We have designed a system to stabilize the gain of a submillimeter heterodyne receiver against thermal fluctuations of the mixing element. In the most sensitive heterodyne receivers, the mixer is ...usually cooled to 4 K using a closed-cycle cryocooler, which can introduce /spl sim/1%; fluctuations in the physical temperature of the receiver components. We compensate for the resulting mixer conversion gain fluctuations by monitoring the physical temperature of the mixer and adjusting the gain of the IF amplifier that immediately follows the mixer. This IF power stabilization scheme, developed for use at the Submillimeter Array, a submillimeter interferometer telescope located on Mauna Kea, HI, routinely achieves a receiver gain stability of one part in 6000 (rms to mean). This is an order of magnitude improvement over the typical uncorrected stability of one part in a few hundred. Our gain stabilization scheme is a useful addition to superconductor-insulator-superconductor heterodyne receivers that are cooled using closed-cycle cryocoolers in which the 4-K temperature fluctuations tend to be the leading cause of IF power fluctuations.
We describe the design and performance of the digital servo and motion control system for the 6-meter diameter parabolic antennas of the Submillimeter Array (SMA) on Mauna Kea, Hawaii. The system is ...divided into three nested layers operating at a different, appropriate bandwidth. (1) A rack-mounted, real-time Unix system runs the position loop which reads the high resolution azimuth and elevation encoders and sends velocity and acceleration commands at 100 Hz to a custom-designed servo control board (SCB). (2) The microcontroller-based SCB reads the motor axis tachometers and implements the velocity loop by sending torque commands to the motor amplifiers at 558 Hz. (3) The motor amplifiers implement the torque loop by monitoring and sending current to the three-phase brushless drive motors at 20 kHz. The velocity loop uses a traditional proportional-integral-derivative (PID) control algorithm, while the position loop uses only a proportional term and implements a command shaper based on the Gauss error function. Calibration factors and software filters are applied to the tachometer feedback prior to the application of the servo gains in the torque computations. All of these parameters are remotely adjustable in software. The three layers of the control system monitor each other and are capable of shutting down the system safely if a failure or anomaly occurs. The Unix system continuously relays antenna status to the central observatory computer via reflective memory. In each antenna, a Palm Vx hand controller displays system status and allows full local control of the drives in an intuitive touchscreen user interface. It can also be connected outside the cabin for convenience during antenna reconfigurations. Excellent tracking performance (0.3 arcsec rms) is achieved with this system. It has been in reliable operation on 8 antennas for over 10 years and has required minimal maintenance.
The Large-Aperture Experiment to Detect the Dark Age (LEDA) was designed to detect the predicted O(100)mK sky-averaged absorption of the Cosmic Microwave Background by Hydrogen in the neutral pre- ...and intergalactic medium just after the cosmological Dark Age. The spectral signature would be associated with emergence of a diffuse Ly\(\alpha\) background from starlight during 'Cosmic Dawn'. Recently, Bowman et al. (2018) have reported detection of this predicted absorption feature, with an unexpectedly large amplitude of 530 mK, centered at 78 MHz. Verification of this result by an independent experiment, such as LEDA, is pressing. In this paper, we detail design and characterization of the LEDA radiometer systems, and a first-generation pipeline that instantiates a signal path model. Sited at the Owens Valley Radio Observatory Long Wavelength Array, LEDA systems include the station correlator, five well-separated redundant dual polarization radiometers and backend electronics. The radiometers deliver a 30-85MHz band (16<z<34) and operate as part of the larger interferometric array, for purposes ultimately of in situ calibration. Here, we report on the LEDA system design, calibration approach, and progress in characterization as of January 2016. The LEDA systems are currently being modified to improve performance near 78 MHz in order to verify the purported absorption feature.
Blood lead concentrations among children aged 6 years and younger become a concern at 10 microg/dL (0.48 micromol/L) or higher. The authors' objective was to determine whether initial blood lead ...concentrations of 10-19 microg/dL (0.48-0.96 micromol/L) declined among children aged 3 years and younger and whether the magnitude of decline was associated with the case management protocol of the state or local childhood lead poisoning prevention program.
The authors analyzed childhood blood lead surveillance data from 1994 through 1995 and case management protocols from six states that reported the results of all blood lead tests. The study included 2109 children aged 2 years or younger who had a venous blood lead concentration of 10-19 microg/dL (0.48-0.96 micromol/L) and a follow-up venous blood lead test within 3 to 12 months.
Overall, blood lead concentrations increased by 0.25 microg/dL (0.01 micromol/L) between the time of the initial elevated blood lead test and the follow-up test, but concentrations declined by 1.96 microg/dL (0.09 micromol/L) among children covered by a case management protocol that included a home visit and by 0.92 microg/dL (0.04 micromol/L) among those covered by a protocol that included a lead source investigation. The decline remained significant after we adjusted for the child's age.
These findings suggest that childhood lead prevention programs should consider focusing their efforts on home visits and lead source investigations.
Efficient operation of a submillimeter interferometer requires remote (preferably automated) control of mechanically tuned local oscillators, phase-lock loops, mixers, optics, calibration vanes and ...cryostats. The present control system for these aspects of the Submillimeter Array (SMA) will be described. Distributed processing forms the underlying architecture and the software is split between hardware platforms in a leader/follower arrangement. In each antenna cabin, a serial network of up to ten independent 80C196 microcontroller boards attaches to the real-time PowerPC computer (running LynxOS). A multi-threaded, gcc-compiled leader program on the PowerPC accepts top-level requests via remote procedure calls (RPC), subsequently dispatches tuning commands to the relevant follower microcontrollers, and regularly reports the system status to optical-fiber-based reflective memory for common access by the telescope monitor and error reporting system. All serial communication occurs asynchronously via encoded, variable-length packets. The microcontrollers respond to the requested commands and queries by accessing non-volatile, rewriteable lookup-tables (when appropriate) and executing embedded software that operates additional electronic devices (DACs, ADCs, etc.). Since various receiver hardware components require linear or rotary motion, each microcontroller also implements a position servo via a one-millisecond interrupt service routine which drives a DC-motor/encoder combination that remains standard across each subsystem.
Background The optimal operation for a patient with a thyroid nodule “suspicious for papillary thyroid cancer (PTC)” on fine-needle aspiration (FNA) is unclear. This study examines the incremental ...cost-utility of thyroid lobectomy with intraoperative frozen section (thyroid lobectomy) versus total thyroidectomy. Methods Cost-utility analysis was performed for patients with a cytologic diagnosis of “suspicious for PTC” on FNA. Patients underwent either initial total thyroidectomy or thyroid lobectomy and, if needed, completion thyroidectomy. The incremental cost-utility ratio (ICUR; US$/quality-adjusted-life-year QALY), was determined from a societal perspective. Results The base-case ICUR of thyroid lobectomy is $90,776/QALY, strongly favoring total thyroidectomy as a more cost-effective modality. On sensitivity analyses, the model is sensitive to the accuracy of frozen section and to the rate of injury to the recurrent laryngeal nerve (RLN). Thyroid lobectomy is more cost-effective only if both frozen section and final pathology are benign in ≥92% of patients (ICUR $47,959/QALY at 92%). With increasing rates of unilateral (>5%) or bilateral (>2%) RLN injury associated with total thyroidectomy, there is a trend toward thyroid lobectomy being more cost effective ($53,127 and $51,325/QALY, respectively). Conclusion In our model, initial total thyroidectomy is cost-effective for patients with a single thyroid nodule suspicious for PTC on FNA. Our results strongly support total thyroidectomy for initial treatment; thyroid lobectomy is preferred only when complications reach unacceptable levels.
We have constructed a five station 12 GHz atmospheric phase interferometer (API) for the Submillimeter Array (SMA) located near the summit of Mauna Kea, Hawaii. Operating at the base of unoccupied ...SMA antenna pads, each station employs a commercial low noise mixing block coupled to a 0.7 m off-axis satellite dish which receives a broadband, white noise-like signal from a geostationary satellite. The signals are processed by an analog correlator to produce the phase delays between all pairs of stations with projected baselines ranging from 33 to 261 m. Each baseline's amplitude and phase is measured continuously at a rate of 8 kHz, processed, averaged and output at 10 Hz. Further signal processing and data reduction is accomplished with a Linux computer, including the removal of the diurnal motion of the target satellite. The placement of the stations below ground level with an environmental shield combined with the use of low temperature coefficient, buried fiber optic cables provides excellent system stability. The sensitivity in terms of rms path length is 1.3 microns which corresponds to phase deviations of about 1 degree of phase at the highest operating frequency of the SMA. The two primary data products are: (1) standard deviations of observed phase over various time scales, and (2) phase structure functions. These real-time statistical data measured by the API in the direction of the satellite provide an estimate of the phase front distortion experienced by the concurrent SMA astronomical observations. The API data also play an important role, along with the local opacity measurements and weather predictions, in helping to plan the scheduling of science observations on the telescope.
We have designed a system to stabilize the gain of a submillimeter heterodyne receiver against thermal fluctuations of the mixing element. In the most sensitive heterodyne receivers, the mixer is ...usually cooled to 4 K using a closed-cycle cryocooler, which can introduce ~1% fluctuations in the physical temperature of the receiver components. We compensate for the resulting mixer conversion gain fluctuations by monitoring the physical temperature of the mixer and adjusting the gain of the intermediate frequency (IF) amplifier that immediately follows the mixer. This IF power stabilization scheme, developed for use at the Submillimeter Array (SMA), a submillimeter interferometer telescope on Mauna Kea in Hawaii, routinely achieves a receiver gain stability of 1 part in 6,000 (rms to mean). This is an order of magnitude improvement over the typical uncorrected stability of 1 part in a few hundred. Our gain stabilization scheme is a useful addition to SIS heterodyne receivers that are cooled using closed-cycle cryocoolers in which the 4 K temperature fluctuations tend to be the leading cause of IF power fluctuations.
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