Narrow-band Low-pass Filter Research Articles

Phase Measurement Method Based on Digital Dual Frequency Comb for High-Precision High-Speed Heterodyne Interferometry

The heterodyne interferometer is widely used as a precise displacement sensor in ultraprecision processing. However, there is a trade-off between measurement speed and measurement precision in the existing interference signal processing methods. In this work, a novel phase measurement method based on digital dual frequency comb is proposed. The proposed method uses the intrinsic frequency comb signal and a narrowband low-pass filter to form a band-pass filter bank, which divides the broadband brought by high-speed movement into multiple narrow bands. The signal-to-noise ratio (SNR) is improved for phase calculation. In addition, two groups of intrinsic frequency comb signals are used simultaneously for phase measurement and the rotation of the output, preventing the inaccurate phase measurement of the input signal whose frequency is the average of two frequencies in the frequency comb signal. The heterodyne laser interferometer can maintain high precision at high measurement speed. The simulation results show that the precision of the proposed method is more than 50% higher than that of the phase-locked loop method. The experiment with a circuit board combined with 100 Msps sampling 16-bit A/D converter shows that the phase measurement precision of the input signal frequency in the bandwidth range of 2.5–37.5 MHz can reach 0.77 mrad, equivalent to the displacement measurement precision of the double-pass interferometer (19 pm) at a measurement speed of less than 2.77 m/s.

A low noise stable radiometer front-end for passive microwave tissue thermometry

ABSTRACTA low noise, high gain, stable radiometer front-end is presented for non-invasive tissue thermometry using custom designed narrow band low noise amplifiers (LNAs) and band pass filter (BPF) with 1.3 GHz centre frequency and − 20 dB pass band of 332 MHz. The fabricated LNAs have >15 dB gain, unconditional gain stability and noise figure (NF) < 1.45 dB in the pass band. The maximum insertion loss of the BPF is <1 dB over 1.2–1.4 GHz with 50 dB suppression in the surrounding communication bands. The cascaded radiometer front-end has measured gain of 45–50 dB, NF < 1.75 dB in the pass band and > 30 dB suppression in the adjacent personal communication bands. The Allan deviation of the total power radiometer indicates long term system stability and presence of Gaussian thermal noise for integration time, . Radiometer measurements of a matched load at room temperature for indicate acceptably low influence of external electromagnetic interference (EMI) and system noise equivalent temperature of 145 K. The brightness temperature measured by the total power radiometer using a resonant slot antenna demonstrates the ability to detect a 0.3°C change in the test load temperature with better than 0.1°C accuracy.

Imitational modeling of synchronous measuring detector with increased noise immunity and precision

In this paper, an example of application, simulation results, noise and precision properties of a synchronous measuring detector with increased noise immunity and precision (not less than 1%) for the operation providing of primary sensors – thermal sensors and bridge sensors, in particular Hall sensors for magnetic field measurements are considered. As a result of the research, it was discovered that increasing the sensitivity of magnetometers based on Hall sensors leads to the allocation of the initial useful signal of the ultra-low level against the background of noise comparable with it. To allocate the output signal of the Hall sensor or thermal sensor in the background of noise, it is proposed to feed it with harmonic current with the further allocation of the information signal using a in-phase detector with a narrowband low-pass filter.

Fast time-varying linear filters for suppression of baseline drift in electrocardiographic signals

BackgroundThe paper presents a method of linear time-varying filtering, with extremely low computational costs, for the suppression of baseline drift in electrocardiographic (ECG) signals. An ECG signal is not periodic as the length of its heart cycles vary. In order to optimally suppress baseline drift by the use of a linear filter, we need a high-pass filter with time-varying cut-off frequency controlled by instant heart rate.MethodsRealization of the high-pass (HP) filter is based on a narrow-band low-pass (LP) filter of which output is subtracted from the delayed input. The base of an LP filter is an extremely low computational cost Lynn’s filter with rectangular impulse response. The optimal cut-off frequency of an HP filter for baseline wander suppression is identical to an instantaneous heart rate. Instantaneous length of heart cycles (e.g. RR intervals) are interpolated between QRS complexes to smoothly control cut-off frequency of the HP filter that has been used.Results and conclusionsWe proved that a 0.5 dB decrease in transfer function, at a time-varying cut-off frequency of HP filter controlled by an instant heart rate, is acceptable when related to maximum error due to filtering. Presented in the article are the algorithms that enable the realization of time-variable filters with very low computational costs. We propose fast linear HP filters for the suppression of baseline wander with time-varying cut-off frequencies controlled by instant heart rate. The filters fulfil accepted professional standards and increase the efficiency of the noise suppression.

The Design of Linear-Phase FIR Multiple-Notch Filters with Variable Notch Frequencies

In this paper, a paradigm is developed to design the linear-phase FIR multiple-notch filters with variable notch frequencies. The design procedure can be proceeded through two steps: First, a linear-phase narrow-band low-pass filter met the given bandwidth and stopband ripple specifications is designed. Second, a tuning procedure is applied to the computed low-pass filter to yield the desired multiple-notch filter. When the notch frequencies are varied, the same tuning procedure can be employed to render the multiple-notch filter with the new set of the notch frequencies. The tuning procedure employed reduces the computational complexity of designing the multiple-notch filter with the new set of the notch frequencies. The simulation results demonstrate the effectiveness of our scheme.

A modified phase generation carrier technique for fiber-optic distributed disturbance sensor

A modified phase generation carrier technique used in fiber-optic distributed disturbance sensor (FDDS) is proposed and investigated. The FDDS locates the disturbance by dual Mach–Zehnder (MZ) interferometers, however, the initializing phase error of the two MZ interferometer causes the location errors. For the modified PGC technique, a narrow band analog low pass filter is designed, which solves the problem of facilitating the digital narrow band low pass filter in high speed A/D acquisition. In addition, a special demodulation signal is proposed to demodulate higher-order harmonic, compensate the time delay of the long optical fiber and enhance the signal to noise ratio. It is found that the location errors caused by phase drift can be eliminated in our experiments.

The Design of Lock-in Amplifier Based on DSP Builder

Lock-in amplifier is widely used to detect weak signals from significant background noise. In this paper, an FPGA-based Lock-in amplifier is implemented on the DSP Builder platform to measure impedance between biological cells and electrodes. To improve the measuring accuracy, a narrow band low pass filter with low cut off frequency was implemented. This low pass filter consists of a five-stage cascaded integrator-comb filter, two half band filters and a FIR low pass filter. Simulation shows that with a system clock of 10MHz, the cut off frequency of the low pass filter is lower than 20Hz. Experiments demonstrate that the implemented lock-in amplifier is able to detect weak signal with strong noise, making it an attractive approach to implement lock-in amplifiers which is more flexible and can meet the needs for specified measurements.

Efficient Channel Delay Estimation for OFDM Systems over Doubly-Selective Fading Channels

In this paper, we propose an efficient channel delay estimation method for orthogonal frequency-division multiplexing (OFDM) systems, especially over doubly-selective fading channels which are selective in both the symbol time domain and subcarrier frequency domain. For the doubly-selective fading channels in single frequency network (SFN), long and strong echoes exist and thus the conventional discrete Fourier Transform (DFT) based channel delay estimation system often fails to produce the exact channel delay profile. Based on the analysis of the discrete-time frequency response of the channel impulse response (CIR) coefficients in the DFT-based channel delay estimation system, we develop a method to effectively extract the true CIR from the aliased signals by employing a simple narrow-band low-pass filter (NB-LPF). The performance of the proposed system is verified using the COST207 TU6 SFN channel model.

Recursive FIR filter structures on FPGA

A new approach to piecewise-polynomial approximation and recursive implementation structures for linear-phase Finite Impulse Response (FIR) filters have been recently proposed. In this paper, we describe hardware prototype implementations of the new structures for all four types of linear-phase FIR filters using a Field Programmable Gate Array (FPGA) based platform. Narrowband lowpass filters and narrowband differentiators are used as design examples to demonstrate the functionality and efficiency of the implementations. The required wordlength and resource usage is analyzed.

High Quality and Low Loss Millimeter Wave Passives Demonstrated to 77-GHz for SiP Technologies Using Embedded Wafer-Level Packaging Platform (EMWLP)

With the increasing demand for system integration to cater to continuously increasing number of I/Os as well as higher operating frequencies, reconfigured wafer-level packaging, or embedded WLP (EMWLP) is emerging as a promising technology for integration. This platform allows integrated passives to be designed in the redistribution layers using the mold compound as a substrate, which significantly improves the passives performance compared to those of on-chip. In this paper, we present low loss passives on EMWLP platform demonstrated in a 5.5-GHz band pass filter targeted for wireless local area network (WLAN) applications. To ascertain the feasibility of designing for low loss millimeter wave passives on EMWLP, transmission lines were designed and their loss characteristics investigated up to 110 GHz, which are reported here. Subsequently we demonstrate for the first time a narrowband low loss 77-GHz band pass filter on EMWLP platform, with a good correlation obtained between simulation and measurement results. In addition, a temperature dependence characterization was performed on the 77-GHz filter, with little variation in the measured filter characteristics observed.

Spatioangular Prefiltering for Multiview 3D Displays

In this paper, we analyze the reproduction of light fields on multiview 3D displays. A three-way interaction between the input light field signal (which is often aliased), the joint spatioangular sampling grids of multiview 3D displays, and the interview light leakage in modern multiview 3D displays is characterized in the joint spatioangular frequency domain. Reconstruction of light fields by all physical 3D displays is prone to light leakage, which means that the reconstruction low-pass filter implemented by the display is too broad in the angular domain. As a result, 3D displays excessively attenuate angular frequencies. Our analysis shows that this reduces sharpness of the images shown in the 3D displays. In this paper, stereoscopic image recovery is recast as a problem of joint spatioangular signal reconstruction. The combination of the 3D display point spread function and human visual system provides the narrow-band low-pass filter which removes spectral replicas in the reconstructed light field on the multiview display. The nonideality of this filter is corrected with the proposed prefiltering. The proposed light field reconstruction method performs light field antialiasing as well as angular sharpening to compensate for the nonideal response of the 3D display. The union of cosets approach which has been used earlier by others is employed here to model the nonrectangular spatioangular sampling grids on a multiview display in a generic fashion. We confirm the effectiveness of our approach in simulation and in physical hardware, and demonstrate improvement over existing techniques.

Calculation of narrowband lowpass filters with finite impulse response

This study provides a review of digital filters known under the name “interpolated finite impulse response filters” (IFIR filters) that make it possible to create narrowband lowpass filters (LPF) with substantially reduced computational load and, consequently, a simpler design as compared with the conventional finite impulse response filters (FIR filters).

Improved narrowband low-pass IIR filters in fixed-point systems [DSP Tips &amp; Tricks

Due to their resistance to quantized-coefficient errors, traditional second-order infinite impulse response (IIR) filters are the fundamental building blocks in computationally efficient high-order IIR digital filter implementations. However, when used in fixed-point number systems, the inherent properties of quantized-coefficient second-order IIR filters do not readily permit their use in narrowband low-pass filtering applications. Narrowband low- pass IIR filters have traditionally had a bad reputation; for example, MATLAB's Signal Processing Toolbox documentation warns: "All classical IIR low-pass filters are ill conditioned for extremely low cutoff frequencies." This article presents a neat trick that can be used to overcome the shortcomings of narrowband second-order low- pass IIR filters, with no increase in filter coefficient bit widths and no increase in the number of filter multiplies per output sample.

Delta operator realizations of direct-form IIR filters

The use of the delta operator in the realizations of digital filters has recently gained interest due to its good finite-word-length performance under fast sampling. We studied efficient direct form structures, and show that only some of them can be used in delta configurations, while others are evidently unstable. In this paper, we focus on the roundoff noise analysis. Of all the direct-form structures, the direct form II transposed (DFIIt) delta structure has the lowest quantization noise level at its output. This structure outperforms both the conventional direct-form (delay) structures, as well as the state-space structures for narrow-band low-pass filters with respect to output roundoff noise. Excellent roundoff noise performance is achieved at the cost of only a minor additional implementation complexity when compared with the corresponding delay realization. Complexity of a signal processor implementation of the DFIIt delta structure, which was found to be the most suitable delta structure for signal processors, is compared with those of the direct form and state-space delay structures. In addition, some hardware implementation aspects are discussed, including the minimization of the internal word length.

A method for coarse frequency acquisition for Nyquist filtered MPSK

In mobile satellite communications, frequency offsets are important to estimate before further processing and detection of the received signal. In this work, we devise a straightforward technique for coarse frequency acquisition through the discrete Fourier transform (DFT) computation for samples of the received waveform with a set of special preamble sequences suitable for any Nyquist pulse shape and which can also be used for timing recovery. The technique can be applied to any L-ary phase shift keying (LPSK) modulation scheme, though we apply it to binary phase shift keying (BPSK) and show how to use it with a LPSK scheme. Also, the analysis takes into account the effect of the receiver's narrowband low-pass filter (LPF) on attenuating the noise power which also results in correlated noise samples. Such a treatment of performance with correlated noise is not considered in the literature as the noise is usually assumed to be uncorrelated for simplicity in analysis. The presented results are obtained through theory and verified by computer simulation. A new discrete Fourier transform (DFT) search algorithm, based on the maximum-likelihood principle, and which takes into account the preamble structure, is introduced and examined.

Transitional Butterworth-Chebyshev recursive digital filters

A design for transitional Butterworth–Chebyshev recursive digital filters is proposed. These filters show gradual transition and, at the same time, maintain the equiripple peaks. An analysis of the cutoff slope of these filters is also presented. It is shown that the first type of filter is suitable for the design of wideband low-pass filters, and the second one for the design of narrowband low-pass filters.

Bounds for floating-point roundoff noise

The dummy multiplier model and the conventional estimates for roundoff noise variance in floating-point implementations are compared. Common upper and lower bounds for the noise estimates are derived and experimental results are presented. Although the two noise estimates have common finite bounds, it is shown that they may differ by more than 10 dB for narrowband lowpass filters. >

Experimental linewidth-insensitive coherent analog optical link

We constructed an experimental linewidth-insensitive coherent analog optical link. The transmitter utilizes an external electro-optic amplitude modulator and a semiconductor laser. The receiver consists of a heterodyne front-end, a wideband filter, square law detector and narrowband lowpass filter. We performed experimental measurements and theoretical analyses of the spurious-free dynamic range (SFDR), link gain and noise figure for both the coherent AM and the direct detection links; we investigated the dependencies of the foregoing parameters on the received optical signal power, laser linewidth, IF bandwidth, and the laser relative intensity noise (RIN). By selecting a wide enough bandpass filter, we made the coherent AM link insensitive to laser linewidth. The coherent AM link exhibits a higher SFDR than the corresponding direct detection link when the received optical signal power is less than 85 /spl mu/W. The noise figure for the coherent link is greater than that for the direct detection link under all conditions investigated. For received optical signal powers greater than 4 /spl mu/W, the link gain for the direct detection link is greater than that for the coherent AM link. The following are the link parameters that have been achieved for the coherent AM link investigated: SFDR=88 dB/spl middot/Hz/sup 2/3/, link gain=-25 dB and noise figure=78 dB; this performance has been obtained with a received optical signal power of 85 /spl mu/W, and a local oscillator power at the photodetector of 228 /spl mu/W. The link performance can be further improved by auxiliary subsystems such as a balanced receiver and impedance matched transmitter and receiver ends; and/or by using better optical and electrical devices like higher power lasers, linearized optical modulators, low-noise and high gain RF amplifiers, and optical amplifiers,. >

A new approach for 2-D narrow-band circularly symmetry filter design

An approach to 2-D narrowband low-pass filter with circular symmetry is presented. This approach uses a cascade structure of a simple 2-D prefilter and a McClellan-transformed 1-D finite impulse response (FIR) equalizer. If the desired filter is not of the narrowband type, the proposed approach loses its effectiveness because one cannot take the advantage of the simple prefilter. >

Roundoff noise in multirate digital filters

The roundoff noise problem for multirate digital filters is difficult because rate changing inside the filter makes both scaling and roundoff noise calculation complicated. With a new multirate filter analysis technique, the transfer function between any two points inside a multirate filter can be easily found, thus, simplifies the scaling and noise calculation. This paper finds theLP scaling and derives the roundoff noise expressions for fixed point implementations of the multistage decimator, interpolator, and multirate narrow-band low-pass filter. It is shown that the noise source at a low sampling rate stage is more important than that at a higher sampling rate stage. Methods to reduce output roundoff noise are discussed.