Integrated Tuning Structure Research Articles

A 3×3 millimeter-wave micromachined imaging array with superconductor–insulator–superconductor mixers

Results from a 3×3 micromachined millimeter-wave focal-plane imaging array with superconducting tunnel junctions as mixing elements are presented. The array operates in a frequency range of 170–210 GHz. The imaging array chip uses relatively large-area 9 μm2 and low-impedance (4–5 Ω) junctions. Integrated tuning structures are implemented to match the devices to the antenna impedance. Noise measurements yielded the lowest double-sideband noise temperature of 52 K (at 190 GHz) from the central element. The lowest noise temperatures from the off-axis elements are in the range of 60–100 K, with a uniform bandwidth of 30 GHz. Antenna beam patterns with an essentially Gaussian profile have been measured for on- and off-axis elements.

Noise and gain in frequency mixers with NbN 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.

Low noise broadband fixed tuned SIS waveguide mixers at 660 and 800 GHz

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.

Characterization of a 680–760 GHz SIS waveguide mixer

A heterodyne waveguide receiver employing 1 µm2 Nb superconducting tunnel junctions with on chip integrated tuning structures is characterized from 680–760 GHz. Several different types of integrated tuning structures are investigated. Lowest DSB receiver noise temperatures of 310 K at 709 GHz and 400 K at 720 GHz are measured. Analysis of the data shows that the loss of the superconducting tuning structures has a major influence on the overall receiver performance. A 25% reduction in receiver noise temperature is observed if the mixer is cooled from 4.2 K to 2 K, which we attribute to the reduced loss of the superconducting microstrip lines at lower temperatures. The calculated performance of the different tuning structures is shown to be in good agreement with the actual receiver noise measurements.

Broadband quasi-optical SIS mixers with large area junctions

A broadband quasi-optical superconducting tunnel junction (SIS) mixer with integrated tuning elements was designed and tested. We are able to achieve very low noise performance using commercially available niobium integrated circuit (IC) technology. The low critical current density (980 A/cm/sup 2/) and large area (12 /spl mu/m/sup 2/) of the commercial SIS's is compensated by the ability to fabricate accurate integrated tuning structures in the mature niobium IC process available from Hypres, Inc. Noise measurements were made in the frequency range from 70 to 105 GHz. The best uncorrected double sideband receiver noise is 38 K at 77 GHz, with receiver noise temperatures less than 100 K from 75 to 102 GHz.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Resonance effects in Josephson tunnel junctions with integrated tuning structures

Large-current steps have been found in the I/V characteristics of superconducting tunnel elements that are integrated within fixed tuning structures. The Josephson frequency at the steps corresponds to the operation frequencies of the different tuning structures at 345, 460, or 600 GHz. Therefore, these current steps are interpreted as resonance effects generated by an internal interaction of the Josephson oscillation with the tuning structure. The result of a spectrometer measurement around 350 GHz is in very good agreement with the resonance frequency obtained from the I/V characteristic. This confirms the interpretation of the current steps. A simple dc measurement is therefore sufficient to determine the actual resonance frequency of the tuning structure. It may be shifted with respect to the expected frequency, due to deviations of the junction size or other parameters from their required values. The exact knowledge of the resonance frequency is important for the application of these integrated tunnel elements as mixers in radioastronomical receivers. In single junctions a splitting of the resonance step into several branches was observed, whereas in two-junction arrays a splitting into two distinct states was found. Irregular switching appeared between these two states. The temperature and magnetic-field dependence of the current steps was investigated.

Simulated performance and model measurements of an SIS waveguide mixer using integrated tuning structures

A three-port approximation of the quantum mixer theory is employed to perform mixer gain calculations at 230 GHz for SIS junctions with integrated tuning structures. In addition, the embedding impedance range of a waveguide mixer mount has been obtained from model measurements and has been included in the gain calculations. The results show that even moderately small junctions can perform well in a waveguide environment when an integrated tuning structure is used. A three-element tuning circuit is presented that would allow broad band operation with a fixed embedding impedance which is important for applications using a planar antenna structure.

An 85-116 GHz SIS receiver using inductively shunted edge junctions

A superconductor-insulator-superconductor (SIS) mixer with a broadband integrated tuning structure is described. The mixer is tunable from 85 to 116 GHz and at 114 GHz has a noise temperature >

Integrated tuning elements for SIS mixers

The use of inductive elements to tune out the junction capacitance in SIS mixers is examined. Two new integrated tuning structures are introduced which overcome the limitations of earlier designs.