Optimum source conductance for high frequency superconductingquasiparticle receivers

The authors have used the quantum theory of mixing for extensive numerical calculations to determine the mixer source conductance G_s required to optimize a superconductor-insulator-superconductor (SIS) quasiparticle heterodyne receiver. The optimum G_s matches an empirical formula which can be understood by a simple derivation. Previous work indicated that G_s should vary inversely with frequency, and this implies that the critical current density of SIS junctions used for mixing should increase as frequency squared, a stringent constraint on the design of submillimeter SIS mixers. On the contrary, it was found that G_s is more weakly dependent upon frequency, and the implications for the design of submillimeter SIS mixers are discussed     

Low noise submillimeter SIS receiver with niobium nitride quasiparticle tunnel junctions

We have developed and tested a submillimeter waveguide SIS mixer with NbN-MgO-NbN quasiparticle tunnel junctions. The two junction array is integrated in a full NbN printed circuit. The NbN film critical temperature is 15 K and the junction gap voltage is 5 mV. The size of the junctions is 1.4 × 1.4 µm and Josephson critical current density is about 1.5 KA/cm² resulting in junction R_NωC product about 40. The inductive tuning circuit in NbN is integrated with each junction in two junction array. A single non contacting backshort was tuned at each frequency in the mixer block. At 306 GHz the minimum DSB receiver noise temperature is as low as 230 K. The sources of the receiver noise and of the limits of the NbN SIS submillimeter mixer improvement are discussed.     

100 GHz mixer vertically integrated (stacked) SIS junction array

A Vertically Integrated Array (stacked array) of single windowSIS junctions (VIA SIS), based on a stacked five layer structure of Nb-AlO_x-Nb-AlO_x-Nb, has been fabricated and tested in a quasi optical mixer configuration at 106 GHz. This particular VIA SIS design has two stacked junctions fabricated by standard tri-layer process employing photolithography, reactive ion and wet etching processes. A simple expression for calculating the specific capacitance of single and arrayed SIS junctions is suggested. Due to the absence of interconnection leads between the individual junctions and reduced overall capacitance, compared to a single SIS junction, has the VIA SIS good future prospects for use in submillimeter wave SIS mixers The VIA SIS may be regarded as a lumped rather than a distributed structure at least up to the gap frequency at 730 GHz for Nb. DC-IV measurements show high quality of the Individual SIS junctions and good reproducibility of the array parameters over the substrate area. The first VIA SIS mixer experiments yielded a receiver noise temperature of 95 K (DSB) at a LO frequency of 106 GHz.     

A Multibeam SIS Mixer Module for a Focal Plane Array Receiver

The integration of many receiver units into a receiver array is a common method of improvement of imaging systems. This approach, well known in the mm band for Schottky mixer arrays, has not so far been developed for Superconductor - Insulator - Superconductor (SIS) junction mixers, which give the best sensitivity in the short mm wave range and in the submm range. We demonstrate for the first time a practical low noise multibeam receiver module using SIS mixer technology. The basis for the integration of several SIS mixers with a common local oscillator source is given by the saturation of the SIS receiver noise dependence upon local oscillator power. The module comprises three identical SIS mixers integrated with a common local oscillator, coupled through a three branch waveguide directional coupler. The multibeam module has been developed for a focal plane array receiver of the 30 meter radio telescope of the Institut de Radioastronomie Millimétrique (IRAM).     

Development of a 385–500 GHz Sideband-Separating Balanced SIS Mixer

A submillimeter (385–500 GHz) low-noise sideband-separating balanced SIS (Superconductor Insulator Superconductor) mixer (Balanced 2SB mixer) with high IRR (Image Rejection Ratio) has been successfully developed, whose SSB (Single SideBand) noise temperature is ～ 200 K (10hf/k) with an image rejection ratio of ≥?～10 dB. Balanced mixers have become a promising technology which would break through the limitation especially in terahertz receivers and heterodyne arrays. However, though there are examples in microwave with relatively worse noise performance, submillimeter and terahertz balanced mixers have rarely been developed in spite of their astronomical importance. The developed balanced 2SB mixer is not only the first one demonstrated at submillimeter frequency range, but also has very low noise, high IRR, wide detectable frequencies (385–500 GHz), and a flat IF output spectrum. The balanced 2SB mixer is composed of three RF hybrids, four DSB (Double SideBand) mixers, two 180° IF hybrids, and an IF quadrature hybrid. Several important performance indicators such as noise temperature, IRR, required LO (Local Oscillator) power, and IF spectra were measured. The measured LO power required for the balanced 2SB mixer was typically ~ 14 dB less than that of the single-ended mixers.     

345 GHZ PROTOTYPE SIS MIXER WITH INTEGRATED MMIC LNA

We report on heterodyne measurements at submillimeter wavelengths using a receiver with a Superconductor-Insulator-Superconductor (SIS) mixer device and a Microwave Monolithic Integrated Circuit (MMIC) cryogenic low noise amplifier (LNA) module integrated into the same block. The mixer characterization presented in this work demonstrates the feasibility of operating a MMIC LNA in close proximity to the SIS device without penalty in mixer performance due to heating effects. Verification of this functionality is crucial for the ongoing development of SuperCam, a 64-pixel focal plane array receiver consisting of eight, 1 × 8 integrated mixer/LNA modules. The test setup included a mixer block modified to accept a MMIC amplifier. Our tests show that the LNA can be operated over a broad range of V_drain voltages from 0.40–1.40 V, corresponding to dissipative powers of 2.6–29 mW. We observe no significant effect on the measured uncorrected receiver noise temperatures in the 345 GHz band.     

A Novel LO Power Injection Method for SIS Mixers

We propose a novel LO power injection method developed for SIS mixers in this paper. Based on the feature of extremely small LO power requirements of SIS quantum mixing, the new method fulfills SIS pumping through a DC/IF route based built-in LO path, which is composed of an additional LO waveguide and the existing microstrip choke filter on the junction substrate. With the new method, traditional external LO diplexers(e.g., crossguide-couplers or beamsplitters) become unnecessary, resulting in a lower loss, compact, and stable receiver system. Experiments at 110- and 230 GHz bands have shown that the present method is efficient in coupling sufficient pumping power to SIS junctions from general LO sources, and the receiver sensitivities have a further improvement of about 10 K. We expect this method is also able to be applied into submillimeter wave band for SIS mixers.     

Analytical prediction for the optimum operating conditions of SIS mixers

Using Tucker's quantum theory of mixing and a quasi five-frequency approximation proposed by Kerr et al., this paper explores the optimum operating conditions of SIS mixers in the frequency region of 100 to 650 GHz. Four parameters (i.e., ΩR_nC_j product, normal state resistance R_n, RF source admittance (R_rf ^?+jB), and IF load resistance R_if) affecting the performance of an SIS mixer have been investigated. Our results indicate that, independent of the absolute value of R_n, the SIS mixer performance is dominated by R_rf/R_n and R_if/R_n; and that the mixer performance becomes quite insensitive to the ΩR_nC_j product, as the mixer operating frequency goes up to submillimeter wavelengths. Concerning all properties of an SIS mixer, the optimum R_rf/R_n value seems proportional to f^1/n (n≈2), and the optimum R_if/R_n and jB/G_n values are relatively independent of frequency, about 0.5 to 1.5 and ?j0.5 to ?j1.0 respectively.     

Simple 1 MM Receivers with a Fixed Tuned Double Sideband SIS Mixer and a Wideband INP MMIC Amplifier

We report on techniques to broaden the intermediate frequency (IF) bandwidth of the Berkeley‐Illinois‐Maryland Array (BIMA) 1mm Superconductor‐Insulator‐Superconductor (SIS) heterodyne receivers by combining fixed tuned Double Side Band (DSB) SIS mixers and wideband Monolithic Microwave Integrated Circuit (MMIC) IF amplifiers. To obtain the flattest receiver gain across the IF band we tested three schemes for keeping the mixer and amplifier as electrically close as possible. In Receiver I, we connected separate mixer and MMIC modules by a 1 ″ stainless steel SMA elbow. In Receiver II, we integrated mixer and MMIC into a modified BIMA mixer module. In Receiver III, we devised a thermally split block in which mixer and MMIC can be maintained at different temperatures–in this receiver module the mixer at 4 K sees very little of the 10–20 mW heat load of the biased MMIC at 10 K. The best average receiver noise we achieved by combining SIS mixer and MMIC amplifier is 45 ‐50 K DSB for ν_LO = 215–240 GHz and below 80 K DSB for ν_LO = 205 ‐ 270 GHz. Over an IF frequency band of 1 – 4 GHz we have demonstrated receiver DSB noise temperatures of 40 – 60 K. Of the three receiver schemes, we feel Receiver III shows the most promise for continued development.     

A planar quasi-optical SIS receiver

A planar, quasi-optical SIS (superconductor-insulator-superconductor) receiver operating at 230 GHz is described. The receiver consists of a 2×5 array of half-wave dipole antennas with niobium-aluminum oxide-niobium SIS junctions on a quartz dielectric-filled parabola. The 1.4-GHz intermediate frequency is coupled from the mixer via coplanar strip transmission lines and 4:1 balun transformers. The receiver is operated at 4.2 K in a liquid helium immersion cryostat. Accurate measurements of the performance of single untuned array receiver elements are reported. A mixer noise temperature of 89 K DSB (double sideband), receiver noise temperature of 156 K DSB and conversion loss of 8 dB into a matched load have been obtained. This mixer noise temperature is approximately a factor of two larger than that of current state of the art waveguide mixers using untuned single junctions a the same frequency     

A 275–370 GHz Receiver Employing Novel Probe Structure

We present the results of the development of a 275–370 GHz, fixed-tuned double sideband (DSB) receiver based on superconductor-insulator-superconductor (SIS) junction mixer. The mixer block uses a full height rectangular waveguide and employs a novel radial-like probe structure with integrated bias-T. The measured uncorrected receiver noise temperature is 30–50 K corresponding to about 2–3 quantum noise across the full frequency band with an IF from 3.8 to 7.6 GHz. The mixer is to be used on the Atacama Pathfinder EXperiment (APEX) submillimeter telescope in Chile.     

Wide-band low noise MM-wave SIS mixers with a single tuning element

Several SIS quasiparticle mixers have been designed and tested for the frequency range from 80 to 115 GHz. The sliding backshort is the only adjustable RF tuning element. The RF filter reactance is used as a fixed RF matching element. A mixer which uses a single 2×2 μm² Pb-alloy junction in a quarter-height waveguide mount has a coupled conversion gain of G_M(DSB)=2.6±0.5 dB with an associated noise temperature of T_M(DSB)=16.4±1.8 K at the best DSB operation point. The receiver noise temperature T_R(DSB) is 27.5±0.8 K for the mixer test apparatus. This mixer provides a SSB receiver noise temperature below 50 K over the frequency range from 91 to 96 GHz, the minimum being T_R(SSB)=44±4 K. Another mixer with an array of five 5×5 μm² junctions in series in a full-height wave-guide mount has much lower noise temperature T_M(DSB)=6.6±1.6 K, but less gain G_M(DSB)=?5.1±0.5 dB.     

Superconductor-Insulator-Superconductor Mixers for the 2 mm Band (129-174 GHz)

We present the design, construction and performance of backshort-tuned Single Side Band (SSB) and of fixed-tuned Double Side Band (DSB) Superconductor-Insulator-Superconductor (SIS) mixers covering the frequency range of 129-174 GHz (2 mm band). Receivers employing these SSB mixers have been continuously operated for astronomical observations on the six antennas of the IRAM Plateau de Bure Intereferometer (PdBI) since 2007 and on the IRAM 30 m Pico Veleta (PV) radio telescope since 2009. The DSB version of the mixer was employed in a prototype of a four-element focal plane array that was tested on the IRAM 30 m radio telescope. Both SSB and DSB mixers employ the same chip and are based on a wideband single ended probe transition from WR6 full-height waveguide to thin-film microstrip line and on a series array of two Nb/Al-AlO_x/Nb junctions. The measured receiver noise for the four-element DSB mixer array pumped by a Gunn oscillator cascaded with a frequency doubler was in the range 25-35 K across the 135-168 GHz LO band. The PdBI and PV receivers equipped with the SSB mixers have measured noise temperatures in the range of 30 K to 60 K and an image sideband rejection below -10 dB over the 129-174 GHz RF band. The measurement results agree well with the predictions obtained through detailed simulations of the SIS receivers based on the standard theory of quantum mixing.     

An SIS waveguide heterodyne receiver for 600 HGz-635 GHz

A waveguide SIS heterodyne receiver using a Nb/AlO_X/Nb junction has been built for astronomical observations of molecular transitions in the frequency range 600 GHz - 635 GHz, and has been successfully used at the Caltech Submillimeter Observatory (CSO). We report double sideband (DSB) receiver noise temperatures as low as 245 K at 600 GHz -610 GHz, and near 300 K over the rest of the bandwidth. These results confirm that SIS quasiparticle mixers work well at submillimeter-wave frequencies corresponding to photon energies of at least 90% of the superconductor energy gap. In addition, we have systematically investigated the effect on the receiver performance of the overlap between first-order and second-order photon steps of opposite sign at these frequencies. The receiver noise increases by as much as 40% in the region of overlap. We infer potential limitations for operating submillimeter-wave Nb/AlO_x/Nb mixers.     

Simple FTS Measurement System for Submillimeter SIS Mixer

We have constructed a simple Fourier Transform Spectroscopy system, and carried out performance measurement of our 640-GHz band SIS mixer devices. This system uses the identical quasi-optics to the one for heterodyne measurement, which allows a direct and quick comparison between FTS and heterodyne responses. With a room-temperature absorber for submillimeter source, instead of a high-temperature source such as Hg lamp, we successfully obtained interferogram with good signal-to-noise ratios. The frequency resolution is moderately coarse (Δν ≈ 17 GHz( due to a limitation on the travel length of scanning mirror for the interferometer, but we found it is useful to investigate broad-band characteristics of SIS mixers.     

Low noise SIS mixer for 230 GHz receivers of plateau de bure interferometer

We present a SIS mixer developed for 200 – 250 GHz band receivers of Plateau de Bure Interferometer. We demonstrate the minimum DSB receiver noise of 20 K at 220 GHz. The average receiver noise of 25 K is possible in 200 – 250 GHz range. The receiver conversion gain and output noise instability of 10^?4 on the time scale of 1 minute is comparable with the Shottky receivers performance. The minimum measured SIS mixer noise of about 10 K is close to the quantum limit. The waveguide SIS mixer with a single backshort has two junction array with inductively tuned junctions. The Nb/Al Oxide/Nb SIS junctions are 2.24 µm² each with the Josephson critical current density of 3.2 KA/cm². The thermal properties of the SIS mixer are studied. The mixer band of the low noise operation is in a good agreement with the design requirements.     

A Low-Noise Terahertz SIS Mixer Incorporating a Waveguide Directional Coupler for LO Injection

We have developed a low-noise heterodyne waveguide Superconductor-Insulator-Superconductor (SIS) mixer with a novel local oscillator (LO) injection scheme for the Atacama Large Millimeter/submillimeter Array (ALMA) band 10, over the frequency range 0.78–0.95 THz. The SIS mixer uses radio frequency (RF) and LO receiving horns separately and a waveguide 10 dB LO coupler integrated in the mixer block. The insertion loss of the waveguide and coupling factor of the coupler were evaluated at terahertz frequencies at both room and cryogenic temperatures. The double-sideband (DSB) receiver noise temperatures were below 330 K (7.5hf/k _B) at LO frequencies in the range 0.801–0.945 THz. The minimum temperature was 221 K at 0.873 THz over the intermediate frequency range of 4–12 GHz at an operating temperature of 4 K. This waveguide heterodyne SIS mixer exhibits great potential for practical applications, such as high-frequency receivers of the ALMA.     

A 400–500 GHz Balanced SIS Mixer with a Waveguide Quadrature Hybrid Coupler

We have developed a 400–500 GHz low-noise balanced SIS (Superconductor Insulator Superconductor) mixer, which is based on a waveguide RF quadrature hybrid coupler. The RF quadrature hybrid was designed and fabricated as a broadband hybrid with good performance at 4 K. The fabricated RF quadrature hybrid was measured at room temperature with a submillimeter vector network analyzer to check amplitude and phase imbalance between two output ports. Then the balanced mixer was assembled with the RF hybrid, two DSB mixers, and a 180° IF hybrid. Several important parameters such as noise temperature, LO power reduction, and IF spectra were measured. The LO power reduction is defined as how much LO power the balanced mixer saves compared with a typical single-ended mixer. The measured noise temperature of the balanced mixer was ~ 55 K at the band center which corresponds to ~ 3 times the quantum noise limit (hf/k) in DSB, and ~ 120 K at the band edges. The noise performance over LO frequency was almost the same as that of the worse DSB mixer used in the balanced mixer. In addition the LO power required for the balanced mixer is ~ 11 dB less than that of the single-ended mixers.     

Submillimeter-wave detection with superconducting tunnel diodes

The review presents a theoretical framework for understanding submillimeter detection using an optical photodiode theory. Both gain and noise in the superconductor-insulator-superconductor (SIS) mixer are described in terms of mixing on a photodiode. The role of impedance matching in the proper design of an SIS mixer is described. A variety of methods for achieving good impedance match at submillimeter frequencies are presented. The state of the submillimeter SIS mixer art as practiced in a variety of laboratories is described and summarized     

Quasi-optical slot antenna SIS mixers

A quasi-optical SIS mixer designed for efficient radiation coupling is described. The mixer uses a twin-slot antenna which has the advantages of a good beam pattern and a low impedance. The radiation and impedance characteristics of the antenna were obtained from a moment-matched calculation. Tapered superconducting microstrip transmission lines are used to carry the radiation from the slot antennas to the tunnel junction. The effective impedance seen by the tunnel junction is quite low, about 4 Ω, which allows micron-size junctions to be used at 500 GHz. The mixers have been fabricated using Nb/Al-oxide/Nb tunnel junctions and a receiver noise temperature of 420 K (DSB) was measured at 490 GHz, which is the best yet obtained for a quasi-optical mixer at this frequency. The comparatively large junction area increases the mixer saturation power and allows strong suppression of noise from the Josephson effect by the application of a magnetic field of modest strength