Bolometric Microwave Power Calibration Techniques at the National Bureau of Standards

A bolometric method of calibrating low-level microwave power measuring devices is currently employed at the National Bureau of Standards in Boulder, Colo. The technique is one of direct comparison between the standard and the unknown, and utilizes NBS working standard bolometer units and self-balancing dc bridges. In general, the unknown is calibrated to an accuracy of one per cent. The following quantities are defined and the associated errors are discussed: 1) Calibration factor of a bolometer unit in combination with a directional coupler, 2) Calibration factor of a bolometer unit, and 3) Effective efficiency of a bolometer unit.     

A K-Band Phase-Locked Transmitter-Receiver System for CW Balanced-Bridge Measurements

A major sensitivity limitation common to most microwave CW balanced-bridge systems must be attributed to bridge distortion noise, caused by residual FM in the transmitted signal. The described superheterodyne transceiver minimizes this limit for a narrowband K-band bridge system by utilizing precise, coherent IF detection. A phase-locked local oscillator provides means for an arbitrary, stable preselection of the in-phase or quadrature component with the help of a calibrated IF delay line. The transmitter is either 1) frequency-stabilized to a sample cavity by means of a high-gain AFC loop, or 2) phase-locked simultaneously to a K-band harmonic of a VHF quartz oscillator and to a tunable VHF oscillator (VFO). This yields flexibility in a wide range of applications, such as measuring small reflection coefficients, dielectric constants, or magnetic tensor susceptibilities (e. g., in ESR spectroscopy). Analytical expressions for phase and amplitude distortions are derived for a bridge containing one high-Q element. In the systems theory of operation, analytical formulas for the noise spectral densities and the loop errors are given, together with numerical examples. The additional receiver noise, due to residual FM and increased bridge power, is demonstrated by means of measured IF-noise spectra. A cavity-Q measurement with ±1 percent accuracy, using 5-?W incident bridge power, proves the system's capability for measurements of small reflection coefficients.     

Automatic Calorimeter System for the Effective Efficiency Measurement of a Bolometer Mount in 35-GHz Band

An automated calorimetric measurement system using a microcomputer has been developed for determining the effective efficiency of a bolometer mount in the millimeter-wave region. This system consists of a calorimetric measurement part, a bolometric measurement part, and a control/data processing part. The system has been constructed and experimented in 35-GHz band. The accuracy of the system has been estimated within ±0.37 percent.     

Transient processes in nonlinear thin-wire bolometers for laser radiation

Transient processes in nonlinear thin-wire bolometers have been analyzed for the case when the main physical parameters of the latter are a linear function of the temperature. By recording the bolometer output signal parameters while switching the optical power, it is possible to increase the power measurement accuracy, to eliminate the systematic error due to conversion nonlinearity, and to determine the relative parameter of nonuniform optical power distribution over the area of the entry aperture of the device. Translated from Izmeritel'naya Tekhnika, No. 9, pp. 29–32, September, 1996.     

Development of a Bolometer Detector System for the NIST High Accuracy Infrared Spectrophotometer

A bolometer detector system was developed for the high accuracy infrared spectrophotometer at the National Institute of Standards and Technology to provide maximum sensitivity, spatial uniformity, and linearity of response covering the entire infrared spectral range. The spatial response variation was measured to be within 0.1 %. The linearity of the detector output was measured over three decades of input power. After applying a simple correction procedure, the detector output was found to deviate less than 0.2 % from linear behavior over this range. The noise equivalent power (NEP) of the bolometer system was 6 × 10⁻¹² W/Hz at the frequency of 80 Hz. The detector output 3 dB roll-off frequency was 200 Hz. The detector output was stable to within ± 0.05 % over a 15 min period. These results demonstrate that the bolometer detector system will serve as an excellent detector for the high accuracy infrared spectrophotometer.     

A Direct-Current-Comparator Bridge for Measuring Shunts up to 20000 Amperes

A direct-current-comparator bridge for calibrating four-terminal resistors or shunts at currents up to 20 000 amperes is described. Measurements can be made at up to full rated current of the shunts so that the effects of the load coefficient are included. The resistor under test is compared with a reference resistor of higher value by measuring the ratio of the currents through the two resistors required to produce equal voltages across them. A comparator bridge with a range of 100 amperes and errors of less than 1 ppm has been described previously. Improvements to this bridge have been made, the main one being a reversing feature, which permits the currents through the resistors to be reversed in a few milliseconds. This makes an accurate measurement easier, particularly if there is a change of resistance due to heating. By connecting a second comparator in cascade, the range has been extended to 20 000 amperes at an overall ratio up to 2 × 106: 1, with only a slight loss of accuracy; the errors may be a few parts per million. Other applications of the measuring system are the accurate measurement of large currents or the calibration of transductors.     

A Self-Balancing High-Voltage Capacitance Bridge

A self-balancing high-voltage capacitance ratio bridge, especially designed for continuous measurement of corona losses on experimental transmission lines, is described. The instrument is derived by combining a negative-feedback amplifier with a high-voltage capacitance bridge based on the current comparator principle. Manual control is provided for the four most significant decades of the bridge, thus permitting the function of the feedback amplifier to be limited to the balancing of small deviations. In this manner the errors originating in the amplifier and the associated indicating or recording instruments are made insignificant in the overall measurement. The accuracy of the built-in current comparator is 0.001 per cent, of the circuitry associated with the amplifier, 1 per cent. The output of the automatic balancing circuit is instrumented to produce a readout which is proportional to the power loss and reactive volt-amperes. Both components are in the form of small dc voltages and thus are suitable for recording.     

Autocorrelation and Power Measurement with Pyroelectric and Dielectric Bolometers

A pulse comparison technique is described which yields the autocorrelation function and the power spectrum of a repetitive time-domain waveform. The autocorrelation function is realized with a sliding short in a coaxial transmission line to provide time delay; a pyroelectric bolometer to provide multiplication through a square law voltage response; and a capacitor to provide integration. Problems of realization of a perfect time delay and integration limitations are considered, and it is found that noise fluctuations yield the main time resolution limitation that is equivalent to 8 ps for 15-V pulses and a 50-s integration time. The pyroelectric voltage-sensing bolometer is then compared to a pyroelectric capacitance sensing bolometer. It is shown that the capacitance sensing bolometer can handle much longer pulse durations than the voltage sensing bolometer. It is also demonstrated that the sensitivities of the two techniques are equivalent in a typical case at a capacitance sensing bolometer bridge voltage of 3 V. Measurement results of the autocorrelation function and power spectrum, using a voltage sensing pyroelectric bolometer, are given for a nominal 15-V, 500-ps time duration, whose baseband pulses have a 100-pps (pulses per second) repetition rate.     

Calibration & measurement facilities for AC high current & high voltage ratio standards at NPL

Regular calibration of the Instrument Transformers (ITs) is very essential for the accurate measurement of power and energy at high voltages. ITs are also used in the measurement of high currents & high voltages at power frequencies. The nominal transformation of an IT depends only on the number of turns of its windings but due to various losses there are always errors in the Instrument Transformers’ ratios. The correct knowledge of their errors requires calibrations of the ITs. In this paper the methods used in calibrating the Current Transformers (CTs) & Potential Transformers (PTs) and the required accuracy have been discussed.     

A Laser Microcalorimeter

A new type of microcalorimeter as the laser power standard in Japan has been developed at the Electrotechnical Laboratory. This is a double calorimeter in which the laser power is measured by a dc substitution method using the thermopile unit, and simultaneous measurement is also made using the Peltier cooling-type calorimeter by dc substitution. The CW power of the gas laser (He-Ne) is controlled with high stability (±0.05 percent) by the automatic power control system using the Faraday rotation component. By this method and equipment, the "effective efficiency" of the standard thermopile unit is determined precisely. The errors in this microcalorimetric technique are investigated with the aid of heat flow analysis and auxiliary experiments. As a result, the absolute accuracy of the CW power measurement of laser beams (6328?, 1.15?) is evaluated with high accuracy better than 0.5 percent. The experiments have been successful at 6328? and 1.15 ?, and this method is considered to be available at the region of infrared and far-infrared wavelength.     

Voltage-Biased Superconducting Transition-Edge Bolometer with Strong Electrothermal Feedback Operated at 370 mK

We present an experimental study of a composite voltage-biased superconducting bolometer (VSB). The tested VSB consists of a Ti-film superconducting thermometer (T(c) ~375 mK) on a Si substrate suspended by NbTi superconducting leads. A resistor attached to the substrate provides calibrated heat input into the bolometer. The current through the bolometer is measured with a superconducting quantum interference device ammeter. Strong negative electrothermal feedback fixes the bolometer temperature at T(c) and reduces the measured response time from 2.6 s to 13 ms. As predicted, the measured current responsivity of the bolometer is equal to the inverse of the bias voltage. A noise equivalent power of 5 x 10(-17) W/ radicalHz was measured for a thermal conductance G ~ 4.7 x 10(-10) W/K, which is consistent with the expected thermal noise. Excess noise was observed for bias conditions for which the electrothermal feedback strength was close to maximum.     

An adaptive window function method for power measurement

An adaptive window function for distorted power measurement is introduced in this paper. It can automatically provide the optimal window function according to the measured signal. Thus, the applied window function is always optimal in the sense of the zeros of its frequency response corresponding to the undesired measured signal spectrum components which will result in the harmful truncation errors. As a result, the harmful truncation errors of asynchronous sampling and the influence of interharmonics in the measured signal spectrum can be totally eliminated. The correctness, accuracy, and applicability of the proposed method have been verified both theoretically and by extensive simulations     

车轨桥中高频耦合振动分析的功率流方法及模型

轮轨滚动激励引起的桥梁振动响应和输入功率是计算桥梁结构辐射噪声的重要参数。时域车轨桥耦合振动分析常用于低频振动分析,但在中高频分析时效率较低。为此,提出一种基于力法原理的频域功率流方法解决这一问题。采用无限长Euler梁或Timoshenko梁建立钢轨部件,采用无限大Kirchhoff板、Mindlin板或有限元模型建立桥梁部件,采用弹簧元件模拟钢轨与桥梁之间的连接扣件,并以弹簧力为未知量建立力法基本方程。对比计算了不同轨桥模型对U梁和箱梁桥振动功率的影响。结果表明：U梁桥面板的剪切效应对桥梁振动功率计算结果影响很大,采用传统的无限大Kirchhoff板模型将导致功率级计算误差达到15 dB,而采用Mindlin板模型可获得良好的计算精度与效率。相对于箱梁实体有限元模型而言,采用Mindlin板模型的误差仍然较大。     

The Effect of Self-Heating on the Measurement of a Temperature Change Using a Bolometric Detector

Even when care is taken to lower as much as possible the Joule power dissipated in a bolometric detector, the self-heating effect is far from being negligible. Consequently, in the steady state, it is of crucial importance to carefully distinguish the cell temperature, the bolometer temperature and the temperature of the part of the helium film which covers it. In the light of these results, when a bolometer is used to measure a small change in a thermal heat flux, the fundamental question concerns the exact definition of the quantity measured by the detector. To illustrate this problem we have considered the amplitude measurement of the third sound temperature oscillation. When the bolometer is operated from a current source, a detailed analysis of the different heat fluxes involved in the problem, provides a relation between the quantity usually measured in the experiments, and the real amplitude of the third sound temperature oscillation: Clearly these two values are noticeably different. Experimental measurements performed at 0.3 K support very well our analysis. Finally we show that a constant temperature bolometer appears much more suited to this kind of measurements.     

线性运动平台多自由度几何运动误差测量技术

线性运动平台是精密制造和测量的基础,其定位精度直接影响着制造和测量的精度。由于制造和装配误差的影响,运动平台存在六个自由度的运动误差,将影响运动平台的定位精度。为了提高运动平台的定位精度,需要准确地测量各项运动误差。传统的测量方法只能对单一误差进行测量,并且测量仪器体积较大,无法集成在运动平台中进行实时误差测量。本文提出了一系列成本低、体积小、易集成的高精度多自由度几何运动误差同时测量技术,可以根据测量需求选择测量系统的自由度,在提高测量精度的同时,还可以实现在线测量,为提高线性运动平台运动精度起到重要作用。     

A Direct Current Comparator Bridge for High Resistance Measurements

A dc comparator bridge, which is suitable for measuring resistors from a fraction of an ohm to several megohms with partper-million accuracy or better is described. An analysis of errors which may occur when measuring high-value resistors and methods used for reducing these errors to negligible proportions are included. The bridge can be connected to make either four-terminal or three-terminal measurements. The four-terminal connection is best for measuring low-value resistors. When it was used for measuring high-value resistors, a systematic bridge error was detected, which was found to be primarily due to leakage paths through the insulation, permitting current to bypass one of the measured resistors. This error is avoided by using the three-terminal connection for measuring high-value resistors. The leakage resistance is then in parallel with a low resistance winding rather than one of the measured resistors. The bridge has automatic reversal and a chart recorder output, which continuously indicates the deviation from the set point. The comparator is adjustable in steps of 10-' of full scale, and measurements can be made at ratios up to 11.111/1.     

A comparative worst-case error analysis of some proposed six-portdesigns

A detailed analysis of the accuracy in complex-reflection coefficient measurement by the six-port technique is discussed. The expression of the relative error in amplitude measurement of the complex reflection due to power-reading errors are derived in worst-case situation using both the basic six-port and sensitivity analysis theories. The precision of measurement of any point in the Smith chart is obtained by reference to lines of constant errors (amplitude and phase) drawn on the Smith chart for various proposed six-port correlators (junctions). A comparison of three proposed six-port designs shows that the Engens six-port design presents the best performance for a given power measurement accuracy. A maximum dynamic range of 60 dB seems to be a limit for six-port reflectometers using a good-quality power detector (α=0.1%)     

The problem of analyzing errors in measuring systems

Summary In the operation of a linear measuring system with its elements subjected to various random external disturbances which produce in the elements additional static errors, transmitted to the output of the system together with the basic errors of the elements, a total static error is produced at the output. The latter error is the one which determines the accuracy of the measuring system under given conditions.If in experimental work the required variable is measured several times by means of a measuring system, whose elements are subjected to various random external disturbances, a statistical analysis of the measurement results may produce a certain compensation of the static measurement errors of this variable.     

Enhancement of accuracy of multi-axis machine tools through error measurement and compensation of errors using laser interferometry technique

The present era is witnessing advancement in digital electronics and microprocessor which enables manufacturing sector capable to produce complex components within small tolerance zone in the tune of nanometre and at one machining center. All motion control systems have some form of position feed back system fitted with the machine. Such systems are not generally accurate due to the errors in the positioning performance of the machine tool which will change over time to time due to wear, damage and environmental effects. The complex structure of multi-axis CNC machine tools produce an inaccuracy at the tool tip caused by kinematic parameter deviations resulting in manufacturing errors, assembly errors or quasi-static errors. Analysis of these errors using a laser measurement system provides the manufacturers a way to achieve better accuracy and hence higher quality output from these processes. In this communication, techniques to measure the linear positional errors of axes of CNC machine tools by a laser interferometer calibration system and accuracy enhancement using the data obtained from the calibration cycle by feeding into the machine’s controller with the help of linear error compensation package are discussed.