Spectrum Of Echo Signal Research Articles

Study of Millimeter-Wave Fuze Echo Characteristics under Rainfall Conditions Using the Monte Carlo Method

Due to the similarity in wavelength between millimeter-wave (MMW) signals and raindrop diameters, rainfall induces significant attenuation and scattering effects that challenge the detection performance of MMW fuzes in rainy environments. To enhance the adaptability of frequency-modulated MMW fuzes in such conditions, the effects of rain on MMW signal attenuation and scattering are investigated. A mathematical model for the multipath echo signals of the fuze was developed. The Monte Carlo method was employed to simulate echo signals considering multiple scattering, and experimental validations were conducted. The results from simulations and experiments revealed that rainfall increases the bottom noise of the echo signal, with rain backscatter noise predominantly affecting the lower end of the echo signal spectrum. However, rain conditions below torrential levels did not significantly impact the detection of strong reflection targets at the high end of the spectrum. The modeling approach and findings presented offer theoretical support for designing MMW fuzes with improved environmental adaptability.

An Efficient Algorithm Based on Spectrum Migration for High Frame Rate Ultrasound Imaging

The high frame rate (HFR) imaging technique requires only one emission event for imaging. Therefore, it can achieve ultrafast imaging with frame rates up to the kHz regime, which satisfies the frame rate requirements for imaging moving tissues in scientific research and clinics. Lu’s Fourier migration method is based on a non-diffraction beam to obtain HFR images and can improve computational speed and efficiency. However, in order to obtain high-quality images, Fourier migration needs to make full use of the spectrum of echo signals for imaging, which requires a large number of Fast Fourier Transform (FFT) points and increases the complexity of the hardware when the echo frequency is high. Here, an efficient algorithm using the spectrum migration technique based on the spectrum’s distribution characteristics is proposed to improve the imaging efficiency in HFR imaging. Since the actual echo signal spectrum is of limited bandwidth, low-frequency and high-frequency parts with low-energy have little contribution to the imaging spectrum. We transform the effective part that provides the main energy in the signal spectrum to the imaging spectrum while the ineffective spectrum components are not utilized for imaging. This can significantly reduce the number of Fourier transform points, improve Fourier imaging efficiency, and ensure the imaging quality. The proposed method is evaluated on simulated and experimental datasets. Results demonstrated that the proposed method could achieve equivalent image quality with a reduced point number for FFT compared to the complete spectrum migration. In this paper, it only requires a quarter of the FFT points used in the complete spectrum migration, which can improve the computational efficiency; thus, it is more suitable for real-time data processing. The proposed spectrum migration method has a specific significance for the study and clinical application of HFR imaging.

Accurate measurement of optical fiber time delay based on frequency domain reflectometry

Optical fiber time delay system has been widely used in optical-controlled phased array antenna, radar distributed network, interferometric optical fiber hydrophone and high-speed photoelectric chip. These applications require high-accuracy and high-stability time delay generated by the system. Time delay measurement directly determines the precision and resolution of the system. Therefore, high-precision time delay measurement method is of great significance in developing the optical fiber delay system. In this paper, progress and problems of optical fiber time delay measurements are discussed. A new method of precisely measuring the time delay in optical fiber is proposed. We use the frequency domain reflectometry (FDR) to avoid the discrepancy between measuring range and measuring precision, which exists in both time-of-flight (TOF) method and phase discrimination approach. An intensity modulated 1064 nm laser signal is used as a light source. The modulation frequency is tuned from 10 MHz to 200 MHz in steps of 0.5 MHz. The spectrum of echo signal is obtained by measuring the amplitudes and phases of echo signals at different frequency points. The delay information is obtained via the inverse fast Fourier transform (IFFT). The precision of delay measurement in our method is determined by step size of frequency variation, and a higher-precision measurement is realized by using interpolation zero algorithm. Since our method is not to modulate the optical frequency, but to control the frequency of the modulation signal loaded on the electro-optic modulator, it is easy to achieve the high-precision and high-linearity frequency modulation. In this paper, theoretical analysis and time delay measurement are used to compare the FDR method with conventional TOF measurement method. The accurate measurement of 33–200 ps is realized, and measurement error is lower than 7 ps. We also design an incremental measurement experiment to study the resolution of the FDR method, which achieves a delay resolution of 3.3 ps. The influence of temperature jitter is analyzed to prove the reliability of experimental results. It proves that the FDR method has a higher measuring accuracy than the TOF method. The time delay measurement precision can be further improved by expanding the modulation bandwidth. Our method is to be applied to an optical fiber delay system to improve the precision and resolution of system delay.

Subsampling Approaches for Compressed Sensing with Ultrasound Arrays in Non-Destructive Testing.

Full Matrix Capture is a multi-channel data acquisition method which enables flexible, high resolution imaging using ultrasound arrays. However, the measurement time and data volume are increased considerably. Both of these costs can be circumvented via compressed sensing, which exploits prior knowledge of the underlying model and its sparsity to reduce the amount of data needed to produce a high resolution image. In order to design compression matrices that are physically realizable without sophisticated hardware constraints, structured subsampling patterns are designed and evaluated in this work. The design is based on the analysis of the Cramér–Rao Bound of a single scatterer in a homogeneous, isotropic medium. A numerical comparison of the point spread functions obtained with different compression matrices and the Fast Iterative Shrinkage/Thresholding Algorithm shows that the best performance is achieved when each transmit event can use a different subset of receiving elements and each receiving element uses a different section of the echo signal spectrum. Such a design has the advantage of outperforming other structured patterns to the extent that suboptimal selection matrices provide a good performance and can be efficiently computed with greedy approaches.

A Class-J Power Amplifier Implementation for Ultrasound Device Applications.

In ultrasonic systems, power amplifiers are one of the most important electronic components used to supply output voltages to ultrasonic devices. If ultrasonic devices have low sensitivity and limited maximum allowable voltages, it can be quite challenging to detect the echo signal in the ultrasonic system itself. Therefore, the class-J power amplifier, which can generate high output power with high efficiency, is proposed for such ultrasonic device applications. The class-J power amplifier developed has a power efficiency of 63.91% and a gain of 28.16 dB at 25 MHz and 13.52 dBm input. The pulse-echo measurement method was used to verify the performance of the electronic components used in the ultrasonic system. The echo signal appearing with the discharged high voltage signal was measured. The amplitude of the first echo signal in the measured echo signal spectrum was 4.4 V and the total-harmonic-distortion (THD), including the fundamental signal and the second harmonic, was 22.35%. The amplitude of the second echo signal was 1.08 V, and the THD, including the fundamental signal and the second harmonic, was 12.45%. These results confirm that a class-J power amplifier can supply a very high output echo signal to an ultrasonic device.

A Mixed Transmitting-Receiving Beamformer With a Robust Generalized Coherence Factor: Enhanced Resolution and Contrast.

Adaptive beamforming has been widely studied for ultrasound imaging over the past few decades. The minimum variance (MV) and generalized coherence factor (GCF) approaches have been validated as effective methods. However, the MV method had a limited contribution to contrast improvement, while the GCF method suffered from severe speckle distortion in previous studies. In this article, a novel ultrasound beamforming approach based on MV and GCF beamformers is proposed to enhance the spatial resolution and contrast in synthetic aperture (SA) ultrasound imaging. First, the MV optimization problem is conceptually redefined by minimizing the total power of the transmitting and receiving outputs. Estimation of the covariance matrices in transmit and receive apertures is carried out and then utilized to determine adaptive weighting vectors. Second, a data-compounding method, viewed as a spatial low-pass filter, is introduced to the GCF method to optimize the spatial spectrum of echo signals and obtain better performance. Robust principal component analysis (RPCA) processing is additionally employed to obtain the final output. Simulation, experimental, and in vivo studies are conducted on different data sets. Relative to the traditional delay-and-sum (DAS) beamformer, mean improvements in the full-width at half-maximum and contrast ratio of 89% and 94%, respectively, are achieved. Thus, considerable enhancement of the spatial resolution and contrast is obtained by the proposed method. Moreover, the proposed method performs better in terms of the computational complexity. In summary, the proposed scheme effectively enhances ultrasound imaging quality.

Radar Simulation of Wake Vortices in Rainy Conditions Based on the Euler-Euler Multiphase Model

The radar monitor of aircraft wake vortex is of great significance to air traffic control. Comparing with the case in clear air, the radar signal of wake vortices in rainy conditions is more easily captured due to the higher levels of signal. To able to analyze radar echo characteristics of evolving wake vortices, the numerical simulation of the evolution of the wake vortices in various rain rates has been performed based on the Euler-Euler frame. The result shows that the rainfall can accelerate the decay of wake vortices. A radar simulator has been represented to process the radar echo signal of raindrops around the wake vortex. The Doppler spectrum of echo signals in radar resolution volumes has been analyzed and the straight vortex and the vortex ring phases show different characteristics of spectrum width. The Doppler width information based on this result of interaction between the wake vortex and rain can be used for assessing the decay phase of wake vortices and monitoring potential hazard for wake vortex encounters.

Unifying Concepts of Statistical and Spectral Quantitative Ultrasound Techniques.

Quantitative ultrasound (QUS) techniques using radiofrequency (RF) backscattered signals have been used for tissue characterization of numerous organ systems. One approach is to use the magnitude and frequency dependence of backscatter echoes to quantify tissue structures. Another approach is to use first-order statistical properties of the echo envelope as a signature of the tissue microstructure. We propose a unification of these QUS concepts. For this purpose, a mixture of homodyned K-distributions is introduced to model the echo envelope, together with an estimation method and a physical interpretation of its parameters based on the echo signal spectrum. In particular, the total, coherent and diffuse signal powers related to the proposed mixture model are expressed explicitly in terms of the structure factor previously studied to describe the backscatter coefficient (BSC). Then, this approach is illustrated in the context of red blood cell (RBC) aggregation. It is experimentally shown that the total, coherent and diffuse signal powers are determined by a structural parameter of the spectral Structure Factor Size and Attenuation Estimator. A two-way repeated measures ANOVA test showed that attenuation (p-value of 0.077) and attenuation compensation (p-value of 0.527) had no significant effect on the diffuse to total power ratio. These results constitute a further step in understanding the physical meaning of first-order statistics of ultrasound images and their relations to QUS techniques. The proposed unifying concepts should be applicable to other biological tissues than blood considering that the structure factor can theoretically model any spatial distribution of scatterers.

Determination of the reflector type from an image reconstructed using echo signals measured with ultrasonic antenna arrays

The application of the digital image focusing (DFA) method to the determination of the types of detected reflectors is considered. For this purpose two antenna arrays (AAs) are used, which are placed on opposite sides of the reflector and using which echo signals are recorded in three acoustic channels in the double-scanning mode. The first and second acoustic channels transmit and receive echo signals using the first and second AA, respectively, and the third channel is tuned so that the first AA transmits pulses and the second AA receives echo signals. Using signals in each channel, many partial images can be reconstructed in a common coordinate system according to different acoustic schemes with allowance for both multiple reflections from irregular boundaries of a tested object and effects of transformation of the wave types. Combining partial images makes it possible to obtain a high-quality image, in which the entire boundary of the reflector is seen and using which an attempt to automate the procedure of evaluating the reflector size and determining its type was made. Such an approach allows one to reduce the subjective influence of an operator on the testing results. It is shown that in order to increase the image resolution, the spectrum of echo signals can be extrapolated by the spectrum splitting method jointly with the construction of an AR model of their spectrum. The results of model experiments that confirm the possibility of determining the reflector type are presented.

The use of radio images for analyzing the results of narrow-band radio location of asteroids

We investigate a method of getting radio images of asteroids by single-position radar with narrowband sounding signal. A model is presented for calculation of power spectra of echo signals of a rotating rough ellipsoid. This model is a useful tool for analyzing the results of radio-location experiments. The accuracy of determination of the shape of an asteroid and its roughness is estimated based on the analysis of two-dimensional radio images.

Radar characteristics of asteroid 33342 (1998 WT24)

The processed data of radar sounding of asteroid 33342 (1998 WT24) with the help of the Yevpatoria-Medicina bistatic radar system are presented. Variations in the power spectra of echo signals with a wavelength of 6 cm are analyzed for two orthogonal polarizations of the echo signal. The obtained data are used for the analysis of the asteroid’s scattering characteristics. The asteroid characteristics for different polarizations are obtained and compared.

Anti-Stokes femtosecond photon echo in a polymer film

An anti-Stokes model is proposed which explains the room-temperature blue shift of the echo-signal spectrum compared to the spectrum of exciting femtosecond pulses in a polymer film doped with dye molecules. The possibility of realising the anti-Stokes regime of coherent laser cooling of such films is analysed.

The possibility of using the maximum-entropy method in ultrasonic nondestructive testing for increasing the resolution of echo signals

The problem of deconvoluting echo signals via regularization according to the maximum entropy method followed by the reconstruction of 2D images via the Fourier transformation synthetic aperture focusing technique (FT-SAFT) is considered. Numerical and experimental simulations have been performed. The possibility of improving the ray resolution of echo signals and reducing the speckle-noise level is demonstrated. The results obtained are compared to the results of constructing an AR model of the spectrum of echo signals.

Extract echo signal in HFR imaging system

Ultrasonic high frame rate (HFR) method was developed by Jianyu Lu in 1997. This method constructs images in frequency domain; the spectrum of space domain objects is calculated based on the spectrum of echo signals, and the image is obtained by using IFFT (inverse fast Fourier transform). Because of this imaging process, the echo signal from each individual scatterer cannot be obtained directly. In this paper, we present a method, which can pick up the echo signal of every scatterer in HFR imaging method. Computer simulation has been performed to verify our method. With the new method we have developed, many other imaging methods, such as Doppler blood flow and elastic imaging, can be easily combined with HFR method to give more information of the imaged objects.

Increasing the signal-to-noise ratio during joint use of the spectrum extrapolation and splitting methods

The possibility of increasing the signal-to-noise ratio by the method of echo-signal spectrum splitting jointly with spectrum extrapolation is considered. The essence of the method proposed is that the echo-signal spectrum known within a given frequency range (fmin, fmax) is divided into several subranges (f min i , f max i ). The construction of the AR model of the echo-signal spectrum allows the spectrum extrapolation from each subrange by an interval (f min e , f max e that far exceeds the initial interval. This means that, with the use of one echo signal, a set of signals can be obtained, the adding together of which increases the signal-to-noise ratio and ensures an ultrafine beam resolution for flaw images. The presented results of processing echo signals, which were obtained in numerical and model experiments, confirm the efficiency of the proposed technique for processing echo signals.

Application of Superresolution Methods for Expert Control of Cylindrical Objects

The possibility of the combined application of the extrapolation procedure to the spectrum of echo signals during construction of its AR model and of the algorithm for the production of flaw images using a multibeam method of projection in the spectral space (PSS) for testing cylindrical objects is shown. Improvement of the quality of flaw images due to a significant decrease in the “side lobe” levels in the point-spread function is demonstrated in a model experiment. These methods may prove to be especially efficient for testing metallurgical flaws in wheel pairs of rolling stock, workpieces for turbine rotors, and similar items.

Echosignal peculiarities in biological mediums

A modified method of Born’s approximation is proposed. The essence of the method is that in real situations, the incident wave gains the object’s speed, but not the medium’s speed; which follows from the Fresnel formula. A solution algorithm for the weak-reflecting bio-organ’s scattering problem is proposed in this task. The algorithm is based on a more precise definition of Born’s approximation. Numerical and experimental research shows that the pathologies can have dimensions of about 1 mm, which can be identified according to echosignal spectrums. It is shown that norm and pathologies, like pectoral glands, are distinguished. If the pathology is less dense, there will be potent destruction in the echosignal spectrum. If pathology is denser, there will be less potent destruction. The proposed method permits one to classify the pathologies in bio-tissues more subtly.

Classification of lake bottom sediments by neural networks using wideband echo signals

The paper presents a method of lake bottom sediment classification by artificial neural networks (ANN) using wideband echo signals. The samples to be classified were acquired by experiments at Lake Geneva. There are five types of sediments, namely, silt, rocks, pebbles, sand, and a mixture of sand and gravel. The pattern features are extracted from spectra of echo signals in subband energy expression. Different subband divisions for frequency-domain feature extraction are compared and it is shown that the contant Q method provides better results in comparison with the constant bandwidth method. Using the constant Q method in association with a BP-type locally connected neural network, 85.1% correct classification in average has been achieved for a testing data set. Wideband echo signals have outstanding superiority for classification in comparison with narrow-band signals. It contains more information representing the physical and architectural features of targets. The neural network ultilizes the information through careful optimization and provides a performance improvement up to 10%.

Cellularity and fibrin mesh properties as a basis for ultrasonic tissue characterization of blood clots and thrombi

This in vitro study was designed to evaluate the ability of ultrasonic tissue characterization (UTC) based on power spectrum analysis of backscattered radio-frequency echo signals to distinguish two prominent variables of thrombi: cellularity (primarily red cell content) and fibrin-mesh density. Six types of clots simulating thrombus components were prepared by varying red-cell and platelet concentrations and shear forces during clotting. Data were acquired with a linear-array transducer, digitized, and analyzed in terms of slope and intercept parameters obtained from normalized power spectra of radio-frequency echo signals. Increased cellularity and fibrin-mesh density both produced lower slope and higher intercept values, which permitted statistically significant discrimination of cellularity and mesh density in the six types of clots analyzed. Shearing forces and (to a lesser degree) platelet concentrations increased fibrin-mesh density. This study suggests that UTC based upon the power spectrum of echo signals may be used to detect and follow compositional differences that have clinical relevance in the diagnosis and follow-up of thrombi.