Accurate Frequency Measurements Research Articles

Split-pulse spectrometer for absolute XUV frequency measurements

A split-pulse spectrometer based on pairs of time-delayed femtosecond pulses can give access to accurate frequency measurements in the extreme ultraviolet (XUV) spectral domain. We demonstrate this approach by measuring the absolute frequency of a single-XUV-photon transition to a bound state of atomic argon excited with the ninth harmonic of an amplified Ti:sapphire laser.

Hyperfine coupling constants of cesium 7D states using two-photon spectroscopy

Doppler-free two-photon transitions of cesium 6S1/2→7D3/2, 5/2 were observed in a thermal stabilized cell. A repeated spectrum of 75 MHz introduced from the side band of an electro-optical modulator served as a frequency marker to improve the accuracy of frequency measurement. The hyperfine magnetic dipole constant A and electrical quadrupole constant B of Cs 7D3/2, 5/2 can be derived from the splitting intervals of the observed spectra. The results are: A=7.36(07) MHz, B=−0.88(87) MHz for the 7D3/2 state, and A=−1.81(05) MHz, B=1.01(1.06) MHz for the 7D5/2 state. These coefficients are improvements for testing high-precision measurements and determining fundamental physical constants.

Optical methods in stringed instrument testing.

Stringed instruments present an experimental challenge for several reasons. Radiated sound is a strong function of dynamic response, so particularly accurate measurements of structural and acoustic resonant frequencies are desirable. However, the structures tend to be very light and sensitive to the additional mass of contacting sensors. Thus, optical methods are attractive. The ready availability of lasers, inexpensive digital cameras, and powerful, inexpensive computers has made optical testing practical. As a result, a range of methods has been applied to musical instruments. There are many ways to organize the methods, but an attractive one is whether they require coherent light for interference effects. A hierarchy of methods is presented along with representative results. The simplest methods simply track motion of a reflected beam across an optical sensor. Coherent methods start with laser vibrometry and move through holography and speckle pattern interferometry. We pay particular attention to an particular application of laser vibrometry and to a low cost speckle pattern interferometer.

A New Airborne Passive DF Method only Based on Frequency Difference

If three antenna units are divided into two set and two baselines are placed at right angles to each other in flight plain, in which the direction of one baseline is parallel to the actual flight direction of air vehicle, the sine and cosine function of target bearing respectively in two baseline directions can be simultaneously obtained according to the analysis principle of the direction cosine change rate. The angulations’ formula only based on Doppler frequency difference can be derived after eliminating the unknown parameters including angular velocity and wavelength by the specific value of two circular functions. The analog calculation shows that the relative error is in direct proportion to the baseline length provided that the incident wave is parallel in derivation. But the error analysis depicts that the measurement accuracy is in direct proportion to the baseline length. Moreover, the measurement error relies on mainly the accuracy of frequency measurement. Furthermore, the derived formula has irregularity in airborne axis direction. However, since the new method is not associated with wavelength, this new DF only based on Doppler frequency difference will be more adapted to passive sounding as compared with phase interference method.

디지털 신호처리기를 이용한 전력계통 주파수 측정에 관한 연구

A frequency in electrical power system changes by the load fluctuation in utility grid, has an influence on a connected generator, and ultimately brings a big trouble in the power system. Therefore, a quick measurement of system frequency and governor control of power system is a very important factor in the reliability and the economic feasibility. Electromagnetic frequency relays in the past had the large power consumption and the difficulty of accurate measurement. After Researched and developed digital relays are very affected by the noise and the distortion, and the recently developed Microprocessor relays have problems of expensive device and time when measuring the frequency at 50[ms]. In this study, An improve algorithm that measures the power system frequency quickly and accurately is suggested, simulated by using Matlab and programmed using C code through DSP6713 KIT. This algorithm is tested to the arbitrary voltage waveform input. The results show that the suggested algorithm is effective in the accurate and quick frequency measurements.

An alternative optical method of determining the unknown microwave frequency by the use of electro-optic materials and semiconductor optical amplifier

There are found different established methods for measuring the frequency of an unknown microwave signal. Several resonating and electronic methods are found where the frequency of a microwave is measured with good performances, where each method has its own advantage. Here in this communication the authors propose a new concept of measuring the frequency of unknown microwave with the joint uses of reflecting semiconductor optical amplifier (RSOA) and electro-optic Pockel cell. To measure the frequency a known microwave source of variable and calibrated frequency is also required. Then with the help of a RSOA and electro-optic material one can find the unknown microwave frequency more accurately than that of any conventional mechanism. This method can extend a high degree of accuracy as optics is used to measure the unknown microwave frequency. As optical signal has million time greater frequency than that of microwave signal therefore a high degree of accuracy of frequency measurement is achieved. The electro-optic material takes the role of phase modulation for splitting an optical wave into several frequencies.

Quantitative determination of allele frequency in pooled DNA by using sequencing method

Quantitative determination of the allele frequency of single-nucleotide polymorphism (SNP) in pooled DNA samples is a promising approach to clarify the relationships between SNPs and diseases. Here, we present such a simple, accurate, and inexpensive method for quantitative determining the allele frequency in pooled DNA samples. Three steps of DNA pooling, PCR amplification and sequencing are involved in this assay. Although direct determination of the allele frequency from the two allele-specific fluorescence intensities is possible, correction for differential response of alleles is important. We explored the effect of differential response of alleles on test statistics and provide a solution to this problem based on heterozygous fluorescence intensities. We demonstrate the accuracy and reliability of this assay on pooled DNA samples with pre-determined allele frequencies from 7.1% to 53.9%. The accuracy of allele frequency measurements is high, with a correlation coefficient of r 2 = 0.997 between measured and known frequencies. We believe that by providing a means for SNP genotyping up to hundreds of samples simultaneously, inexpensively, and reproducibly, this method is a powerful strategy for detecting meaningful polymorphic differences in candidate gene association studies.

New Knudsen effusion apparatus with simultaneous gravimetric and quartz crystal microbalance mass loss detection

A new Knudsen effusion apparatus, enabling simultaneous gravimetric and quartz crystal microbalance mass loss detection, is described. This device allows the measurement of vapour pressures of small sample mass (50 to 100) mg over a wide temperature range (350 to 650) K using very short effusion time intervals. The performance of the apparatus was checked by measuring the vapour pressures of anthracene, benzanthrone, and 1,3,5-triphenylbenzene, between (0.1 and 1) Pa, over temperature intervals of 20 K. The derived standard molar enthalpies of sublimation and vapour pressures are in excellent agreement with the mean of the available literature values and with the recommended values. The new working methodology and design of this apparatus allows the measurement of high quality vapour pressure data due to: accurate temperature measurement and control; improvement in vacuum thermal contact between the effusion cell and the oven metal block; optimisation of the quartz crystal sensor head microbalance position; efficient temperature control of the quartz crystal microbalance head; accurate measurement of the resonance crystal frequency using an impedance circuit analyser methodology.

An improved phasor based algorithm for accurate frequency measurement

A novel algorithm, called the improved phasor based algorithm (IPBA), to estimate the frequency of a power system is proposed. The IPBA calculates the difference of phase angles using transformed consecutive points of a DFT to reduce the leakage effects. The proposed algorithm has the following merits: it has higher precision than a conventional phasor based algorithm; it is immune to inter-harmonics as well as harmonics; it has simple and easy implementation; it has good performance both in steady and dynamic states. What is more, it can keep high precision in a very wide frequency deviation range, for example, 30–70 Hz for a 50 Hz system. The performance of the IPBA is compared with a phasor based algorithm through simulation experiments.

Hardware-based simple, fast and accurate measurement of power system frequency using a hybrid system

All digital methods developed for frequency measurement suffer from one or more factors such as superimposed noise, non-linear static characteristics, high-order harmonics and slow response because of complex computations. The measurement speed of conventional analogue methods is constant and depends on the input signal frequency. Additionally, as a result of deviated and/or spurious zero-crossings caused by the DC component, noise and high-order harmonics in the AC waveform, it is hard to attain the desired accuracy with conventional analogue methods. In this study, a phase-locked loop and microcontroller-based hybrid frequency measurement method developed by the modification of conventional zero-crossing is presented. The input signal, with harmonics and noise, and the frequency can be measured in a fast and accurate way with the proposed method, whose real-time application is extremely simple; this is demonstrated in a comparable manner to the studies in the literature. The measurement method presented in this study is fast, cheap, reliable, accurate and suitable for real-time applications.

Spectral Moment Analysis of Unilateral Vocal Fold Paralysis

Many acoustic measures have been used to assess and track the voices of patients with voice problems. Some of these measures rely on the accurate measurement of fundamental frequency to produce reliable results. Patients with voice disorders often produce voices with considerable quasiperiodicity or aperiodicity. There are other measurements that do not depend on the accurate tracking of fundamental frequency by computing the spectrum of the sound and comparing different parts of the spectrum. The moments of the spectral distribution may also be important measurements to use in patients with voice problems. Several studies have reported good results using these measures to track treatment progress. In this study, spectral moments were used to assess the effectiveness of two treatment approaches in patients with unilateral vocal fold paralysis (UVFP). Twenty-six patients with UVFP and dysphonia (16 female and 10 male patients) were studied. Thirteen underwent surgery to improve their voice, whereas the other 13 received voice therapy. The patients were recorded at three time intervals: before the start of treatment, about 1 month after treatment has been completed, and at 3 months after treatment. They produced three types of speech material, vowels /ah/ and /oo/ and a simple sentence. The first four spectral moments (mean, standard deviation, skewness, and kurtosis) were computed from the long-term average spectrum. Severity of voice dysphonia was rated on a 11-point scale ranging from 0 (normal) to 10 (aphonic). There were no statistical differences between males and females for any of the four moments. There was also no difference between the two treatment types. There were differences among the three types of speech material for moments 1 and 3. There were also differences for moments 1, 2, and 3 for the three treatment conditions with most of the differences occurring between the pretreatment and first posttreatment condition. Severity of dysphonia decreased significantly from the pretreatment to either of the two posttreatment conditions. Spectral moments appear to be viable acoustic measurements to use to assess the effects of treatment on the voice of patients with UVFP.

Eigenvalue based inverse model of beam for structural modification and diagnostics: examples of using

In the work, in order to solve the inverse problem, i.e. the problem of finding values of the additional quantities (mass, elasticity), the beam inverse model was proposed. Analysis of this model allows finding such a value of additional mass (elasticity) as a function of its localization so that the free vibration frequency changes to desirable value. The criteria for choice of the “proper” pair (mass - its position), including the criterion allowing changing the position of the vibration node of the second mode of the free vibrations, were given. Analysis of the influence of uncertainties in the determination of the additional quantity value and its position on the desired free vibration frequency was carried out, too. The proposed beam inverse model can be employing to identification of the beam cracks. In such a case, the input quantity is free vibration frequency measured on the damaged object. Each determined free-vibration frequency allows determining the flexibility curve for the spring modeling crack as a function of its position. The searched parameters of the crack (its depth and position) are indicated by the common point of two arbitrary curves. Accuracy of crack parameters determination depends on accuracy (uncertainty) of frequency measurement. Only some regions containing the searched crack parameters can be obtained in such a situation.

Microwave frequency measurement using a finite impulse response system that incorporates Lorentzian profile-weighted optical taps

We propose an advanced photonic microwave processing scheme useful for the accurate measurement of incoming microwave signal frequency over a wide frequency bandwidth. Our scheme is based on a finite impulse response system using Lorentzian profile-weighted multiple optical wavelength taps. Unlike the conventional schemes based on two optical taps, our scheme allows for well-defined, linear mapping of the incoming microwave frequency onto the output electrical power utilizing a log scale. In order to verify the principle, an experimental demonstration was carried out by using a multiwavelength channel beam, which was produced by the combination of a broadband, continuous wave supercontinuum and a digital micromirror device-based spatial light modulator. An excellent frequency measuring performance over a range of 1–9 GHz was achieved.

Atomic clocks and coherent population trapping: Experiments for undergraduate laboratories

We demonstrate how to construct and operate a simple and affordable apparatus for producing coherent effects in atomic vapor and for investigating their applications in time-keeping and magnetometry. The apparatus consists of a vertical cavity surface emitting diode laser directly current-modulated using a tunable microwave oscillator to produce multiple optical fields needed for the observation of coherent population trapping. This effect allows very accurate measurement of the transition frequency between two ground state hyperfine sublevels, which can be used to construct a coherent population trapping-based atomic clock.

Lattice clocks embrace ytterbium

Accurate frequency measurements of a narrow optical clock transition in 171Yb atoms trapped in an optical lattice establish this system as a serious contender in the quest to develop increasingly accurate atomic clocks.

A Real-Time Smart Sensor for High-Resolution Frequency Estimation in Power Systems

Power quality monitoring is a theme in vogue and accurate frequency measurement of the power line is a major issue. This problem is particularly relevant for power generating systems since the generated signal must comply with restrictive standards. The novelty of this work is the development of a smart sensor for real-time high-resolution frequency measurement in accordance with international standards for power quality monitoring. The proposed smart sensor utilizes commercially available current clamp, hall-effect sensor or resistor as primary sensor. The signal processing is carried out through the chirp z-transform. Simulations and experimental results show the efficiency of the proposed smart sensor.

An Improved Taylor Method for Frequency Measurement in Power Systems

The frequency of a power system is an important operational parameter for the safety, stability, and efficiency of the power system. Accurate and fast measurement of frequency is a goal required in many applications, for example, for load shedding. An improved Taylor approximation method (ITAM) algorithm is proposed, and a comparison of the new algorithm with the conventional Taylor approximation method (TAM) algorithm is presented in this paper under the assumption of the presence of a direct-current (DC) offset component, higher order harmonics, and random noise in the power system signal. The proposed ITAM has the characteristics of low computational and simple hardware requirements, as well as more accurate measurement performance. Another key feature of the algorithm is a good tracing ability for the frequency change and measurement of rate of change of the frequency of a power system.

Experimental test of a two-dimensional approximation for dielectric microcavities

Open dielectric resonators of different shapes are widely used for the manufacture of microlasers. A precise determination of their resonance frequencies and widths is crucial for their design. Most microlasers have a flat cylindrical geometry, and a two-dimensional approximation, the so-called method of the effective index of refraction, is commonly employed for numerical calculations. Our aim has been an experimental test of the precision and applicability of a model based on this approximation. We performed very thorough and accurate measurements of the resonance frequencies and widths of two passive circular dielectric microwave resonators and found significant deviations from the model predictions. From this we conclude that the model generally fails in the quantitative description of three-dimensional dielectric resonators.

Precision measurements of quantum defects in the nP3/2 Rydberg states of 85Rb

Rydberg states are used in our one atom Maser experiment because they offer a large dipole moment and couple strongly to low numbers of microwave photons in a high-Q cavity. Here, we report the absolute frequencies of the P3/2 states for principal quantum numbers n = 36–63. These measurements were made with a three step laser excitation scheme. A wavemeter was calibrated against a frequency comb to provide accurate absolute frequency measurements over the entire range, reducing the measurement uncertainty to 1 MHz. We compare the spectroscopic results with known frequency measurements as a test of measurement accuracy.

Demonstration of a multipulse interferometer for quantum kicked-rotor studies

We implemented a multipulse interferometer scheme that allows us to study a quantum kicked rotor by observing dephasing of momentum coherence. Our study shows that momentum coherence can be nearly perfectly preserved under conditions where the mean energy as a function of the kick number is known to increase without bound. The accompanying width narrowing of these coherences may provide a new method for accurate measurement of the recoil frequency.