Showing papers on "Harmonic published in 1997"

Generation of coherent soft x rays at 2.7 nm using high harmonics

Abstract: Ultrafast laser pulses from a Ti:sapphire laser centered at 800nm, with 26fs pulse duration, were used to generate coherent soft-x-ray harmonics, at wavelengths down to 2.7nm (460eV) in He, and 5.2nm (239eV) in Ne. In He, discrete harmonic peaks are observed up to order221, and unresolved harmonic emission is observed up to order297. These wavelengths are well within the {open_quotes}water window{close_quotes} region of x-ray transmission. Our work represents the shortest wavelength coherent light generated to date. The harmonic cutoff from all the noble gases is consistent with analytic theory. {copyright} {ital 1997} {ital The American Physical Society}

Class-F power amplifiers with maximally flat waveforms

Abstract: Class-F power amplifiers (PA's) employ harmonic-frequency resonators to shape their drain or collector waveforms to improve efficiency. Generally, the output network must present the drain with either an open or short circuit at the harmonic frequencies. At VHF and higher frequencies, the drain capacitance, lead inductance, lead length, and dispersion make implementation of reasonably ideal tuned circuits difficult. However it is possible to control the impedances at a finite number of harmonics. This note first derives the basic relationships among the Fourier coefficients of the waveforms and the performance of the amplifier. Fourier coefficients for maximally flat waveforms are then derived for inclusion of up to the fifth harmonic. Amplifier performance is then tabulated as a function of which harmonics are included in the voltage and current waveforms. Efficiency increases from 50% of class A toward 100% as harmonics are added. Power-output capability increases by up to 27%.

Nonlinear propagation applied to the improvement of resolution in diagnostic medical ultrasound

Abstract: Medical B-mode scanners operating under conditions typically encountered during clinical work produce ultrasonic wave fields that undergo nonlinear distortion. In general, the resulting harmonic beams are narrower and have lower sidelobe levels than the fundamental beam, making them ideal for imaging purposes. This work demonstrates the feasibility of nonlinear harmonic imaging in medical scanners using a simple broadband imaging arrangement in water. The ultrasonic system comprises a 2.25-MHz circular transducer with a diameter of 38 mm, a membrane hydrophone, also with a diameter of 38 mm, and a polymer lens with a focal length of 262 mm. These components are arranged coaxially giving an imaging geometry similar to that used in many commercial B-scanners, but with a receiver bandwidth sufficient to record the first four harmonics. A series of continuous wave and pulse-echo measurements are performed on a wire phantom to give 1-D transverse pressure profiles and 2-D B-mode images, respectively. The refle...

A new imaging technique based on the nonlinear properties of tissues

Abstract: Finite amplitude sound propagating in a medium undergoes distortion due to the nonlinear properties of the medium. The nonlinear distortion produces harmonic (and subharmonic) energy in the propagating signal. The amplitudes used by commercial medical scanners during routine diagnostic scanning are in most cases finite and thus within the range that produces nonlinear distortion. Thermoviscous absorption of tissue which is frequency dependent rapidly dissipates this harmonic energy. This has led to the widely held assumption that nonlinear distortion was not a significant factor in medical diagnostic imaging. However, the wide dynamic range, digital architecture, and the signal processing capabilities of modern diagnostic ultrasound systems make it possible to utilize this tissue generated harmonic energy for image formation. These images often demonstrate reduced nearfield artifacts and improved tissue structure visualization. Previously, those images were believed to be the result of transmitted second harmonic energy. It is shown that the nonlinear properties of tissue are the major contribution of harmonic images.

Passive harmonic mode locking in soliton fiber lasers

Abstract: We show that low time jitter of a harmonic passively mode-locked laser can be obtained at repetition rates close to acoustic eigen-frequencies of the optical fibre.

Study of the spatial and temporal coherence of high order harmonics

Abstract: We apply the theory of high-order harmonic generation by low-frequency laser fields in the strong field approximation to the study of the spatial and temporal coherence properties of the harmonics. We discuss the role of dynamically induced phases of the atomic polarization in determining the optimal phase matching conditions and angular distributions of harmonics. We demonstrate that the phase matching and the spatial coherence can be controlled by changing the focusing parameters of the fundamental laser beam. Then we present a detailed study of the temporal and spectral properties of harmonics. We discuss how the focusing conditions influence the individual harmonic spectra and time profiles, and how the intensity dependence of the dynamically induced phase leads to a chirp of the harmonic frequency. This phase modulation can be used to control the temporal and spectral properties of the harmonic radiation. Temporally, the harmonic chirped pulse can be recompressed to very small durations. Spectrally, chirping of the fundamental beam may be employed to compensate for the dynamically induced chirp and to control the individual harmonic spectrum. Finally, we discuss the short pulse effects, in particular nonadiabatic phenomena and the possibility of generating attosecond pulses. Comment: Latex file with 37 pages, 25 postscript figures. to appear in Advances in Atomic, Molecular and Optical Physics

A novel overmodulation technique for space vector PWM inverters

Abstract: In this paper, a novel overmodulation technique for space-vector pulsewidth modulation (PWM) inverters is proposed. The overmodulation range is divided into two modes depending on the modulation index (MI). In mode I, the reference angles are derived from the Fourier series expansion of the reference voltage which corresponds to the MI. In mode II, the holding angles are also derived in the same way. The strategy, which is easier to understand graphically, produces a linear relationship between the output voltage and the MI up to six-step operation. The relationship between those angles and the MI can be written in lookup tables or, for real-time implementation, can be piecewise linearized. In addition, harmonic components and total harmonic distortion (THD) of the output voltage are analyzed. When the method is applied to the V/f control of an induction motor, a smooth operation during transition from the linear control range to the six-step mode is demonstrated through experimental results.

Measuring non-axisymmetry in spiral galaxies

Abstract: We present a method for measuring small deviations from axisymmetry of the potential of a filled gas disc. The method is based on a higher order harmonic expansion of the full velocity field of the disc. This expansion is made by first fitting a tilted-ring model to the velocity field of the gas disc and subsequently expanding the velocity field along each ring into its harmonic terms, We use epicycle theory to derive equations for the harmonic terms in a distorted potential. The phase of each component of the distortion can vary with radius. We show that if the potential has a distortion of harmonic number m, the velocity field as seen on the sky exhibits an m - 1 and m + 1 distortion. As is to be expected, the effects of a global elongation of the halo are similar to an m = 2 spiral arm. The main difference is that the phase of the spiral arm can vary with radius, Our method allows a measurement of epsilon(pot) sin 2 phi(2), where epsilon(pot) is the elongation of the potential and phi(2) is one of the viewing angles. The advantage of this method over previous approaches to measure the elongations of disc galaxies is that, by using H I data, one can probe the potential at radii beyond the stellar disc, into the regime where dark matter is thought to be the dominant dynamical component. The method is applied to the spiral galaxies NGC 2403 and 3198 and the harmonic terms are measured up to ninth order. The residual velocity field of NGC 2403 shows some spiral-like structures. The harmonic analysis indicates that the m = 3 term is dominant, with an average value of similar to 0.02v(c). This is consistent with an average ellipticity of the potential of epsilon(pot) sin 2 phi(2) = 0.064 +/- 0.003, but spiral arms may couple significantly to this result. In the harmonic analysis of the kinematics of NGC 3198 the m = 2 and m = 3 terms are strongest (similar to 0.01v(c)), The inferred average elongation of the potential is 0.019 +/- 0.003. Since the amplitude of the elongation is coupled to the viewing angles and may be influenced by spiral arms, more galaxies should be examined to separate these effects from true elongation in a statistical way.

Inverter harmonic reduction using Walsh function harmonic elimination method

Abstract: A pulse-width-modulated (PWM) inverter using the Walsh function harmonic elimination method is proposed in this paper. By using the Walsh domain waveform analytic technique, the harmonic amplitudes of the inverter output voltage can be expressed as functions of switching angles. Thus, the switching angles are optimized by solving linear algebraic equations instead of solving nonlinear transcendental equations. The local piecewise linear relations between the switching angles and the fundamental amplitude can be obtained under an appropriate initial condition. By searching all feasible initial conditions, the global solutions are obtained. The relations between switching angles and fundamental amplitude can be approximated by straight-line curve fitting. Thus, on-line control of fundamental amplitude and frequency is possible for the microcomputer-based implementation. The developed algorithm can be applied to both bipolar and unipolar switching schemes. The theoretical predictions are confirmed by computer simulations and DSP-based hardware implementation.

Ultrasonic diagnostic imaging of response frequency differing from transmit frequency

Abstract: An ultrasonic diagnostic imaging system and methods are described which produces ultrasonic images from harmonic echo components of a transmitted fundamental frequency. Preferably, a programmable digital filter is used to pass harmonic echo components for image processing to the exclusion of fundamental frequency signals. In a preferred embodiment, artifacts are removed by producing decorrelated replicas of the harmonic signals, which are then combined and used for imaging. To produce an image in the presence of depth dependent attenuation of high frequency echo signals, both fundamental and harmonic echo signals are processed and used to produce an image blended from components of both fundamental and harmonic echo signals.

Second-harmonic tomography of tissues.

Abstract: A novel noninvasive second-harmonic-generation tomographic method of mapping the structure of animal tissues by use of 100-fs laser pulses at 625nm is described. Subsurface structures were measured with this approach, which is potentially a symmetry-sensitive tool for optical histological reconstruction.

Ultrabroadband phase-matched optical parametric generation in the ultraviolet by use of guided waves

Abstract: We present what is believed to be the first experimental demonstration of guided-wave phase-matched frequency mixing and harmonic conversion in gases. Broad-bandwidth ultrafast pulses, tunable around 270 nm, were generated from an ultrafast Ti:sapphire amplifier system using 2ω+2ω-ω parametric wave mixing in a capillary waveguide. We achieved nonresonant phase matching by coupling both the fundamental and the second-harmonic light into the lowest-order mode. The output 3ω pulses have an energy of >4 µJ at a 1–kHz repetition rate. Simple extensions of this method can generate higher-energy 10–20-fs pulses tunable throughout the vacuum ultraviolet.

Active power filter control circuit based on the instantaneous active and reactive current i/sub d/-i/sub q/ method

Abstract: This paper presents the control circuit of a shunt active power filter based on the instantaneous active and reactive current i/sub d/-i/sub q/ method The presented method is compared with the instantaneous active and reactive power p-q method under several mains voltage conditions and for different harmonic injection highpass filters The i/sub d/-i/sub q/ method as an excellent harmonic compensation performance and presents the advantage of being frequency-independent The DC voltage regulation system is analysed and its synthesis is performed Extensive simulation results are used to show the stability of the voltage regulation system and to establish the superior performance of i/sub d/-i/sub q/ method based compensator

Second harmonic generation in a photonic crystal

Abstract: Phase matched second harmonic generation is observed experimentally in a centrosymmetric crystalline lattice of dielectric spheres of optical dimensions. The inversion symmetry is broken locally at the surface of each sphere in such a way that the scattered second harmonic light interferes constructively leading to a nonvanishing macroscopic field. Phase matching of the fundamental and second harmonic waves in such periodic lattice is observed to be naturally provided by the bending of the photondispersion curve at the edge of the Bragg reflection band of a given set of lattice planes.

Active Vibration Control of Distributed Systems Using Delayed Resonator With Acceleration Feedback

Abstract: The Delayed Resonator (DR) and Dual Frequency Fixed Delayed Resonator (DFFDR) are newly introduced control techniques for active vibration absorption. Both methods propose a delayed position feedback within the absorber section of the structure to impart ideal resonance features to the absorber. When installed on an oscillating primary body, they form notch filters at their resonance frequencies attenuating the response of the primary structure. The DR absorber is shown to be real-time tunable to time varying disturbance frequencies. In this article, a number of new issues are considered. First, the basic theory is modified for acceleration feedback instead of position, which was originally proposed for the DR methodology. Second, the new absorption methods are implemented on distributed parameter structures which are under high frequency excitation (around 1 KHz). Stability of the combined structure is studied on a reduced order multi-degree-of-freedom primary structure together with the DR absorber. Experimental tests are conducted on a steel beam to verify the analytical findings. Piezoelectric actuators are used both to generate harmonic disturbances and to implement the control. The correspondence observed between the theoretical and experimental results is encouraging. The efficiency of the DR and DFFDR absorption techniques is demonstrated.

A new frequency tracking and phasor estimation algorithm for generator protection

Abstract: Digital generator protection is a complex and difficult problem. Analog and solid state methods have been successfully applied to generator protection in the past and implementation of these functions in a digital device is a continuing trend. This paper explores a new method to implement frequency tracking and phasor estimation in a numerical relay. A new algorithm is presented which utilizes a variable window discrete Fourier transform (DFT) for frequency tracking. Use of the DFT to compute the phasor estimates at a frequency other than the assumed frequency is outlined first. Next, a new technique for tracking the frequency is outlined. The paper concludes with testing of the new algorithm.

Ultrasound imaging method and system for transmit signal generation for an ultrasonic imaging system capable of harmonic imaging

Abstract: An improvement to the method for harmonic imaging including the steps of (a) transmitting ultrasonic energy at a fundamental frequency and (b) receiving reflected ultrasonic energy at a harmonic of the fundamental frequency is provided. For the improvement, the transmitting step includes the step of transmitting a uni-polar waveform or a waveform characterized by an amplitude change rate of 8 or fewer times pre carrier cycle, said waveform comprising an envelope shape rising gradually to a respective maximum value and falling gradually from the respective maximum value.

Analytic Stability Theory for Faraday Waves and the Observation of the Harmonic Surface Response

Abstract: We present an analytic expression for the onset of the Faraday instability, which is applicable to a wide frequency range covering both shallow gravity and deep capillary waves. For sufficiently thin fluid layers, the surface oscillates in harmonic rather than subharmonic resonance with the forcing. An experimental confirmation of this result is reported.

A method for sensorless on-line vibration monitoring of induction machines

Abstract: This paper proposes a method for sensorless on-line vibration monitoring of induction machines based on the relationship between the current harmonics in the machine and their related vibration harmonics. Initially, the vibration monitoring system records two baseline measurements of current and vibration with the machine operating under normal conditions. The baseline data is then evaluated to determine the critical frequencies to monitor on-line. Once these frequencies are determined, the baseline vibration measurement is simply used to scale the current harmonic signal to an estimated vibration level. Based on theoretical analysis, simulation results, and the experimental results shown here, a linear relationship between the current harmonics and vibration level can be assumed. The results of two experiments on a three-phase 230 V, 10 HP induction motor operating under no load are discussed and show the feasibility of this method for sensorless on-line vibration monitoring.

Active EMI filter for switching noise of high frequency inverters

Abstract: In high frequency inverter fed AC motor systems, high frequency leakage current generated by high dv/dt at switching times flows to the ground wire through the stray capacitor between motor windings and core. This paper proposes an active EMI filter to compensate common mode current using high frequency transistors as active elements and a high frequency common mode CT. It works just as a power active filter in the harmonic compensation in the power system. The leakage current is actively suppressed to under 1/50 of the uncompensated circuit. The proposed method is also effective to decrease the EMI more than 40 dB/spl mu/V.

Phase-locked high-order harmonic sources

Abstract: We demonstrate that two harmonic sources generated independently in a xenon gas jet using the same picosecond Nd:YAG laser are locked in phase. The experiment is performed by separating a laser beam into two parallel beams focused at different locations under the nozzle of a gas jet, and therefore producing two independent sources of harmonic radiation, and studying the pattern obtained in the far field when the radiations from these sources interfere. A high and robust fringe visibility is obtained.

The effect of rotor design on sensorless speed estimation using rotor slot harmonics identified by adaptive digital filtering using the maximum likelihood approach

Abstract: Adaptive digital filtering has been demonstrated as an effective technique for extracting a real-time, sensorless, speed signal from rotor slot harmonics (RSHs) embedded in the line current waveform of induction motor drives. It is known that sensorless speed estimation techniques using RSHs may exhibit poor performance with certain motor designs. This paper examines the reasons for that poor performance which reflects differences in the magnitude of the slot harmonic signals consequent upon rotor design. Experimental results for a 30 kW motor with 6 different rotors are presented. The significant parameters are the number of rotor slots, skew and the accuracy of construction. It is clearly shown how inferior performance can arise. Conversely, improvements in reliability of speed estimation and transient response can be obtained by recognising those aspects which provide an enhanced signal, by minimising the background noise of the inverter/machine, or by enhancing the adaptive filter. The recursive maximum likelihood technique is presented as an improved algorithm for tuning the digital filter which aids transient response and reliability of speed estimation. Real time, experimental transient performance is demonstrated for the different rotors used in this paper and the performance failure of a particular slot combination demonstrated.

A new frequency domain arc furnace model for iterative harmonic analysis

Abstract: This paper presents a new frequency domain arc furnace model for iterative harmonic analysis by means of a Newton method. Powerful analytical expressions for harmonic currents and their derivatives are obtained under balanced conditions of the power system. The model offers a three-phase configuration where there is no path for homopolar currents. Moreover, it contemplates continuous and discontinuous evolution of the arc current. The solution obtained is validated by means of time domain simulations. Finally, the model was integrated in a harmonic power flow where studies have been performed in a network with more than 700 busbars and 7 actual arc furnace loads.

Designing harmonic filters for adjustable speed drives to comply with IEEE-519 harmonic limits

Abstract: This paper discusses the application of the revised IEEE-519 Harmonics standards to typical industrial facilities employing adjustable speed drives (ASDs). The harmonic generation characteristics of ASDs are described, Requirements for control of the harmonic currents are developed as a function of the ASD characteristics, overall plant loading level, power system characteristics, and power factor correction requirements. Filter design procedures are presented for controlling the harmonic currents injected onto the power system.

High-efficiency power amplifier integrated with antenna

Abstract: Two class B GaAs field-effect transistor (FET) power amplifiers integrated with patch antennas have been designed and fabricated at 2.48 GHz. Both amplifiers are integrated with patch antennas, which serve as load and radiator. In one case, a standard patch design was used with random harmonic termination. In another case, a modified patch design was used, which allows the tuning of the second harmonic. In this case the antenna has an additional function of a filter. An increase of 7% in the power-added efficiency (PAE) and 0.5 dB in the output power was achieved through the second harmonic tuning.

Circuit, audio system and method for processing signals, and a harmonics generator

Abstract: A circuit, audio system and method are presented for processing an audio signal, in which a frequency band is selected, harmonics are generated from the selected signal by a harmonics generator, wherein the harmonics are scaled by a level detected in at least a part of the spectrum of the audio signal related to the selected frequency band. Furthermore, a harmonic generator is presented for generating arbitrary harmonics of an input signal.