Time-domain Measurement System Research Articles

Time Domain Measurement System for Electric Field of Radio Interference Generated by Corona Discharge

When corona discharge occurs around the wire, it will be accompanied by the movement of space charge, and then generate corona discharge effects such as radio interference and audible noise. In order to reveal time domain characteristics of the electric field of radio interference generated by corona discharge, a set of time domain measurement systems for radio interference, including oscillator antenna, sampling resistance, and data acquisition system, is developed. Based on the characteristics of corona-generated radio interference and antenna theory, the size of the antenna, the equivalent circuit model, and the selection of sampling resistance are discussed carefully to satisfy the requirements for the measurement of corona-generated radio interference. To verify the effectiveness of the measurement system, an experimental platform with a single corona source is built, and the time domain waveforms of corona current and radio interference are simultaneously measured. Besides, the attenuation law of radio interference is obtained by using a time domain measurement system. The results show that the amplitude of the measured voltage of radio interference decreases exponentially with distance. The time domain measurement system presented in this paper can lay a solid foundation for the time-domain evaluation of the radio interference level generated by corona discharge.

A broadband optical fiber transmission-based time domain measurement system for nanosecond-level transient electric field.

A novel nanosecond transient electric field (E-field) measurement system is developed in this paper to measure the E-field pulse caused by the operation of the high-voltage switch (switching E-field pulse) in the substation. An electrically small rod antenna is used as the receiving antenna and is matched by the operational amplifier with high input impedance to achieve broadband frequency response and stable working performance. A broadband analogoptical fiber transmission system is further designed based on the high-frequency circuit model of the electronic components. Unlike the traditional frequency domain E-field measurement methods, the developed measurement system can directly output the time domain waveform of the switching E-field pulse. It also has the advantages of adjustable sensitivity, portability, and anti-electromagnetic interference. The calibrated measurement bandwidth ranges from 200Hz to 680MHz. Furthermore, the switching E-field pulse in an ultra-high voltage substation is measured and analyzed to verify the effectiveness of the fabricated measurement system.

Enhanced diffuse optical tomographic reconstruction using concurrent ultrasound information.

Multimodal imaging is an active branch of research as it has the potential to improve common medical imaging techniques. Diffuse optical tomography (DOT) is an example of a low resolution, functional imaging modality that typically has very low resolution due to the ill-posedness of its underlying inverse problem. Combining the functional information of DOT with a high resolution structural imaging modality has been studied widely. In particular, the combination of DOT with ultrasound (US) could serve as a useful tool for clinicians for the formulation of accurate diagnosis of breast lesions. In this paper, we propose a novel method for US-guided DOT reconstruction using a portable time-domain measurement system. B-mode US imaging is used to retrieve morphological information on the probed tissues by means of a semi-automatical segmentation procedure based on active contour fitting. A two-dimensional to three-dimensional extrapolation procedure, based on the concept of distance transform, is then applied to generate a three-dimensional edge-weighting prior for the regularization of DOT. The reconstruction procedure has been tested on experimental data obtained on specifically designed dual-modality silicon phantoms. Results show a substantial quantification improvement upon the application of the implemented technique.This article is part of the theme issue ‘Synergistic tomographic image reconstruction: part 2’.

Temperature compensated differential acoustic sensor for CO2 sensing

Temperature must be accounted for in order to provide accurate measurements in acoustic wave resonator based gas sensors. We present a flexible printed flexural plate wave (FPW) sensor for CO2 monitoring employing temperature compensation. The FPW sensor is based on a differential interdigitated transducers (IDTs) structure consisting of an input IDT and two identical output IDTs with same distance from the input IDT. Among the two outputs of the differential structure, one is sensitive to the target gas CO2 and the other one is insensitive to the target gas. The difference between the two output signals’ resonant frequencies is considered as the sensor’s differential response. Experiments at 25 ∘C and 45 ∘C show that the sensor’s differential response changes linearly with CO2 concentration over 20–3000 ppm range. The differential responses at both temperatures are very similar showing the ability of the sensor’s structure to compensate for the temperature change. Sensitivity and limit of detection for CO2 sensing are measured to be 543 Hz ppm−1 and 5.6 ppm, respectively. Further, the sensor shows good repeatability and low hysteresis. Sensor’s responses are mostly measured by measuring the scattering parameter, forward transmission (S21) between each output IDT and input IDT of the sensor in frequency domain. In the last section, a prototype time domain measurement system is demonstrated to show the feasibility of the time domain measurements with the sensors. The sensor with the potential to be printed with roll-to-roll printing can be a good candidate for CO2 monitoring where temperature varies.

Study on the influence of coil configuration on electromagnetic characteristics of inductively coupled plasma superimposed frequency selective surface

Aiming at the limitation of frequency selective surface (FSS) used in radome, a new type of active frequency selective surface (AFSS) based on inductively coupled plasma (ICP) is proposed. The structure consists of a square plasma discharge cavity with a size of 20 × 20 × 5 cm3 and a FSS with broadband bandpass response. By comparing the simulation results of COMSOL Multiphysics with the experimental results of microwave interference diagnosis, the influence of coil configuration on the characteristic parameters of ICP source is analyzed. The transmissivity of ICP-FSS with different coil configurations was measured by time domain measurement system. The experimental results show that the bandwidth of passband of the structure is 6.4 GHz–19.1 GHz when the ICP source is not excited. When the ICP source is excited, the ICP-FSS structure presents different stopband transmission characteristics under different coil configurations. The bandwidth of stopband widens as the RF power increases, and the transmission coefficient valley also increases, which can effectively reduce the detection probability of the radar in the off state. Therefore, the proposed AFSS based on inductively coupled plasma is an effective candidate for radome application.

A Referenceless Antenna Measurement System Based on Software-Defined Radio [Measurements Corner

This article introduces a low-cost time-domain near-field measurement system based on softwaredefined radio (SDR). The method is based on using a low-cost receiver to retrieve by Fourier techniques the amplitude and phase of the signal radiated by antennas. The uncertainties introduced by the receiver are modeled and compared with empirical data. Moreover, a setup based on a reference antenna is explored as a possible solution to fix the phase reference of the measurement. The limitations and optimization of the setup are analyzed for different geometries. Some postprocessing techniques are demonstrated to be suitable to overcome the inherent errors introduced by the setup. Once the amplitude and phase are recovered, diagnostic techniques or near-field to far-field transformation can be applied. The results demonstrate the potential and good accuracy of the method for fast and lowcost characterization of antennas when the reference is not accessible.

High-temperature time domain measurement system for solid and molten materials

A low-cost time domain system for measuring dielectric relaxation in the frequency range of 1 Hz–1 MHz of solid and molten materials at high temperatures is described in detail. A parallel plate electrode sample cell is described that can hold molten material by capillary action. In order to avoid electrical breakdown or a change in electrode distance due to opposite charge buildup, the step voltage can be set to the mV range. At this level interference noise affects the measurements and active filtering is required. The resulting signal is captured using a digital storage oscilloscope. An algorithm for reducing the measured data prior to data processing is discussed. The system is calibration free and its upper temperature is limited only by the construction materials of the furnace and electrodes. Molten NaCl is presented as a test sample.

A Time-Domain Measurement System for UWB Microwave Imaging

A fully automatic time-domain measurement system for ultra-wideband (UWB) microwave imaging is presented. The system uses a pair of movable antennas to transmit and receive custom UWB pulse designs with each antenna able to be independently controlled to rotate about a region of interest. A full measurement, consisting of both incident and total field collections, is completed within 20 min using custom software. The collected data is processed to reconstruct images of three test objects. Imaging results demonstrate the system can localize and recognize different shaped objects, and differentiate multiple objects. The precision of antenna positioning and the impact of random noise on the microwave signal are investigated. The maximal positioning error for the antennas allows for accurate use of the system up to 7 GHz. A comparison between the time-domain system and a frequency-sweep method shows both methods allow for accurate imaging results. The signal-to-noise ratio and image quality may be improved with time-domain results when properly selecting a time window for the received signal.

Measurement and analysis of time-domain characteristics of corona-generated radio interference from a single positive corona source

The corona-generated electromagnetic interference commonly known as radio interference (RI) has become a limiting factor for the design of high voltage direct current transmission lines. In this paper, a time-domain measurement system is developed to measure the time-domain characteristics of corona-generated RI from a single corona source under a positive corona source. In the experiments, the corona current pulses are synchronously measured through coupling capacitors. The one-to-one relationship between the corona current pulse and measured RI voltage pulse is observed. The statistical characteristics of pulse parameters are analyzed, and the correlations between the corona current pulse and RI voltage pulse in the time-domain and frequency-domain are analyzed. Depending on the measured corona current pulses, the time-domain waveform of corona-generated RI is calculated on the basis of the propagation model of corona current on the conductor, the dipolar model for electric field calculation, and the antenna model for inducing voltage calculation. The well matched results between measured and simulated waveforms of RI voltage can show the validity of the measurement and calculation method presented in this paper, which also further show the close correlation between corona current and corona-generated RI.

Experimental Demonstration of In‐Place Calibration for Time Domain Microwave Imaging System

AbstractIn this study, the experimental demonstration of in‐place calibration was conducted using the developed time domain measurement system. Experiments were conducted using three calibration methods—in‐place calibration and two existing calibrations, that is, array rotation and differential calibration. The in‐place calibration uses dual receivers located at an equal distance from the transmitter. The received signals at the dual receivers contain similar unwanted signals, that is, the directly received signal and antenna coupling. In contrast to the simulations, the antennas are not perfectly matched and there might be unexpected environmental errors. Thus, we experimented with the developed experimental system to demonstrate the proposed method. The possible problems with low signal‐to‐noise ratio and clock jitter, which may exist in time domain systems, were rectified by averaging repeatedly measured signals. The tumor was successfully detected using the three calibration methods according to the experimental results. The cross correlation was calculated using the reconstructed image of the ideal differential calibration for a quantitative comparison between the existing rotation calibration and the proposed in‐place calibration. The mean value of cross correlation between the in‐place calibration and ideal differential calibration was 0.80, and the mean value of cross correlation of the rotation calibration was 0.55. Furthermore, the results of simulation were compared with the experimental results to verify the in‐place calibration method. A quantitative analysis was also performed, and the experimental results show a tendency similar to the simulation.

Channel measurement and modeling for the 15-GHz radio band in an indoor corridor environment

Traditional mobile communication systems mainly work on the licensed frequency band near or below 3 GHz; however, this band is becoming increasingly crowded. On the other hand, there are abundant unlicensed spectrum resources in higher frequency bands, e.g., 6 GHz, 15 GHz, and 28 GHz, and if those bands are applied, the current spectrum shortage problem could be effectively alleviated. However, the wireless channel characteristics and models are important but still unknown, and thus in this study, extensive measurements and modeling have been conducted to study the characteristics of the high-frequency 15-GHz band. Specifically, a PN(Pseudo Noise) sequence based time-domain measurement system was built and applied to measure the propagation characteristics of the LOS (Line-of-Sight) and NLOS (Non-Line-of-Sight) scenarios in an indoor corridor at 15 GHz. Then, in-depth analysis and modeling on the large-scale characteristics of wireless channels, the relationship between distance and path loss, the path loss exponent, and the shadow fading standard variance are provided. Moreover, the relationship between received power and different elevation angles was studied. In the measurement, two 25-dBi horn antennas with a 10 half-power beam width are used to change elevation angles in the transmitting terminal and azimuth angles in the receiving terminal for all measurement points. The findings and results in this work will serve as a reference and basis for future theoretical studies of the 15-GHz band.

Improvement of dynamic losses in pulse CW characterisation of non‐linear RF devices

A time-domain (TD) measurement system that improves dynamic losses for pulsed radiofrequency characterisation of non-linear devices is presented. This system allows the measurement of voltage and current TD waveforms at very high sampling rate at both ports of the device under test. The fully calibrated system is used to accurately characterise transients in a 40 W GaN wideband high power amplifier under pulsed signal excitation.

Decomposition of Electromagnetic Interferences in the Time-Domain

Electromagnetic interferences are potentially very complex signals formed by the superposition of transient (broadband) and continuous wave (narrowband) components with significant randomness in both amplitude and phase. Decomposing the electromagnetic interference measured in the time domain into a set of intrinsic mode functions is useful to gain insights of the process that generates the interference. Evaluating the intrinsic mode functions contributes to improving the measurement capabilities of the time-domain electromagnetic emissions measurement systems based on the general-purpose oscilloscopes. In this paper, a combination of techniques that includes empirical mode decomposition and transient mode decomposition is used to separate the main components of complex electromagnetic disturbances. This approach requires no prior information on the spectral content of the measured EMI and it does not perform a domain transformation. Examples of electromagnetic interference decomposition verify the effectiveness and the accuracy of the proposed approach. Finally, a discussion on the advantages, practical applications, limitations, and drawbacks of the described techniques is addressed.

On the Statistical Properties of the Peak Detection for Time-Domain EMI Measurements

This paper presents a discussion on the inherent characteristics of the measurements performed with time-domain electromagnetic interference measurement systems in regards of the detection of the maximum emissions levels. In that sense, some relevant statistical properties of the frequency components of the maximum emissions levels in the amplitude spectrum are investigated using the extreme value theory to provide a model based on the Gumbel probability distribution and estimates for its parameters, expected value, variance, and Cramer-Rao bounds. The results suggest that using the expected maximum value of the emissions levels instead of the just the observed maximum value improves the measurement repeatability and also reduces the uncertainty in the results. This paper presents an additional insight measure that enhances our understanding of the statistical behavior of the measured EMI and of the time-domain measurement process itself.

Dependence of the colored frequency noise in spin torque oscillators on current and magnetic field

The nano-scale spin torque oscillator (STO) is a compelling device for on-chip, highly tunable microwave frequency signal generation. Currently, one of the most important challenges for the STO is to increase its longer-time frequency stability by decreasing the 1/f frequency noise, but its high level makes even its measurement impossible using the phase noise mode of spectrum analyzers. Here, we present a custom made time-domain measurement system with 150 MHz measurement bandwidth making possible the investigation of the variation of the 1/f as well as the white frequency noise in a STO over a large set of operating points covering 18–25 GHz. The 1/f level is found to be highly dependent on the oscillation amplitude-frequency non-linearity and the vicinity of unexcited oscillation modes. These findings elucidate the need for a quantitative theoretical treatment of the low-frequency, colored frequency noise in STOs. Based on the results, we suggest that the 1/f frequency noise possibly can be decreased by improving the microstructural quality of the metallic thin films.

Terahertz Imaging With a Time-Reversed Finite Difference Time-Domain Method

A novel approach to three-dimensional terahertz (THz) imaging based on a time-reversed finite difference time-domain (FDTD) method is presented. Time-reversed convolutional perfectly matching layers (CPML) were implemented as boundaries to truncate the simulation domain. The proposed computationally efficient technique is capable of producing detailed images using a much smaller set of measured data compared to conventional terahertz imaging methods. A coarser grid of transmitter/receiver locations is needed, saving significant data acquisition time. Transmission and reflection imaging configurations in far-field were investigated. Simulations as well as experimental results, including comparisons with conventional methods, are presented. Our approach was validated experimentally using a time-domain terahertz measurement system.

Improvement in Spectral Properties of Quantization Noise of Harmonic Signal Using Multiresolution Quantization

Multiresolution quantization (MQ) is used in modern equipment designed for wideband spectrum measurements with high dynamic range. Such time-domain measurement systems significantly reduce measurement time. Only basic theoretical analyses of the possible influences of MQ on the fidelity of the measured spectrum have been performed until today. Therefore, the real contribution of the method to the quality of measurement should be clarified. In this paper, a new model of quantization noise is proposed for MQ. It was derived from a model of power spectral density previously used for uniform quantization. A system with two different quantization steps used for the quantization of harmonic input signal is considered. The model suitability for such conditions is proved by simulation and confronted with experimental results. Multiresolution measurements were acquired using a dual-channel data acquisition card and personal-computer postprocessing.

A Large Aperture UWB Antenna Array for Real Beam Radar Imaging

The development of four-element ultra-wideband (UWB) comb taper slot antenna array with 18 cm element spacing for real beam radar imaging is described. The four-element UWB array system with optimum element spacing is analyzed by energy pattern. A wideband double ridge horn antenna is used as the transmitting antenna, the developed large aperture UWB array is used as the receiving antenna. The transmitting antenna and the receiving antenna are combined with impulse time domain measurement system to achieve real beam radar imaging. The receiving impulse signals at various positions are processed by the time delay and sum algorithm. The examples of several aluminum cans have been verified in the resolution and compared with using the UWB array as a receive antenna and the double ridge horn as a transmit antenna in the test setup. The crossrange resolution of UWB antenna array is better than wideband double ridge horn antenna because the beam width of UWB array is narrower.

4-Channel, time-domain measurement system using track and hold amplifier for the characterization and linearization of high-power amplifiers

We propose in this paper a 4-channel time-domain test bench for the characterization and linearity enhancement of microwave power amplifiers (PAs). The proposed time-domain measurement system utilizes four track and hold amplifiers (THAs) for direct subsampling of radiofrequency (RF) signals. The use of wideband THAs to replace samplers or mixers enables reducing analog IF circuit complexity. It permits direct digitization of RF signals like CW, two-tone and pulsed modulated signals, bringing more flexibility in the receiver's performance by enhancing the dynamic range and bandwidth. This test bench is capable of completely extracting the phase, amplitude, and transfer characteristics of non-linear devices excited with CW or modulated RF signals. In this work, two-tone transfer characteristics of a 50 W GaN HEMT Nitronex PA were extracted and processed for applying digital pre-distortion linearization to enhance linearity performance. Time-domain envelope and carrier waveforms (voltage and current) along with third-order inter-modulation distortion (IMD) products with and without digital pre-distortion scheme are also presented.

Measurement of luminescence decays: High performance at low cost

The availability of inexpensive ultra bright LEDs spanning the visible and near-ultraviolet combined with the availability of inexpensive electronics equipment makes it possible to construct a high performance luminescence lifetime apparatus (∼5 ns instrumental response or better) at low cost. A central need for time domain measurement systems is the ability to obtain short (∼1 ns or less) excitation light pulses from the LEDs. It is possible to build the necessary LED driver using a simple avalanche transistor circuit. We describe first a circuit to test for small signal NPN transistors that can avalanche. We then describe a final optimized avalanche mode circuit that we developed on a prototyping board by measuring driven light pulse duration as a function of the circuit on the board and passive component values. We demonstrate that the combination of the LED pulser and a 1P28 photomultiplier tube used in decay waveform acquisition has a time response that allows for detection and lifetime determination of luminescence decays down to ∼5 ns. The time response and data quality afforded with the same components in time-correlated single photon counting are even better. For time-correlated single photon counting an even simpler NAND-gate based LED driver circuit is also applicable. We also demonstrate the possible utility of a simple frequency domain method for luminescence lifetime determinations.