Variable Bandpass Research Articles

Periodically correlated time series and the Variable Bandpass Periodic Block Bootstrap.

This research introduces a novel approach to resampling periodically correlated time series using bandpass filters for frequency separation called the Variable Bandpass Periodic Block Bootstrap and then examines the significant advantages of this new method. While bootstrapping allows estimation of a statistic's sampling distribution by resampling the original data with replacement, and block bootstrapping is a model-free resampling strategy for correlated time series data, both fail to preserve correlations in periodically correlated time series. Existing extensions of the block bootstrap help preserve the correlation structures of periodically correlated processes but suffer from flaws and inefficiencies. Analyses of time series data containing cyclic, seasonal, or periodically correlated principal components often seen in annual, daily, or other cyclostationary processes benefit from separating these components. The Variable Bandpass Periodic Block Bootstrap uses bandpass filters to separate a periodically correlated component from interference such as noise at other uncorrelated frequencies. A simulation study is presented, demonstrating near universal improvements obtained from the Variable Bandpass Periodic Block Bootstrap when compared with prior block bootstrapping methods for periodically correlated time series.

Novel Time-Saving Technique to Measure the 2D Complex Profile of Spectral Transmittance of Linearly Variable Filters and Assessing Homogeneity of Optically Transparent Materials

Emerging technologies for coating new types of filters, such as linearly variable bandpass filters, urges new kind of measurement systems requiring accurate and time-saving spatially resolved transmittance measurements. On the other hand, one of the main uses of coated filters is standardization use; e.g., as certified reference materials (CRMs) for transmittance and absorbance standards. In this case, the homogeneity assessment is of great interest. To do such measurements with low uncertainty, spatial scanning methods using reference double-beam spectrophotometers are routinely used. High time consumption and relatively high-cost are disadvantages of this traditional method. In this work, we propose a new system based on an imaging technique. The newly developed system comprises a quartz tungsten halogen (QTH)-based illuminator, with collimator and coupling optics, a double-grating monochromator, customized integrating sphere, and scientific highly digitized, high-resolution and dynamic range Si-based CCD (monochrome) camera. The system is time-saving: in one-shot we may measure the spatial transmittance distribution of the samples (e.g. filters) without separately measuring the reference (air), which has many instability issues. Validation measurements using standard filters are presented. Systematic errors may be studied and corrected for. Many types of filters and liquids, for instance chemical reference materials (RMs), may be measured and reported. Uncertainty of spatial, spectral and radiometric components are studied carefully and estimated. The combined uncertainty evaluation has shown that relatively low uncertainty levels may be achieved, specially in the visible range (U < 1 %, k=2 ). Spatially-resolved data with different curves at different positions (spatial points) are shown in the spectral range of 380 nm through 950 nm. The combined uncertainties at some spectral points are presented and proved to be sample- and wavelength-dependent at all the points in the field-of-view (FOV) of the CCD camera.

Generalised strategy for stabilising minimax-type variable bandpass filter

ABSTRACT This paper first develops a novel transformation-based strategy for guaranteeing the stability of recursive-type variable digital filters, and then presents a new type of functions called less-than-unity (LTU) functions required in performing stability-guaranteed parameter transformations (LTU transformations). The LTU transformations can be viewed as the generalised existing one. Based on the LTU transformations, a recursive bandpass filter with variable passband-width (PBW) is attained using the minimax criterion so that the stability is ensured. The recursive bandpass filter has continuously PBW and fixed passband centre frequency. To meet the stability condition of the recursive variable PBW bandpass filter (variable-PBW filter), the LTU parameter transformations are incorporated into the minimisation of the maximum amplitude-response error. It can be proved that incorporating the LTU transformations into the design process definitely produces a recursive variable-PBW filter whose stability is absolutely guaranteed. For verifying the ensured stability and demonstrating the high design accuracy, detailed computer simulations are included. The illustrative example verifies that the obtained variable-PBW filter not only has definitely ensured stability, but also its approximation accuracy is extremely high.

A Novel Quad-Band Branched Monopole Antenna with a Filter Suppressing Higher Order Modes

This paper proposes a novel quad-band branched monopole antenna with a filter. The proposed antenna has a simple configuration in which branch-elements are added to a basic configuration consisting of a mast and dielectric wires. The antenna is characterized by performances such as wideband impedance matching, gain stabilization, and gain enhancement. Wideband impedance characteristics satisfying the voltage standing ratio of less than 2 are obtained by exciting a parallel resonance at the lowest band and multi-resonance at high bands. The filter suppressing higher order modes is used for gain stabilization, so that averaged gains above 5 dBi are obtained at the quad-band. The antenna has a high gain of 11.1 dBi because the branch-elements work as an end-fire array antenna at the highest band. Furthermore, it is clarified that an operating frequency is switched by using a variable bandpass filter at the lowest band. Last, a scale model of the antenna is fabricated and measured, then the effectiveness of the proposed antenna is demonstrated.

Development and Validation of a Single-Variable Comparison Stimulus for Matching Strained Voice Quality Using a Psychoacoustic Framework.

Acoustic and perceptual quantification of vocal strain has been a vexing problem for years. To increase measurement rigor, a suitable single-variable matching stimulus for strain was developed and validated, based on the matching stimulus used previously for breathy and rough voice qualities. A set of 21 comparison stimuli for a single-variable matching task (SVMT) was synthesized based on a speech-shaped sawtooth waveform mixed with speech-shaped noise. Variable bandpass filter gain in mid-to-high frequencies achieved a wide range of computed sharpness (in constant sharpness steps) and served as the independent variable for the SVMT. Ten natural /ɑ/ stimuli with a wide range of the primary voice quality of strain and a minimum of breathiness or roughness were selected and assessed using the SVMT. Natural voice samples and synthetic comparison stimuli were also assessed using a perceptual magnitude estimation (ME) task. ME data validated the correspondence of the set of comparison stimuli to varying perceived strain. Perceived strain magnitudes of the comparison stimuli increased significantly and linearly with computed sharpness (r 2 = .99). A linear regression revealed that strain matching values were significantly predicted by computed sharpness (r 2 = .96) and perceived strain magnitudes (r 2 = .95) of the natural voice stimuli. The perception of vocal strain is strongly associated with computed sharpness and is captured accurately and precisely using an SVMT, in which the independent variable is the bandpass filter gain (in steps of equal sharpness) applied to the comparison stimuli.

Photoabsorption of potassium clusters isolated in helium droplets: From discrete electronic transitions to collective resonances

Photoabsorption spectra of neutral potassium clusters in helium nanodroplets are recorded by depletion spectroscopy using a supercontinuum laser source equipped with a variable bandpass filter. The results provide insight into the evolution of electronic spectra from molecular transitions to localized surface plasmon resonances in nanometer-sized clusters. Size selected spectra for potassium clusters ${\mathrm{K}}_{n}$ are acquired from $n=2--110$. Indications for the transition from molecularlike absorption to a collective resonance are already observed below $n=20$. For larger clusters, a splitting of the plasmon mode into two components is observed at around $n=600$. Supported by simulations, this is explained by the presence of nonspherical potassium nanoparticles grown via multicenter aggregation inside the helium droplets. The presented supercontinuum laser-based experimental approach provides a new route for the mass-selective spectroscopic characterization of different materials and material combinations isolated in helium nanodroplets in a size regime ranging from single atoms and molecules to small sub-10-nm particles.

Enhancing scanning electrochemical microscopy's potential to probe dynamic co-culture systems via hyperspectral assisted-imaging.

Precise determination of boundaries in co-culture systems is difficult to achieve with scanning electrochemical microscopy alone. Thus, biological scanning electrochemical microscope platforms generally consist of a scanning electrochemical microscope positioner mounted on the stage of an inverted microscope for correlated electrochemical and optical imaging. Use of a fluorescence microscope allows for site-specific fluorescence labeling to obtain more clearly resolved spatial and electrochemical data. Here, we construct a unique hyperspectral assisted-biological scanning electrochemical microscope platform to widen the scope of biological imaging. Specifically, we incorporate a variable fluorescence bandpass source into a biological scanning electrochemical microscope platform for simultaneous optical, spectral, and electrochemical imaging. Not only does this platform serve as a cost-effective alternative to white light laser imaging, but additionally it provides multi-functional analysis of biological samples. Here, we demonstrate the efficacy of our platform to discern the electrochemical contribution of site-specific cells by optically and spectroscopically resolving boundaries as well as cell types within a complex biological system.

Application of static masking technique in magnetron sputtering technology for the production of linearly variable filters

Variable filters are key components for compact spectral imagers. In this paper, we present a method for the fabrication of linearly variable filters based on Bühler HELIOS machine (plasma assisted reactive magnetron sputtering). These filters are obtained by producing a variation of the thickness of all the layers of the coating, using adapted masks placed in between the sputtering targets for the low and high refractive index materials and the substrates. Variable band pass filter from 550 up to 1000 nm is demonstrated.

Development of measurement system and analysis method for characterization of linear variable bandpass filters.

An automated measurement system was developed to characterize the spatial gradient, linearity of the spatial gradient, bandwidth and transverse uniformity of a linear variable filter (LVF). To demonstrate this, the LVF fabricated in our group has been measured and analyzed. Simulations for beam spot size effects on measurements were performed for various LVF spectral peak profiles with results indicting significant averaging effect due to beam spot size and this is consistent with experiment results. Moreover, to fit the peak profile more accurately, a modified Pearson VII function was proposed and demonstrated high capability to express complex shapes of peaks mathematically. This provides a methodology for deconvoluting the original LVF peak profile from a measured averaged peak profile and has been verified using actual measured data.

Variable cutoff frequency FIR filters: a survey

Many signal processing applications require digital filters with variable frequency characteristics, especially the filters with variable bandwidth. Due to their linear phase and inherent stability, variable bandwidth finite impulse response (FIR) filters are the popular choice in majority of the applications. Once a variable cutoff frequency (VCF) FIR lowpass filter is designed, variable bandwidth bandpass / highpass / bandstop filters and reconfigurable filter banks can be realized from the same. In this paper, we present a comprehensive review of the existing variable cutoff frequency FIR filter design techniques, including the developments in the recent two decades. We provide the basic concepts, design and architectural details for each of these techniques and the major developments / incremental works thereof. Qualitative as well as quantitative comparisons are provided to assist the reader in choosing the most suitable VCF filter design technique for a particular application.

Low-cost hyper-spectral imaging system using a linear variable bandpass filter for agritech applications.

Hyperspectral imaging for agricultural applications provides a solution for non-destructive, large-area crop monitoring. However, current products are bulky and expensive due to complicated optics and electronics. A linear variable filter was developed for implementation into a prototype hyperspectral imaging camera that demonstrates good spectral performance between 450 and 900 nm. Equipped with a feature extraction and classification algorithm, the proposed system can be used to determine potato plant health with ∼88% accuracy. This algorithm was also capable of species identification and is demonstrated as being capable of differentiating between rocket, lettuce, and spinach. Results are promising for an entry-level, low-cost hyperspectral imaging solution for agriculture applications.

In situ Performance analysis for Noise Suppression in Hearing prosthesis

Human beings, wearing hearing prosthetics will have hard time in segregating voice of interest from background noise in surrounded noisy environment, such that the intelligibility of the hearing prosthetics has to be improved. In this paper, a variable bandwidth band pass filter is proposed so that the intelligibility of the speech will get enhanced. The proposed algorithm is also analyzed in different noisy environments. This work is implemented in LAB View and Field Programmable Gate Arrays.

Focus on photonics, spectroscopy, and spectrometry

Focus on photonics, spectroscopy, and spectrometry

Snapshot Multispectral Imaging

AbstractA multispectral imaging concept based on a multi‐aperture system approach using a customized microlens array combined with a slanted continuously variable bandpass filter and a silicon‐based image sensor helps to overcome the restrictions of scanning techniques or wafer‐level coated detectors.

Deposited poly-Si as on-demand linewidth compensator for on-chip Fabry–Perot interferometer and vertical linear variable optical filter bandpass and passband manipulation

Deep reactive ion etching (DRIE) is an important process for etching vertical structures for microelectromechanical systems. Due to the sidewall profile of some photoresists as well as effects from upstream processes, bulk micromachined structures, to a certain extent, could differ from expected. Concerning photonics applications, minute deviations from the intended design might alter its optical characteristics. The most popular approach is to introduce a compensation factor during mask design. However, such a method is not robust enough to accommodate batch variations due to varying process conditions.In one particular example specific to this work, the simulated passband for a Si-air Fabry–Perot interferometer configuration was 3.67 μm. However, post DRIE the passband was measured to be 3.40 μm. To resolve this discrepancy, linewidth compensation using low-pressure chemical vapor deposition (LPCVD) poly-Si is presented. When 170 and 194 nm of poly-Si were separately deposited, the passbands redshifted to 3.54 and 3.57 μm, respectively. With the LPCVD poly-Si layer being highly conformable, the full width half maximum remains unchanged at 80 nm. An on-chip linear variable optical filter was demonstrated with a compensation of 194 nm poly-Si. It was observed that the working range redshifted from 3.0–3.9 μm to 3.3–4.5 μm.

A 1V 1.4 mW multi-band ZigBee receiver with 64 dB SFDR

A low voltage low power receiver supporting 780/868/915/2400 MHz ZigBee bands is presented in this paper. The receiver exploiting low-IF architecture consists of a RF-to-BB (baseband) current reuse front-end, a Gm-C based variable gain complex band-pass filter (CBPF) for image rejection and four stages of limiting amplifiers which saturate IF signal for demodulation. The proposed ZigBee receiver chip is implemented in TSMC 180 nm CMOS technology with metal-insulator-metal (MIM) capacitors. PCB measurement results show that the receiver has 45.9 dB conversion gain, 8.5 dB NF and −33.5 dBm out-of-band IIP3 at sub-GHz bands. When working in 2.4 GHz band, the gain is 38.4 dB, noise figure (NF) is 16.7 dB and the out-of-band IIP3 is −28.2 dBm. The S11 bandwidth (S11 < −10 dB) can cover the entire ZigBee spectrum (700–2500 MHz). The receiver consumes 1.42 mW from a 1 V DC supply and the die size is 1.41 mm2 including all pads.

A bandpass sigma-delta modulator with the function of adjustable center-frequency

A bandpass sigma-delta modulator with the function of adjustable center-frequency is present in this paper. A method used to convert 2-order low-pass modulator to 4-order bandpass modulator is proposed to realize the design. Center-frequency that can be set to 1/6, 1/4 and 1/3 of sampling frequency is controlled according to the digital logic voltage. The bandwidth of pass-band is 200 Hz. Test results show that the average SNR is about 80 dB at different center-frequency when the sampling frequency is 64 kHz. The input signal can reach to −3 dBFs. The complexity to design a variable center-frequency bandpass modulator is greatly reduced.

Switched‐bandwidth SAW‐based bandpass filters with flat group delay

Tunable surface-acoustic-wave-(SAW)-based bandpass filters (BPFs) with flat in-band group delay (Tg) and variable passband bandwidth (BW) are reported. They are based on N hybrid acoustic-wavelumped-element-resonator (AWLR) modules shaped by one lumpedelement resonator and K RF-switched SAW resonators that are arranged in a Gaussian-type source-to-load impedance-inverter network. Fractional bandwidth (FBW) tuning is achieved by reconfiguring the number of SAW resonators within the AWLR modules. Major advantages of the proposed filter concept when compared to conventional SAW BPFs are as follows: (i) they do not depend on the electromechanical coupling coefficient (k t 2 ) of the SAW resonators - FBW>k t 2 can be realised -, (ii) they do not require lossy elements or SAW resonators with different frequencies, and (iii) they can be tuned. For experimental-validation purposes, a 433.9 MHz two-pole/fourtransmission-zero prototype was built and measured. It exhibited flatin-band-T g passbands with discretely-tunable BW between 0.18 and 0.45 MHz (i.e. 2.5:1 tuning ratio).

Compressive Spectral Imaging via Polar Coded Aperture

A compressive spectral imager based on a polar coded aperture and a continuous variable circular bandpass filter is proposed for spinning munitions. As the imager rotates with the munition, compressive projections are sequentially captured with embedded spatial and spectral modulation. The polar coded aperture design is introduced, aiming at optimizing the sensing process. Both discrete and continuous rotation models of the proposed imager are derived and used to characterize the compressive imager. Computer simulations validate the computational models and the reconstruction algorithm.

Realisation and optical engineering of linear variable bandpass filters in nanoporous anodic alumina photonic crystals.

We present the first realisation of linear variable bandpass filters in nanoporous anodic alumina (NAA-LVBPFs) photonic crystal structures. NAA gradient-index filters (NAA-GIFs) are produced by sinusoidal pulse anodisation and used as photonic crystal platforms to generate NAA-LVBPFs. The anodisation period of NAA-GIFs is modified from 650 to 850 s to systematically tune the characteristic photonic stopband of these photonic crystals across the UV-visible-NIR spectrum. Then, the nanoporous structure of NAA-GIFs is gradually widened along the surface under controlled conditions by wet chemical etching using a dip coating approach aiming to create NAA-LVBPFs with finely engineered optical properties. We demonstrate that the characteristic photonic stopband and the iridescent interferometric colour displayed by these photonic crystals can be tuned with precision across the surface of NAA-LVBPFs by adjusting the fabrication and etching conditions. Here, we envisage for the first time the combination of the anodisation period and etching conditions as a cost-competitive, facile, and versatile nanofabrication approach that enables the generation of a broad range of unique LVBPFs covering the spectral regions. These photonic crystal structures open new opportunities for multiple applications, including adaptive optics, hyperspectral imaging, fluorescence diagnostics, spectroscopy, and sensing.