Design parameters and measured performance of the IRAM 30-mMillimeter Radio Telescope

The “Millimeter Radio Telescope” (MRT) is operated by the Institute for Radio Astronomy in the millimeter range (IRAM) and is located at 2850-m altitude in the Sierra Nevada, near Granada, Spain. It is a reflector antenna of 30-m diameter with a surface accuracy of 0.08 mm and a pointing accuracy of better than 2 arcsec. The telescope is equipped with sensitive receivers for the atmospheric windows between 0.8- and 7-mm wavelength. The authors describe the optics layout of the receiver and calibration system, which allows simultaneous observations at a number of frequencies. The special design aspects of the antenna, in particular the control of thermal deformations and the achievement of a high reflector and pointing accuracy are described. The authors compare the design computations with the characteristics of the telescope, derived from several years of operation and optimization. The success of the design is demonstrated by observational experience. The authors conclude the paper with a short review of some of the astronomical results obtained with the telescope     

Coma correction of a wobbling subreflector

Radio astronomical observations at millimeter wavelengths with Cassegrain telescopes often use on-source/off-source measurements with a wobbling subreflector to eliminate the influence of the Earth's atmosphere. The tilt of the subreflector produces the required beam throw, but also a coma-like wavefront deformation noticeable as a degradation of the beam pattern and a reduction of the main beam power. We demonstrate, with measurements made with the IRAM 30-m telescope at 1.3 mm wavelength (230 GHz) of the quasi-point-like source Mars and the extended source Saturn, that it is possible to eliminate (to a large extent) this wavefront deformation when using a near-focus corrector, on the 30-m telescope conveniently located on the co-rotating Nasmyth mirror     

Surface adjustment of the IRAM 30 m radio telescope

The techniques used to set and stabilise the surface of the IRAM 30 m radio telescope to a final root mean square accuracy of about 50 mm are described. This involved both phase retrieval and phase coherent holography using a variety of radiation sources at several frequencies. A finite-element model was utilised in improving the temperature control system for the telescope structure. The factors influencing the ultimate surface accuracy are discussed.     

Thermal behavior of millimeter wavelength radio telescopes

The passive and/or active thermal control of significant structural components, for instance the backstructure and the feed legs, of the IRAM 15-m MM and 30-m MM wavelengths telescopes is discussed, and their thermal behavior is illustrated. The design of the thermal protection was supported by dynamic time-dependent model calculations, which are explained and compared with in situ recorded temperatures of the telescope components     

Dynamic and steady state analysis of a three phase buck rectifier

Current source rectifiers among other alternatives, offer several advantages over line commutated rectifiers. Advantages include displacement power factor control and reduced line current harmonic distortion. This paper analyzes the current source rectifier (CSR) in transient and steady state, the models are developed in a synchronous reference frame. The load behavior is characterized for two load conditions, resistive load or, in general, increasing current for increasing voltage, and constant output power, decreasing output current for increasing voltage. Constant power operation can occur for a converter system supplying a pulse width modulation (PWM) inverter with high dynamics. Several static converter characteristics such as power factor, real and reactive power are analyzed for both types of load. Transient characteristics are analyzed for both types of load by exact small-signal model with full set of equations     

法国电信业务融合战略

2002年,全球扩张和竞争3G牌照使法国电信背上了沉重的债务。今天,在融合环境下法国电信又面临来自各领域、价值链上利益相关者的竞争。然而,法国电信已经从战略上做好了积极应对的准备,正如法国电信集团战略与开发高级执行副总裁GeorgesPenalver对其融合战略的诠释：要调整网络、人员及结构来适应新的市场格局,淡化价值链的边界、快速实现产品和服务的创新、成为全球性的运营商。     

Microprocessor Control of DC/AC Static Converters

Microprocessor control of power electronic systems offers the possibility of improvements in reliability, maintenance and servicing, and increased control flexibility. This paper describes several pulsewidth modulation (PWM) waveforms where a predetermined number of unwanted harmonics can be canceled. The advantages and disadvantages of the different methods of delaying the signals in small microprocessor controlled PWM inverter drive systems are considered. Several examples of single-phase and three-phase PWM inverters, with the laws which govern the commutation angles, output waveforms, and their respective frequency spectra are presented.     

Geometric Analysis and Manufacturing Considerations for Optimizing the Characteristics of a Twisted Pair

The geometry of a twisted pair largely determines its electrical characteristics. To improve and refine the value of these characteristics according to preset values, the optimization of the manufacturing processes requires comprehensive knowledge of twisted pair geometry and of how electrical magnitudes are affected by the construction features of the twisted pair. This paper studies the relation between the length of a twisted pair cable and the length of each of the wires that compose the cable, by analyzing concepts such as pitch angle and radius of the helix. In addition, it examines the deformations and irregularities that can occur in the twisted pair during the manufacturing process and their effects on the geometry of the twisted pair. Results showed in this paper are a part of a larger research project carried out in association with the R&D department of a cable manufacturing company, and these results are being applied into the design department of this company.