this theory, to Nb tunnel junctions up to 500 GHz. The noise temperature of an array with a lower gap voltage, is 220 K at 495 GHz. This frequency is 85 % of the reduced gap frequen- cy, indicating that Nb SIS mixers can be used up to at least the gap frequency of 680 GHz. I. INTRODUCTION
The noise and gain of a heterodyne waveguide mixer using Nb/Al/sub 2/O/sub 3//Nb superconducting tunnel junctions were measured in the 400-500-GHz frequency range. Three different arrays of two junctions in series are analyzed. The minimum receiver noise temperature is 120 K DSB at 480 GHz, measured with an array having integrated tuning stubs. The authors compare data of the pumped I-V curves with the Werthammer-Tucker theory and demonstrate an excellent agreement at frequencies up to 500 GHz. For an array without integrated tuning stubs. a mixer noise temperature of 90+or-30 K and a DSB mixer gain of -12.5+or-0.6 dB were measured. A comparison of the measured gain versus bias voltage with the quantum theory of mixing shows good qualitative agreement, indicating the applicability of this theory to Nb tunnel junctions up to 500 GHz. The noise temperature of an array with a lower gap voltage is 220 K at 495 GHz. This frequency is 85% of the reduced gap frequency, indicating that Nb superconductor insulator-superconductor (SIS) mixers can be used up at least the gap frequency of 680 GHz.>
The noise and gain of a heterodyne waveguide mixer employing Nb/Al2O3/Nb superconducting tunnel junctions with an on-chip integrated tuning element are measured and analyzed at 680–750 GHz and at 840 GHz. The lowest receiver noise temperatures are 400 K (double side band) at 720 GHz and 1500 K (3000 K including the beam splitter loss) at 840 GHz. We compare data of the pumped I–V curves with the quantum theory of mixing and demonstrate good agreement at frequencies well above the gap frequency.
The authors have made a detailed study of the gain and noise of a SIS (superconductor-insulator-superconductor) heterodyne receiver at 345 GHz. An array of two Nb-Al/sub 2/O/sub 3/-Nb SIS junctions in series are used as the mixing element. The array is operated in a waveguide mount with a backshort and an E-plane tuner. The best receiver noise temperature achieved is 140 K DSB (double sideband). The embedding impedances were determined by fitting theory to the measured pumped curves. High-quality fits were obtained, providing the first detailed test of the Tucker-theory at frequencies above 300 GHz. The impedances found by this method are in very good agreement with impedances measured in a scale model at 3.3 GHz. From these embedding impedances, the gain and noise of the mixer were calculated over a full bias range using the Tucker theory in the three-port low-IF approximation. The measured dependence of mixer gain and noise on bias voltage, pump power and embedding impedance is in good agreement with theory. However the absolute values show discrepancies that appear to be independent of the bias parameters of the mixer.>
We report on low-noise heterodyne mixing with NbN/A1N/NbN tunnel junctions in the submillimeter-wave region for the first time. The receiver consists of the quasi-optical NbN SIS mixer with an integrated tuning circuit in which a radial short stub tuner is incorporated to resonate out the junction capacitance for RF matching and a balanced IF circuit for broadband IF matching. The prepared NbN SIS junction has a current density of 20 kA/cm 2 and is about 1 I1111 in diameter, supplying a small coC JRN product (o)CJRN = 3 at 300 GHz). The junctions showed good dc I-V characteristics, excellent submillimeter-wave responses and sensitive heterodyne mixing responses. From 254 to 350 GHz the average receiver noise temperature measured using by the standard Y-factor method was about 250 K (DSB) at 5 K. The lowest receiver noise temperature, 200 K (DSB), was obtained at around 303 GHz. Comparing NbN/A1N/NbN and Nb/A10x/Nb tunnel junction performance with the same tuning circuit showed that the frequency dependence of the receiver noise temperature agreed well in the two receivers. These results suggest that our well-controlled NbN SIS junctions can be used for terahertz mixer elements instead of Nb SIS junctions.
The measured gain and noise of a SIS waveguide mixer at 345 GHz have been compared with theoretical values, calculated from the quantum mixer theory using a three port model. As a mixing element, we use a series array of two Nb-Al2O3-Nb SIS junctions. The area of each junction is 0.8 sq microns and the normal state resistance is 52 omega. The embedding impedance of the mixer has been determined from the pumped DC-IV curves of the junction and is compared to results from scale model measurements (105 x). Good agreement was obtained. The measured mixer gain, however, is a factor of 0.45 plus or minus 0.5 lower than the theoretical predicted gain. The measured mixer noise temperature is a factor of 4-5 higher than the calculated one. These discrepancies are independent on pump power and are valid for a broad range of tuning conditions.
We present measurements and simulations of mixer performance around 660 GHz and around 800 GHz. We use Nb-Al/sub 2/O/sub 3/-Nb tunnel junctions with areas of 0.9 /spl mu/m/sup 2/ and 0.7 /spl mu/m/sup 2/, and RA-products of 14.5 /spl Omega/(/spl mu/m)/sup 2/ and 13 /spl Omega/(/spl mu/m)/sup 2/ for 660 GHz and 800 GHz. Both junctions have an integrated tuning structure made of niobium that consists of a series resonant stub and a quarter lambda transformer. The waveguide mixerblock has no additional adjustable tuning elements. It contains just a waveguide cavity and a substrate channel across it. A horn is carefully adjusted to the cavity and flanged to the block. The measured receiver noise temperatures from 630-690 GHz are below 190 K with a best value of 120 K at 655 GHz. From 780-820 GHz they are below 950 K with a best value of 780 K at 792 GHz. When the operating temperature is reduced from 4.2 K to 2.5 K, a reduction in noise temperature from 830 K to 660 K is observed at 810 GHz. The mixer performance is simulated using the quantum theory of mixing. The simulated performance shows a fairly good agreement with the measured one.