Acquisition Bandwidth Research Articles

Under-Sampling Digital Predistortion of Power Amplifier Using Multi-Tone Mixing Feedback Technique

An under-sampling digital predistortion based on a multi-tone mixing feedback technique (MTM-DPD) is presented in this article. In contrast to the under-sampling digital predistortion (DPD) based on conventional receivers, the proposed method could simultaneously reduce the sampling rate and acquisition bandwidth using a multi-tone mixing (MTM)-based feedback channel. The MTM-DPD is implemented with an evolved direct learning architecture, where the coefficients of a predistorter are estimated based on the aliased feedback signal, preprocessed input signal, and preprocessed kernels of the DPD model. To preprocess the input signal and kernels, the behavioral model of the MTM-based feedback channel and its corresponding identification algorithm are analyzed in detailed. Experimental results show that for a gallium nitride (GaN)-based power amplifier (PA) with 40-MHz input, under the excitation of a 17 tone LO signal with frequency interval 7.68 MHz, the MTM-DPD with sampling rate 15.36 MSPS and acquisition bandwidth 15.36 MHz could improve the adjacent channel power ratio (ACPR) from −30.1 to −46.5 dBc and the error vector magnitude (EVM) from 9.13% to 1.76%. Compared with the conventional full sampling DPD (FS-DPD), the MTM-DPD could reduce the cost of analog-to-digital converters (ADCs) in feedback channel significantly while maintaining satisfactory linearization performance.

A Low Sampling Rate Memory-Grouped Method for Digital Predistortion With Constrained Acquisition Bandwidth

A novel memory-grouped method is proposed for digital predistortion (DPD) of power amplifiers (PAs) with constrained acquisition bandwidth (BW) and extremely low sampling rate. By grouping the basis functions with different memory delays, a two-step algorithm can reduce overfitting in the constrained-BW technique. As a result, excellent DPD performance can be achieved even when the acquisition BW is constrained to several times narrower than the BW of the original input signal. Thus, using the memory-grouped method, a very low-speed analog-to-digital converter (ADC) can be used to sample the constrained feedback signal without aliasing, significantly reducing the cost of configuring a high-speed ADC. The performance of the proposed method is compared with the existing methods in different scenarios. Experimental results show that when the feedback BW is less than the BW of the original input signal, the proposed method can still achieve good performance, while the traditional methods fail to work. The experiment results also imply that the memory-grouped method can reduce the sampling rate by more than ten times.

Bandwidth compensation in swept laser source using the asymmetric sinusoidal modulation

Broadband, high-speed wavelength-swept laser sources can substantially enhance applications in optical coherence tomography (OCT), metrology, fiber-optic sensing, and so on. Fiber Fabry-Perot tunable filters (FFP-TF) are active wavelength selection media for wavelength-swept laser sources because of their wide free spectral range, high finesse values, and more compactness. However, FFP-TF is irreversible and conventionally driven using a sinusoidal electronic waveform, thereby limiting the required acquisition bandwidth. Here, we introduce a new approach based on bandwidth compensation of the continuous-output, asymmetric sinusoidal-mode driving of FFP-TF, where output wavelength is tuned coincidentally to the forward and backward sweeps during the driving cycle. A detailed structure framework and the comparison of the sweep range of the swept laser source under convention sinusoidal and asymmetric sinusoidal-mode driving is discussed. A swept laser source with a sweep rate of 66.7 kHz over a 161 nm range at a center wavelength of 1310 nm is demonstrated. The result shows that the asymmetric sinusoidal-mode is a suitable driving medium for FFP-TF for swept laser sources with increased output bandwidth.

19F MRI Imaging Strategies to Reduce Isoflurane Artifacts in In Vivo Images

PurposeIsoflurane (ISO) is the most commonly used preclinical inhalation anesthetic. This is a problem in 19F MRI of fluorine contrast agents, as ISO signals cause artifacts that interfere with unambiguous image interpretation and quantification; the two most attractive properties of heteronuclear MRI. We aimed to avoid these artifacts using MRI strategies that can be applied by any pre-clinical researcher.ProceduresThree strategies to avoid ISO chemical shift displacement artifacts (CSDA) in 19F MRI are described and demonstrated with measurements of 19F-containing agents in phantoms and in vivo (n = 3 for all strategies). The success of these strategies is compared to a standard Rapid Acquisition with Relaxation Enhancement (RARE) sequence, with phantom and in vivo validation. ISO artifacts can successfully be avoided by (1) shifting them outside the region of interest using a narrow signal acquisition bandwidth, (2) suppression of ISO by planning a frequency-selective suppression pulse before signal acquisition or by (3) preventing ISO excitation with a 3D sequence with a narrow excitation bandwidth.ResultsAll three strategies result in complete ISO signal avoidance (p < 0.0001 for all methods). Using a narrow acquisition bandwidth can result in loss of signal to noise ratio and distortion of the image, and a frequency-selective suppression pulse can be incomplete when B1-inhomogeneities are present. Preventing ISO excitation with a narrow excitation pulse in a 3D sequence yields the most robust results (relative SNR 151 ± 28% compared to 2D multislice methods, p = 0.006).ConclusionWe optimized three easily implementable methods to avoid ISO signal artifacts and validated their performance in phantoms and in vivo. We make recommendation on the parameters that pre-clinical studies should report in their method section to make the used approach insightful.

An Intelligent In-Shoe System for Gait Monitoring and Analysis with Optimized Sampling and Real-Time Visualization Capabilities.

The deterioration of gait can be used as a biomarker for ageing and neurological diseases. Continuous gait monitoring and analysis are essential for early deficit detection and personalized rehabilitation. The use of mobile and wearable inertial sensor systems for gait monitoring and analysis have been well explored with promising results in the literature. However, most of these studies focus on technologies for the assessment of gait characteristics, few of them have considered the data acquisition bandwidth of the sensing system. Inadequate sampling frequency will sacrifice signal fidelity, thus leading to an inaccurate estimation especially for spatial gait parameters. In this work, we developed an inertial sensor based in-shoe gait analysis system for real-time gait monitoring and investigated the optimal sampling frequency to capture all the information on walking patterns. An exploratory validation study was performed using an optical motion capture system on four healthy adult subjects, where each person underwent five walking sessions, giving a total of 20 sessions. Percentage mean absolute errors (MAE%) obtained in stride time, stride length, stride velocity, and cadence while walking were 1.19%, 1.68%, 2.08%, and 1.23%, respectively. In addition, an eigenanalysis based graphical descriptor from raw gait cycle signals was proposed as a new gait metric that can be quantified by principal component analysis to differentiate gait patterns, which has great potential to be used as a powerful analytical tool for gait disorder diagnostics.

Optical Vernier sampling using a dual-comb-swept laser to solve distance aliasing

Optical interferometry using comb-swept lasers has the advantage of efficiently reducing the acquisition bandwidth for high-speed and long-range detection. However, in general, the use of a comb-swept laser involves a critical limitation in that the absolute distance cannot be measured, and, thus, multiple layers cannot be distinguished when measuring each position. This is because of the distance ambiguity induced by optical aliasing, in which there is periodic repetition of the frequency of an interferometric signal owing to discrete spectral sweeping, which does not occur in conventional optical interferometry that uses a continuous swept laser. In this paper, we introduce an optical Vernier sampling method using a dual-comb-swept laser to measure the absolute distances in a multi-layer target. For this, we designed a new type of dual-comb-swept laser to include two different free spectral ranges (FSRs) in separated wavelength bands to provide a stable lasing condition. Using a principle similar to that of a Vernier caliper for length measurement, the two different FSRs can be used to recover a higher frequency of an optical interferometric signal to measure longer distances from different layers in a target. Using the dual-comb-swept laser in optical interferometry, we solved the optical aliasing issue and measured the absolute distances of three layers separated over 83 mm using a point-scanning imaging setup and the simultaneous absolute distance of the top surfaces separated over 45 mm using a full-field imaging setup at 14 and 8 times lower acquisition bandwidth than a conventional continuous swept laser that is based on optical interferometry.

Time-expanded phase-sensitive optical time-domain reflectometry

Phase-sensitive optical time-domain reflectometry (ΦOTDR) is a well-established technique that provides spatio-temporal measurements of an environmental variable in real time. This unique capability is being leveraged in an ever-increasing number of applications, from energy transportation or civil security to seismology. To date, a wide number of different approaches have been implemented, providing a plethora of options in terms of performance (resolution, acquisition bandwidth, sensitivity or range). However, to achieve high spatial resolutions, detection bandwidths in the GHz range are typically required, substantially increasing the system cost and complexity. Here, we present a novel ΦOTDR approach that allows a customized time expansion of the received optical traces. Hence, the presented technique reaches cm-scale spatial resolutions over 1 km while requiring a remarkably low detection bandwidth in the MHz regime. This approach relies on the use of dual-comb spectrometry to interrogate the fibre and sample the backscattered light. Random phase-spectral coding is applied to the employed combs to maximize the signal-to-noise ratio of the sensing scheme. A comparison of the proposed method with alternative approaches aimed at similar operation features is provided, along with a thorough analysis of the new trade-offs. Our results demonstrate a radically novel high-resolution ΦOTDR scheme, which could promote new applications in metrology, borehole monitoring or aerospace.

Time-Domain Characterization and Linearization of a Dual-Input Power Amplifier Using a Vector Network Analyzer as the Receiver

A newly calibrated testbed based on a commercial vector network analyzer (VNA) is proposed for the fast in-band vector signal characterization and linearization of nonlinear multiport devices. The testbed can acquire both CW and wideband modulated periodic signals without breaking contact with the DUT, in up to five channels. The time-domain measurements are made possible by calibrating all the receivers in the frequency domain. This calibration provides a phase and amplitude correction for the modulation tones of all the incident and reflected waves in the acquisition bandwidth. The measurement bandwidth is determined at calibration by the bandwidth of the periodic OFDM signals used as a phase standard. It is independent of the VNA receivers' bandwidth. A 500-MHz 100 000-tones 64-QAM OFDM constellation is recovered with 0.48% error vector magnitude (EVM). As an application, a dual-input hybrid Doherty-outphasing power amplifier (HD-OPA) is characterized at 2.08 GHz. A CW signal is used to determine the outphasing angle and input power levels, which provides the best drain and power-added efficiencies for each output power. The dynamic response of the dual-input HD-OPA is acquired on five channels by using the same testbed. Various periodic OFDM signals, with 24 000 tones, are measured before and after predistortion linearization in a fraction of a second.

Time Domain Discrete Fourier Domain Mode Locked Laser With k-Space Uniform Comb Lines

Frequency comb swept lasers or wavelength stepped swept lasers with comb filters have been proposed in optical domain subsampling or circular-ranging optical coherence tomography (OCT) to achieve high axial scan rate and extended imaging range with a given acquisition bandwidth. However, the comb filters lack flexibility in varying the free spectral range (FSR). In this article, we propose and demonstrate a novel discrete Fourier domain mode locked (FDML) laser by using intracavity optical intensity modulation. Swept signals with comb lines uniformly distributed in the frequency domain is generated with tailor-made pulse patterns according to the sweep trace of the swept filter. Discrete k -space uniform swept signals with different FSRs and pulse durations are generated in the same laser. The FDML laser discretized by temporal modulation is more stable than that with a comb filter. The discrete FDML lasers will be potential sources of OCT and the fast-developing swept source LiDAR.

High Resolution Laser Self-Mixing Displacement Sensor Under Large Variation in Optical Feedback and Speckle

Self-Mixing interferometry (SMI) signal characteristics are highly dependent on both the operating optical regime and target surface. In this paper, a method is proposed to overcome some identified limitations of the even power scalable algorithm (EPSA), such as the required operating regime to be weak feedback. In addition, by using the up-sampling techniques, the number of stages involved in the even-power scalable algorithm can be drastically increased without any data acquisition bandwidth modification. Here, 10 successive stages have been successfully implemented to achieve a theoretical resolution of ${\lambda } _{\sf 0}/ {\sf 2}^{\sf 13}$ . It is further shown that the proposed method can handle and process weak, moderate and strong feedback regime as well as speckle affected SMI signals more efficiently. Lastly, FPGA based hardware emulation of EPSA is also done for later embedded implementation of this high-resolution algorithm. FPGA synthesis results show that the designed system can measure maximum target velocity up to 1.18 m/s while consuming total power of 1.4W.

Spaser Nanoparticles for Ultranarrow Bandwidth STED Super-Resolution Imaging.

Super-resolution microscopy, as a powerful tool of seeing abundant spatial details, typically can only distinguish a few distinct targets at a time due to the spectral crosstalk between fluorophores. Spaser (i.e., surface plasmon laser) nanoprobes, which confine lasing emission into nanoscale, offer an opportunity to eliminate such obstacle. Here, realized is narrow band stimulated emission depletion (STED) nanoscopy on spaser nanoparticles by collecting the coherent spasing signals. Demonstrated are the physics concept and feasibility of erasing spaser emission by using a depletion beam to suppress the population inversion, which lays the foundation of spaser-based STED super-resolution. Thanks to the small size (47 nm) and narrow spectral linewidth (3.8 nm) of the spaser nanoparticles, a 74 nm spatial resolution in STED imaging within an acquisition bandwidth of 10 nm is finally obtained. These spaser nanoparticles, if multiplexing with different wavelengths, in principle, allow for spectral-multiplexed imaging, sensing, cytometry, and light operation of a large number of targets all at once.

Large-Temporal-Numerical-Aperture Parametric Spectro-Temporal Analyzer Based on Silicon Waveguide

The parametric spectro-temporal analyzer (PASTA) system has been demonstrated as a flexible tool in single-shot spectrum measurements, especially for ultrafast non repetitive phenomena with arbitrary waveforms. However, the highly nonlinear fiber (HNLF) based PASTA is subject to a limited spectral resolution across a limited observation bandwidth, because the inherent dispersion and dispersion slope of the HNLF restrict the temporal numerical aperture (NA) of the time lens of current PASTA systems. Therefore, in this work, we propose and experimentally demonstrate a PASTA based on a dispersion-engineered silicon waveguide with a much lower accumulated dispersion and slope to improve the temporal NA. Leveraging the short interaction length and dispersion-engineered waveguide, a broadband phase-matching condition can be obtained, and the limited converted pump bandwidth can be overcome by implementing the time lens on the silicon waveguide. Compared to the HNLF-based PASTA, the silicon waveguide improves the temporal NA by a factor of 2.5: a 2.5-nm bandwidth pump can theoretically achieve an ultrahigh optical resolution of 1.3 pm (limited to 20 pm because of the acquisition bandwidth limit) over a 21-nm observation bandwidth. Moreover, the silicon waveguide-based PASTA presents a new way to integrate the whole system because of the waveguide configuration, and is promising for real-time measurements, which has not been possible with most conventional optical spectrum analyzers.

A Spectrum-Sensing DPD Feedback Receiver With $30\times$ Reduction in ADC Acquisition Bandwidth and Sample Rate

This paper presents a feedback receiver for the digital predistortion (DPD) of radio frequency (RF) power amplifiers (PAs). In contrast to conventional architectures, the receiver samples in the frequency domain. By randomly selecting a frequency bin in each sampling interval, the PA model is identified with substantially lower acquisition bandwidth and sampling rate. The receiver prototype is implemented in a 28-nm FD-SOI technology and samples at 4MS/s instead of the full Nyquist rate of 168MS/s for a 20-MHz Long Term Evolution (LTE) signal with spectral regrowth. The prototype is shown to linearize a 4-W small base station power amplifier from an adjacent channel leakage ratio (ACLR) of −35 dBc to −47.7 dBc, after processing only about 250 samples.

Performance Analysis of Analog Intermittently Nonlinear Filter in the Presence of Impulsive Noise

In this paper, an adaptive nonlinear differential limiter (ANDL) is proposed to efficiently alleviate the impact of impulsive noise (IN) in a communication system. Unlike existing nonlinear methods, the ANDL is implemented in the analog domain where the broader acquisition bandwidth makes outliers more detectable and consequently it is easier to remove them. While the proposed ANDL behaves like a linear filter when there is no outlier, it exhibits intermittent nonlinearity in response to IN. Therefore, the structure of the matched filter in the receiver is modified to compensate the filtering effect of the ANDL in the linear regime. In this paper, we quantify the performance of the ANDL by deriving a closed-form analytical bound for the average signal-to-noise ratio at the output of the filter. The calculation is based on the idea that the ANDL can be perceived as a time-variant linear filter whose bandwidth is modified based on the intensity of the IN. In addition, by linearizing the filter time parameter variations, we treat the ANDL as a set of linear filters where the exact operating filter at a given time depends upon the magnitude of the outliers. The theoretical average bit error rate is validated through simulations and the performance gains relative to classical methods such as blanking and clipping are quantified.

Ultrafast discrete swept source based on dual chirped combs for microscopic imaging.

An inertial-free, ultrafast frequency comb source based on two chirped optical frequency combs (OFCs) is proposed and experimentally demonstrated. The high linearity frequency sweeping is realized by the Vernier effect between the two OFCs rather than any mechanical motion component, so that good stability and reliability are ensured and no recalibration or resampling process is required. Swept rate up to 1 MHz is realized while keeping a narrow instantaneous linewidth of 0.03 nm, thanks to the extra-cavity frequency sweeping method. The wavelength step is proportional to the swept rate (3.8 pm at 10 kHz), and can be tuned by changing the repetition rate difference between the two OFCs. This swept source is applied for high-speed wavelength encoded imaging and achieves 4.4-μm spatial resolution at a 329-kHz frame rate. Compared with the traditional time-stretch microscopy, the signal acquisition bandwidth decreased from 3.8 GHz to below 90 MHz to achieve the same spatial resolution. Furthermore, the exposure time for a specific wavelength is much longer due to the discrete sweeping feature, which is a benefit for higher sensitivity. This discrete swept source provided a promising low-cost option for high-speed biomedical imaging systems and high-accuracy spectroscopy.

New Nonlinear Second-Order Phase-Locked Loop with Adaptive Bandwidth Regulation

Synchronization of large acquisition bandwidth brings great challenges to the traditional second-order phase-locked loop (PLL). To address the contradiction between acquisition bandwidth and noise suppression capability of the traditional PLL, a new second-order PLL coupled with a nonlinear element is proposed. The proposed nonlinear second-order PLL regulates the loop noise bandwidth adaptively by the nonlinear module. When a large input–output phase error occurs, this PLL reduces the frequency offset quickly by taking advantage of the large bandwidth. When the phase error is reduced by the loop control, the proposed PLL suppresses noises by using the small bandwidth to increase the tracking accuracy. Simulation results demonstrate that the tracking speed of the proposed PLL is increased considerably, and its acquisition bandwidth is increased to 18.8 kHz compared with that of the traditional second-order PLL (4 kHz).

Bandwidth enhancement in Hall probe by X-Hall DC biasing

The spinning-current technique is the state-of-the-art method for offset cancellation in Hall-effect probe. This technique achieves the best results in terms of offset reduction but limits the acquisition bandwidth to less than 1 MHz; therefore, it precludes the use of purely Hall-effect sensors in broadband current measurement. We present the X-Hall probe, a DC bias approach combined with an octagonal 8-contact morphology of the Hall-effect probe, which overcomes the bandwidth limitations of spinning-current-operated Hall sensors while reducing the offset. A prototype of the X-Hall probe is realized in CMOS-like technology and can potentially achieve a bandwidth wider than 40 MHz with an acceptable residual offset.

Challenges for high rate signal processing for the NUMEN experiment

The objective of the research and development activities, regarding the upgrade of the MAGNEX focal plane detector (FPD) and the development of the γ detector array for the NUMEN project, is the construction of new detectors capable to fulfil the requirements of high event rate, radiation tolerance and data acquisition and transmission bandwidth deriving from the upgrade of the Superconducting Cyclotron at INFN Laboratori Nazionali del Sud. The design of the front-end (FE) and read-out (RO) electronics has been performed in parallel with that of the new tracker. The design of the new segmented anode and the architecture of the front-end and read-out electronics are presented.

Ultrafast time-stretch microscopy based on dual-comb asynchronous optical sampling.

The ultrafast time-stretch microscopy based on a single-pixel detector has become a hotspot of the research, owing to its high sensitivity compared to those pixel sensors. However, gigahertz or tens of gigahertz acquisition bandwidth is required for this scheme, resulting in great expense for the whole imaging system and hindering its wide applications. In this Letter, a dual-comb asynchronous optical sampling is applied for the conventional time-stretch microscopy, whose ultrafast temporal axis is magnified by 9200 times. The acquisition bandwidth requirement is thus greatly relaxed, and 320kHz bandwidth successfully resolves 2.3μm spatial resolution with tens of kilohertz frame rate.

The low-frequency seismic vibrator: design and experimental verification

Extending the Vibroseis acquisition bandwidth towards low frequencies has become an increasing trend in land seismic exploration. It is clear that adding low-frequency content to seismic data can be beneficial to enabling waveform inversion for velocity model determination or refinement, improving the resolution of seismic data and obtaining structural information of deeper reservoirs. These geophysical benefits have been highlighted by many authors, for example Baeten et al. (2013) and Mahrooqi et al. (2012). However, using the Vibroseis method to acquire low frequency seismic data becomes very challenging. Owing to mechanical and hydraulic limitations, most conventional seismic vibrators cannot produce sufficient low-frequency force for transmission of seismic energy into deep ground below 10 Hz. Because of this fact many sweep design techniques aimed at enhancing the Vibroseis low-frequency contents have been developed (Bagaini 2006, 2008; Sallas 2010; Baeten 2011). These low frequency sweeps can enable conventional vibrators to shake as close as possible to their low-frequency mechanical limitations. Unfortunately, this approach requires a lower drive level and hence a slower sweep rate resulting in a longer sweep length. Therefore, the generation of extra low-frequency bandwidth with low frequency sweep methods usually has an impact on productivity.