Digital Phase Comparator Research Articles

Multi-chord IR-visible two-color interferometer on KSTAR.

Major parts of an IR-visible two-color interferometer (TCI) on KSTAR have been upgraded for the multi-chord operation: (1) a diode-pumped-solid-state (DPSS) laser (660nm) replacing the former HeNe laser (633nm), (2) vacuum-compatible vibration isolator with titanium retro-reflectors, and (3) full digital phase comparator for multi-chord real-time density signals. The commercial compact DPSS laser suits the multiple chord configuration with its strong beam power (500 mW) and long coherent length (>100 m). Ti retro-reflectors are mounted on vacuum-compatible vibration isolators. The isolators are essential for the visible beams to avoid any fringe skips due to their short wavelength, considering the speed of the mechanical vibration (up to hundreds of μm). Field-programmable-gate-array (FPGA) modules count the entire fringes fast enough with a signal output rate up to 1.25MHz, solving the fringe skip issues. The FPGA module enables the full digital processing of the phase comparator with a CORDIC algorithm after the sampling rate of 160 MS/s for the 40MHz intermediate frequency of each beam. The full digital signals are transferred to the main plasma control system in real-time. Stable single-input-single-output operation of the KSTAR density control was demonstrated with the TCI. The real-time density profile control is also promising in the near future, with multiple actuators such as pellets and gas puffings.

Zaštita tronamotajnog energetskog transformatora zasnovana na faznoj komparaciji struja

This paper presents a new algorithm for three-winding transformer protection against internal short circuits. The algorithm is based on the application of the digital phase comparison of currents measured at the primary, secondary and tertiary terminals of the power transformer. The applied digital phase comparison does not calculate the angle between the two currents compared in phase, but uses the method of integration of the product of two signals within one basic semi-period of the signals. In order to provide selective protection in the case where the power transformer has only one of its three windings connected to the active network and the other two windings feeding passive networks, an overcurrent function has been added to the digital phase comparator. The paper shows that the application of the new algorithm can successfully detect the internal faults of the three-winding transformer and ensure timely tripping of protection. The signals obtained by computer simulations were used to verify the operation of the proposed algorithm. The algorithm and test results are given in the paper. Index Terms- power system protection, power transformer, digital phase comparison, fault discrimination.

A new dispersion interferometer on HL-2A.

In order to avoid a fringe jump caused by high plasma density and pellet injection [Y. Zhou et al., Rev. Sci. Instrum. 87, 11E107 (2016)], a new CO2 dispersion interferometer is designed and commissioned on HL-2A for average line-density measurement and density feedback control. The second harmonic technology in this system eliminates the phase shift caused by mechanical vibration. Signals are processed by a digital phase comparator and can be monitored in real time. A series of experiments are conducted to study the characteristics of the system such as a second harmonic coefficient and long-term stability. The resolution of density measurement is less than 8 × 1017/m3, and the experiment result on HL-2A demonstrates the interferometer's capability to track plasma density evolution with rapid change. The system also shows good stability against mechanical vibrations.

The algorithm for the detection of loss of excitation of synchronous generators based on a digital-phase comparator

The paper presents an algorithm that is used to detect the loss of excitation (LOE) of synchronous generators. The algorithm is implemented on the principle of a digital phase comparison in the time domain. The digital phase comparator calculates the integral of the instantaneous power of the input signals. The algorithm uses the currents and corresponding line voltages obtained by the measurements available at the terminal of the generator as their input signals. The input signals are phase-compared on the basis of the value of loss of excitation index after which the LOE is detected. The software package MATLAB/Simulink includes the test power system along with the synchronous generator which simulates the phenomenon of LOE. The algorithm has been tested for total and partial LOE and stability power swing for different operating modes of a synchronous generator. The test results show good properties of the proposed algorithm.

Asynchronous Phase Comparator for Characterization of Devices for PMUs Calibrator

This paper reports recent progress in developing a new asynchronous digital phase comparator for the precision measurement of phase difference of voltage ratio devices and calibration of functional elements of phasor measurement units (PMUs) calibrator. The phase error of the proposed digital comparator is below 300 nrad at 50 Hz and 100 $\mu $ rad at 100 kHz with applied voltages ranging between 500 mV and 3 V, whereas the phase error of cables and connectors was estimated to be 4 $\mu \text{rad}$ at 1 MHz. Besides resistive dividers, the phase comparator has been employed for the characterization of frequency behavior of phase difference between the output and input of voltage and transconductance amplifiers for a PMUs calibrator. The system can also be an important tool for phase-frequency characterization of devices employed for specific wideband power measurements.

Prototyping and low-level testing of a 13-MHz digital radio-frequency control system for the PEFP 120-keV heavy ion implanter

The Proton Engineering Frontier Project (PEFP) finished a radio-frequency (RF) implanter. Due to mechanical vibration and temperature slow change in environment, the resonance frequency of the implanter’s cavity always drift. For obtaining an optimized beam output performance, a digital frequency control system was designed to regulate the driver frequency for tracking the frequency variation of the cavity. This paper presents the frequency control scheme and digital algorithm implementations. Traditional analog parts, such as phase comparators, analog proportional-integral (PI) controllers, and voltage-controlled oscillator (VCO), in lieu of the above components, a software code was created. The control logic code was written in VHSIC hardware description language (VHDL) language and was implemented on a field programmable gate array (FPGA) board, which was hosted in a versa module europa (VME) mainframe. The main digital control units, including IQ sampling digital phase comparator, digital PI controller, and numerical controlled oscillator (NCO) units were introduced. The algorithm implementation of the control logic, the loop behavior simulation, and the low-level RF experimental test results will be presented.

Design and implementation of a 150 GHz single-channel millimeter wave interferometer on Joint TEXT tokamak

A simple, single-channel millimeter-wave interferometer system has been designed, fabricated, and installed on the J-TEXT tokamak. For the plasma density anticipated on J-TEXT, a solid-state source operating at 150 GHz has been chosen to minimize errors due to both vibration along the beam path and refraction in the plasma. The new aspect of the interferometer design is to use a subharmonic mixer for detection with a frequency doubled 150 GHz source. It employs a single source which is bias-tuned and modulated with a sawtooth wave form up to 100 kHz in order to generate the intermediate frequency. The 12.5 GHz voltage-controlled oscillator is multiplied to 75 GHz before a final doubler raises it to 150 GHz. A portion of the 75 GHZ power is used for the local oscillator (LO) and is directly connected to the LO input of the subharmonic mixer. The phase is evaluated by a digital phase comparator using a software-based algorithm. Detection noise limits the minimum resolvable phase change with the interferometer to ±0.05 fringe, which corresponds to an averaged electron density change along the chord of ±1.1 × 10(17) m(-2). The maximum measurable electron density is expected to be ∼9 × 10(19) m(-3). A comparison of preliminary results from the millimeter wave interferometer with that from the far-infrared hydrogen cyanide laser (wavelength of 337 μm) interferometer shows good agreement during the pulse flat-top period. The millimeter wave interferometer system will be used as a part of the density feedback control system in the future.

Wideband digital phase comparator for high current shunts

A wideband phase comparator for precise measurements of phase difference of high current shunts has been developed at INRIM. The two-input digital phase detector is realized with a precision wideband digitizer connected to the outputs of the shunts under comparison through a pair of symmetric active guarded transformers. Data are first acquired asynchronously, and then transferred from on-board memory to host memory. Because of the large amount of data collected the filtering process and the analysis algorithms are performed outside the acquisition routine. Most of the systematic errors can be compensated by a proper inversion procedure.The system is suitable for comparing shunts in a wide range of currents, from 2 A up to 100 A, and frequencies ranging between 500 Hz and 100 kHz. Expanded uncertainty (k = 2) less than 50 µrad, for frequency up to 100 kHz, is obtained in the measurement of the phase difference of a group of 10 A shunts, provided by some European NMIs, using a digitizer with sampling frequency up to 1 MHz. An enhanced version of the phase comparator employs a new digital phase detector with higher sampling frequency and vertical resolution. This permits the contribution to the uncertainty budget of the phase detector to be decreased by a factor two from 20 kHz to 100 kHz. Theories and experiments show that the phase difference between two high precision wideband digitizers, coupled as a phase detector, depends on multiple factors derived from the analogue and digital imprint of each sampling system.

The algorithm for directional element without dead tripping zone based on digital phase comparator

This paper describes the algorithm for directional element without dead tripping zone. Instead of calculating phase angle between the phasors of voltage and current the algorithm uses digital phase comparator. The procedure is based on calculation of normalized average power within one semi-period of the input voltage and current signals. Before usage the samples are passed through the Fourier filter to remove high-order harmonics and/or direct component. The dead tripping zone is eliminated by introduction of a voltage delay which is used for determination of direction. In addition, special attention is paid to the problem of synchronization which appears when signal frequency deviates from the assumed value. Sensitivity of the algorithm can be easily adjusted in accordance with the parameters of the network in which a directional element is used. Performances of the method are tested by several computer-generated signals. Since the calculations used in this algorithm are based on simple mathematical operations, the high speed of operation is achieved.

Design of far-infrared three-wave polarimeter-interferometer system for the J-TEXT tokamak

A multichannel three-wave far-infrared polarimeter-interferometer will be constructed on the J-TEXT tokamak (R=1.05 m, a=0.27 m, B(T)≤3 T, and I(P)≤350 kA) for current density profile and electron density profile measurements. The system will adopt the three-wave polarimeter configuration which was first introduced on RTP. Three 432.5 μm HCOOH lasers pumped by three CO(2) lasers separately will be adopted, which could generate high output power, nearly 50 mW at each cavity. Two of them will be counter-rotating circularly polarized to probe the Faraday angle, while the third laser will be used as a local oscillator to get the phase shift caused by electron density. Excellent port access (600×76 mm(2)) and high laser power would promise a profile measurement across the whole plasma section with good signal quality. A high-speed digital phase comparator with a few (∼2) μs temporal resolution will be developed, so that fast changes of current or density profile could be measured. Six channels will be installed in the first stage.

Continuous‐time filter featuring Q and frequency on‐chip automatic tuning

AbstractThe on‐chip automatic tuning of digitally programmable continuous‐time Gm‐C filters with a wide operation range employing a hybrid analog‐digital master‐slave scheme is reported in this work. The master system is implemented with a voltage‐controlled filter (VCF) with the same programmable transconductance cells as the main filter to enhance the matching between master and slave. The frequency tuning method, for both analog and digital control, is based on a digital phase comparator included in the phase‐locked loop achieving high‐precision tunability. The digital frequency tuning is obtained with an error of ±4% and the maximum settling time when changing between two consecutive digital words is 0.3 µs. Once the digital word is fixed, the analog tuning finely adjusts the value of the characteristic frequency to the reference and corrects for possible temperature variations with a settling time of 0.2 µs. The analog frequency control covers the whole discrete step with a maximum error of ±5%. Furthermore, a magnitude‐locked loop (MLL) Q‐tuning block is simultaneously applied to the system in order to precisely control the quality factor of the Gm‐C filter by using an appropriate envelope detector. According to the obtained results, both post‐layout simulation and experimental results, the adopted scheme has resulted as a useful and adequate solution to implement the complete auto‐tuning system of digitally programmable continuous‐time filters, including frequency and Q‐control. Copyright © 2008 John Wiley & Sons, Ltd.

A Digital Phase Comparator for Frequency Modulated Laser Interferometer

A Digital Phase Comparator for Frequency Modulated Laser Interferometer

ECCTD 2007 Special Issue ‘Bridging Technology Innovations to Foundations’

ECCTD 2007 Special Issue ‘<i>Bridging Technology Innovations to Foundations</i>’

Multichannel interferometer system for the helically symmetric experiment

A multichannel millimeter-wave interferometer system has been designed, fabricated and installed on the helically symmetric experiment (HSX), located at the University of Wisconsin, Madison. The interferometer system will view the plasma cross section along nine adjacent chords with 1.5 cm spacing. With this arrangement, coverage will span from the low-field side plasma scrape-off layer to well past the magnetic axis. For the plasma densities anticipated on HSX, a solid-state source operating at 288 GHz will be utilized. At this frequency refraction will be manageable, being less than the channel spacing. The source will be bias-tuned and modulated with a sawtooth wave form at 750 kHz in order to generate the intermediate frequency necessary for the heterodyne detection scheme. The signals will be measured using Schottky-diode corner-cube mixers. The interferometer will have sensitivity nedl≈8×1011 cm−2, being able to measure density changes &amp;lt;1%. Initially, the phase will be evaluated using analog electronics with bandwidth &amp;lt;10 kHz providing real-time line-integrated output. A digital phase comparator scheme will also be implemented whereby the measured wave forms are directly digitized and the phase evaluated using a software-based algorithm. This will increase the time response up to the modulation frequency of 750 kHz. Improved time response will permit measurement of high-frequency density fluctuations along with “fast changes in” the equilibrium profile.

Application of a digital phase comparator technique to interferometer data

Substantial improvements have been made to increase the frequency bandwidth of the UCLA multichannel interferometer system on TEXT-Upgrade. Recently, a digital phase comparator technique was implemented that has several advantages over the conventional analog phase comparator. The probe and reference wave forms are directly digitized and the phase computed in software using a numerical demodulation algorithm. For the purpose of reducing the data load, aliasing of the laser difference frequency is employed to vary the time response according to experimental demands.

A high-performance digital phase comparator

A digital phase comparator has been developed that is especially well suited to making precision phase measurements at high frequency. The circuit consists of two standard phase comparators that are interconnected so that their outputs are averaged to give the desired phase measurement. A clipping circuit consisting of back-to-back Schottky diodes is used to square up the output pulses, which improves the linearity. A variable-modulus digital prescaler is used to change the range from ±1 fringe to ±32 fringes. Instruments using this circuitry are being used to measure transient phase signals from a microwave interferometer that measures the plasma line density on the magnetic-fusion tandem mirror experiment. The resolution is less than 0.1 degree at a carrier frequency of 40 MHz with a maximum nonlinearity of ~0.5%.

Frequency offset locking of AlGaAs semiconductor lasers

Frequency offset locking was proposed as a reliable electrical negative feedback technique for tracking and sweeping of a semiconductor laser frequency. A frequency stabilized laser was used as a master laser, whose residual frequency fluctuations were 140 (kHz) at the integration time (τ) of 100 ms <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\leq \tau \leq 100</tex> s. A digital phase comparator of a large dynamic range of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2\pi \times 2^{11}</tex> (rad) was employed in the feedback loop to reduce the phase fluctuations of the beat signal between the master and slave lasers. Performances of frequency tracking and sweeping of the slave laser were quantitatively evaluated, and the results are: residual frequency fluctuations of the beat signal were reduced as low as 11 (Hz) at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\tau = 100</tex> s, which meant that the residual frequency fluctuations of the slave laser were almost equal to those of the master laser, i.e., the slave laser frequency tracked accurately to the master laser frequency. Both the capture range and lock range of the beat frequency were 1.22 GHz. Frequency tunable range of the slave laser was 36.6 GHz under the condition of frequency offset locking, in which the slave laser frequency fluctuations were maintained as low as the one given above.

Heterodyne CO2-laser interferometer with direct phase read-out

A CO2-laser interferometer with heterodyne detection has been built to measure electron line densities of plasmas in the range of 10(18) to 5 x 10(22) m(-2). Heterodyne operation is accomplished by using two counteracting acousto-optic modulators at a difference frequency of 500 kHz. A digital phase comparator is used to obtain direct read-out of the phase shift. The time resolution is 3.5 micros.