Development of the SRAO 1024-Channel Digital Auto-Correlator

We developed a 1024-channel digital auto-correlation spectrometer for the Seoul Radio Astronomy Observatory (hereafter the SRAO-1KACS). The SRAO-1KACS has two main modules: the IF-to-baseband converter (IFBC) module and the 1024-channel auto-correlator (1KACR) module. The input frequency range of the IFBC module is from 1.5 to 1.55 GHz with a dynamic range of –4 +3 dBm. The 1KACR module performs calculations of auto-correlation coefficients by the accumulation and modulo-2-counting method in 3-level configuration. The system is controlled by a Linux-based personal computer. The SRAO-1KACS provides 3 different observational modes: 50, 25, and 12.5 MHz bandwidth modes. The channel losses are 20%, 12%, and 8% for each bandwidth mode, respectively. Various tests were executed including lab tests and astronomical tests. Lab tests were performed for a 1.5625MHz sinusoidal wave input and for a white noise source. We also executed astronomical tests in 12CO J=1–0 emission line at 115.2712 GHz, which showed that SRAO-1KACS can be used at astronomical observatories.     

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).     

Minimum noise temperature of a practical SIS quantum mixer

Noise temperature of a SIS quantum mixer has been calculated as function of local oscillator voltage and signal source conductance on the basis of a measured I–V characteristic. Applying Tucker's quantum theory of mixing /1/, it is shown that the SIS mixer is quantum noise limited. Using cryogenic intermediate frequency amplifier, receiver noise temperature of 20 K seems to be possible at mm wavelength.     

The Development of a Dual Channel SIS Receiver of Trao Telescope

We developed a low noise dual channel receiver with 100GHz and 150GHz band, which is used to make the simultaneous observation with two bands. The SIS mixers are used in both bands. The constructed dewar for the receiver has a performance with a vacuum of 10^–8torr and a temperature of 4.2K. The receiver noise temperature is 50K(DSB) for 100GHz band and 80K(DSB) for 150GHz band, respectively. In order to achieve the simultaneous observations, the quasioptical system is precisely designed, and also evaluated by measurements in the laboratory. The relative pointing offset between two bands is 3. We have observed the various sources using the receiver since October 1998.     

Fanatic: An SIS radiometer for radio astronomy from 660 to 695 GHz

FANATIC is a compact radiometer optimized for radio astronomy from about 660 to 695 GHz ( 455 – 432 µm). We observed a large number of molecular and atomic spectral lines from galactic and extragalactic sources duringFANATIC's first run on the James Clerk Maxwell Telescope in early March 1994. Double sideband receiver temperatures during observations were about 800 K (25hv/k). The heart of the receiver is a two-junction Nb/AlO_x/Nb SIS array fed by a sandwiched V-Antenna. The junction array and antenna are fabricated together at IRAM's Grenoble SIS laboratory. Each junction has a normal resistance of R_n 10 , an area of 2 µm², an individual radial stub circuit to resonate the capacitance, and a /4 transformer to match to the antenna. The solid-state local oscillator is a mm-wave Gunn oscillator followed by a doubler and tripler. The LO diplexer is a Martin-Puplett interferometer, which insures that there is always abundant LO power for operation and speedy tuning. The receiver and telescope coupling optics, LO, dewar, and calibration system fit on an 0.6 × 0.8 m optical breadboard.A preliminary version of this paper appeared in theProceedings of the Fifth International Space Terahertz Technology Symposium.     

A Low Noise NbTiN-Based 850 GHz SIS Receiver for the Caltech Submillimeter Observatory

We have developed a niobium titanium nitride (NbTiN) based superconductor-insulator-superconductor (SIS) receiver to cover the 350 micron atmospheric window. This frequency band lies entirely above the energy gap of niobium (700 GHz), a commonly used SIS superconductor. The instrument uses an open structure twin-slot SIS mixer that consists of two Nb/AlN/NbTiN tunnel junctions, NbTiN thin-film microstrip tuning elements, and a NbTiN ground plane. The optical configuration is very similar to the 850 GHz waveguide receiver that was installed at the Caltech Submillimeter Observatory (CSO) in 1997. To minimize front-end loss, we employed reflecting optics and a cooled beamsplitter at 4 K. The instrument has an uncorrected receiver noise temperature of 205K DSB at 800 GHz and 410K DSB at 900 GHz. The degradation in receiver sensitivity with frequency is primarily due to an increase in the mixer conversion loss, which is attributed to the mismatch between the SIS junction and the twin-slot antenna impedance. The overall system performance has been confirmed through its use at the telescope to detect a wealth of new spectroscopic lines.     

Characterization of the IF Noise of a Submillimeter SIS Receiver

A fitting method is presented here for the accurate characterization of the IF noise contribution of a sub-millimeter SIS receiver. By fitting the mixer's IF output power response and junction's IV curve of an SIS mixer without LO pumping, we can obtain the IF noise contribution, the physical temperature of the isolator connected just behind the SIS mixer, the output mismatching of the mixer, and the total gain of the IF chain. Differing from a conventional method, which only uses the normal-state (linear) branch of the junction's IV curve, the method proposed here also includes the nonlinear portion around the gap voltage. The dynamic resistance in this portion is varied dramatically, providing us a good probe to characterize the output mismatching of the mixer, as well as other parameters.     

A 650-GHz Band SIS Receiver for Balloon-Borne Limb-Emission Sounder

A superconducting low-noise receiver has been developed for atmospheric observations in the 650-GHz band. A waveguide-type tunerless mixer mount was designed based on one for the 200-GHz band. Two niobium SIS (superconductor-insulator-superconductor) junctions were connected by a tuning inductance to cancel the junction capacitance. We designed the R_nC_j product to be 8 and the current density to be 5.5 kA/cm². The measured receiver noise temperature in DSB was 126-259 K in the frequency range of 618-660 GHz at an IF of 5.2 GHz, and that in the IF band (5-7 GHz) was 126-167 K at 621 GHz. Direct detection measurements using a Fourier transform spectrometer (FTS) showed the frequency response of the SIS mixer to be in the range of about 500-700 GHz. The fractional bandwidth was about 14%. The SIS receiver will be installed in a balloon-borne limb-emission sounder that will be launched from Sanriku Balloon Center in Japan.     

High Doping Density Schottky Diodes in the 3MM Wavelength Cryogenic Heterodyne Receiver

The paper describes a 3mm cryogenic mixer receiver using high doping density (“room-temperature”) Schottky diodes. The measured equivalent noise temperature T_eq of the diodes is 109 K at 20 K, which is much higher than the T_eq of the low doping density (“cryogenic”) diodes. In spite of this, the double-sideband (DSB) noise temperature of the cryogenic receiver developed is 55 K at 110 GHz, owing to the low conversion loss of the mixer and ultra-low noise of the PHEMT IF amplifier. This is the lowest noise temperature ever reported for a Schottky diode mixer receiver. The results obtained are useful for the development of submm receivers in which high doping density Schottky diodes are used.     

Surface Adjustment of the SRAO 6-M Antenna Based on Near-Field Radio Holography at 86 GHz

In an attempt to measure and correct the surface profile of the SRAO 6- antenna, we carried out a with-phase holographic experiment at 86 GHz with a transmitter in the near field. Unlike other previous experiments, for cost-effectiveness, we utilized harmonic mixers for frequency conversion together with a microwave synthesizer for local oscillators, while still maintaining the S/N ratio above the required value of 30 dB. We measured the complex beam patterns over ±30 _FWHM both in azimuth and elevation and Fourier-transformed them to obtain the surface profile at 10-cm spatial resolution. It is found that the surface accuracy of the antenna is improved from 100 m to better than 30 m. The edge taper is also estimated to be 11.2 dB in good agreement with the design value, 12 dB. To verify the improvement, we observed the Venus and found that the aperture and beam efficiencies are 72% and 76%, respectively, and are roughly independent of frequency over the whole 3-mm band.     

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 m² 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 m² 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.Contribution of the U.S. Government, not subject to copyright     

Scaled model measurements of embedding impedances for SIS waveguide mixers

Several scaled models have been used to determine the contributions of various waveguide mount parameters to the embedding impedance of a mm-wave SIS mixer. Measured effects of waveguide height, substrate orientation and width, junction location, lead inductance and RF-filter impedance are presented and discussed.     

Heterodyne mixing at 70 GHz with superconductor-insulator-superconductor tunnel junctions

A laboratory heterodyne receiver working at 70 GHz was built up using superconductor-insulator-superconductor tunnel junction as mixing element. Single sideband conversion loss L_C as low as 1.92±0.23 and mixer noise temperature T_M of less than 100 K have been achieved while local oscillator pump power is 4·10^–8W.     

Millimeter-Wave Radiometer for the Measurement of Stratospheric ClO Using a Superconductive (SIS) Receiver Installed in the Southern Hemisphere

A new sensitive radiometer for atmospheric minor constituents has been developed by Nagoya University and the National Institute for Environmental Studies (NIES). The instrument equipped low-noise superconducting (SIS) mixer with a system noise temperature of 200K (SSB) at 204GHz. The SIS mixer is operated in a single side band mode with a side band ratio grater than 10dB by adjusting the two tunable backshorts of the mixer. Since any additional SSB filters are not necessary, we can simplify the quasi-optical system and reduce the standing waves which makes difficult to achieve a flat spectral baseline. We installed the observation system at Las Campanas Observatory, Chile in December 1999, and started the test observation from 2000 October. We have detected significant ClO emission at 204.3 GHz with an actual observation time of only 4 hours. This is the first detection of the ClO at 40km altitude from the mid-latitude region of the southern hemisphere. The Nagoya-NIES system has enabled continuous monitoring of ClO in the stratosphere with a time resolution of 4 or 5 hours.     

Analysis of SIS heterojunctions for use as mm-wave mixers exhibiting coversion gain

We apply the quantum formulation of heterodyne mixer theory to SIS heterojunctions (junctions between dissimilar superconductors). Conversion gain is predicted over a wide range of mm-wave frequencies in the 3-port Y-mixer model by exploiting the naturally occurring region of negative conductance in the DC I-V characteristic. In the signal frequency range 50–250 GHz this region persists in the pumpedjunction I–V characteristic for local oscillator power <1 nW and leads to a negative conductance at the mixer's IF port.     

A quasiparticle SIN mixer for the 230 GHz frequency range

We report on quasiparticle mixing in the frequency range 220 to 230 GHz using SIN junctions. The lowest double sideband receiver noise temperature measured was about 230 K. This result shows that the SIN junction provides an interesting alternative for high frequency heterodyne receivers where pair tunneling in SIS junctions could give rise to interference from Josephson effects.     

Five hundred meter aperture spherical radio telescope (FAST)

Five hundred meter aperture spherical radio telescope (FAST) will be the largest radio telescope in the world. The innovative engineering concept and design pave a new road to realizing a huge single dish in the most effective way. Three outstanding features of the telescope are the unique karst depressions as the sites, the active main reflector which corrects spherical aberration on the ground to achieve full polarization and a wide band without involving a complex feed system, and the light focus cabin driven by cables and servomechanism plus a parallel robot as secondary adjustable system to carry the most precise parts of the receivers. Being the most sensitive radio telescope, FAST will enable astronomers to jumpstart many of the science goals, for example, the neutral hydrogen line surveying in distant galaxies out to very large redshifts, looking for the first shining star, detecting thousands of new pulsars, etc. Extremely interesting and exotic objects may yet await discovery by FAST. As a multi-science platform, the telescope will provide treasures to astronomers, as well as bring prosperity to other research, e.g. space weather study, deep space exploration and national security. The construction of FAST itself is expected to promote the development in high technology of relevant fields.     

Analysis of Blocking Effect for MM-Wave Receiver

In this paper, the blocking effect that takes place in MM-wave receiver is analyzed; the methods that prevent and avoid the blocking effect are put forward. The results analyzed and the methods put forward possess certain useful significance for many MM-wave receivers in which the RF amplifiers are contained at present.     

Adaptive optics in astronomy

Since Galileo’s first observations in 1609, telescopes have grown dramatically in size. Larger telescopes collect more light, allowing astronomers to detect fainter sources and to look further back in time towards the birth of the universe. The angular resolution of these telescopes, however, has been limited by turbulence in the earth’s atmosphere. This limitation can be dramatically reduced with the use of adaptive optics (AO) to measure and correct the blurring introduced by atmospheric turbulence. AO is now routinely used for science observations on the world’s largest telescopes and is providing a much more detailed view of our universe.