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Modern measurement systems must have the ability to perform measurements in high-level noise environments. The field with a particularly high content of noise is biomedicine. The traditional approach to solving this problem is to inject very high energy signals in order to minimize the impact of noise on the measurement (Magnetic resonance imaging, X-rays). Unfortunately, this means that these systems are very large, expensive, difficult to maintain and often contain toxic materials. This paper presents research aimed at finding a non-invasive method that uses low-intensity signals that are completely harmless to humans, and at the same time allows extracting the measured signal from a highly noised environment. Noise suppression was achieved with a lock-in amplifier. [ABSTRACT FROM AUTHOR]

Purpose: The purpose of this study is to propose a framework employing Lock-in amplifier for fault diagnosis of rolling bearings using vibration signals, addressing the challenges posed by noise interference and inaccurate estimation of Lock-in amplifier's reference frequency. Methods: By studying the properties of the pulse chains triggered by faults, the proposed framework utilizes the Short-time Variance of vibration signals to determine the bearing fault characteristic frequency, providing a reliable reference frequency for the Lock-in amplifier. This approach aims to overcome the limitations of previous methods reliant on geometric relationships for fault frequency estimation. Results: It has been proved that Short-time Variance can effectively identify sparse pulses in vibration signals. The Short-time Variance spectrum effectively characterizes bearing fault information and outperforms the benchmark method envelope spectrum in high-noise environments. Validation of the proposed framework is conducted using simulated signals and experimental data sourced from a public dataset. The results demonstrate the effectiveness of the framework in accurately diagnosing bearing faults through vibration signals. Conclusion: This study confirms that directly applying theoretical fault characteristic frequencies as the reference frequency for a Lock-in amplifier can have a significant negative impact on diagnostic performance. It has been concluded that the proposed framework offers a robust approach to fault diagnosis of rolling bearings. By leveraging Lock-in amplifiers and the Short-time Variance, it provides accurate fault detection, thus ensuring the safe and reliable operation of mechanical equipment. [ABSTRACT FROM AUTHOR]

Field programmable gate arrays (FPGAs) have not only enhanced traditional sensing methods, such as pixel detection (CCD and CMOS), but also enabled the development of innovative approaches with significant potential for particle detection. This is particularly relevant in terahertz (THz) ray detection, where microbolometer-based focal plane arrays (FPAs) using microelectromechanical (MEMS) resonators are among the most promising solutions. Designing high-performance, high-pixel-density sensors is challenging without FPGAs, which are crucial for deterministic parallel processing, fast ADC/DAC control, and handling large data throughput. This paper presents a MEMS-resonator detector, fully managed via an FPGA, capable of controlling pixel excitation and tracking resonance-frequency shifts due to radiation using parallel digital lock-in amplifiers. The innovative FPGA architecture, based on a lock-in matrix, enhances the open-loop readout technique by a factor of 32. Measurements were performed on a frequency-multiplexed, 256-pixel sensor designed for imaging applications. [ABSTRACT FROM AUTHOR]

The autofluorescence of erythrocyte porphyrins has emerged as a potential method for multi-cancer early detection (MCED). With this method's dependence on research-grade spectrofluorometers, significant improvements in instrumentation are necessary to translate its potential into clinical practice, as with any promising medical technology. To fill this gap, in this paper, we present an automated ratio porphyrin analyzer for cancer screening (ARPA-CS), a low-cost, portable, and automated instrument for MCED via the ratio fluorometry of porphyrins. The ARPA-CS aims to facilitate cancer screening in an inexpensive, rapid, non-invasive, and reasonably accurate manner for use in primary clinics or at point of care. To accomplish this, the ARPA-CS uses an ultraviolet-excited optical apparatus for ratio fluorometry that features two photodetectors for detection at 590 and 630 nm. Additionally, it incorporates a synchronous detector for the precision measurement of signals based on the Walsh-ordered Walsh–Hadamard transform (WHT)w and circular shift. To estimate its single-photodetector capability, we established a linear calibration curve for the ARBA-CS exceeding four orders of magnitude with a linearity of up to 0.992 and a low detection limit of 0.296 µg/mL for riboflavin. The ARPA-CS also exhibited excellent repeatability (0.21%) and stability (0.60%). Moreover, the ratio fluorometry of three serially diluted standard solutions of riboflavin yielded a ratio of 0.4, which agrees with that expected based on the known emission spectra of riboflavin. Additionally, the ratio fluorometry of the porphyrin solution yielded a ratio of 49.82, which was ascribed to the predominant concentration of protoporphyrin IX in the brown eggshells, as confirmed in several studies. This study validates this instrument for the ratio fluorometry of porphyrins as a biomarker for MCED. Nevertheless, large and well-designed clinical trials are necessary to further elaborate more on this matter. [ABSTRACT FROM AUTHOR]

The main objective of the work is to investigate the capacity of a single magnetoresistance (MR) element to measure AC electrical currents. An instrumentation system is presented to characterize individually the four active elements of an MR bridge current sensor preserving their internal connections. The system suggests the possibility to sense electrical currents using only one element of the bridge opening the way to design new MR sensors based on this concept. Sensitivity, offset and non-linearity deviation were obtained using bridges of tunnel (TMR)- and giant (GMR)-based MR technologies. The single element embedded in a Wheatstone bridge configuration is used for practical current measurements in a 50 Hz line. An electronic circuitry is proposed to measure alternating (AC) currents with a single MR element, including a lock-in amplifier and an interface to properly convert the signal to its root mean square (rms) value with a resolution of 250 mA peak in the 125 A range. [ABSTRACT FROM AUTHOR]

Eddy current inspection has been extensively employed in non-destructive testing of various conductive materials. The coil probe, as a mainstream sensor in the eddy current detection system, inevitably encounters interference from external signals while transmitting its own signal. Therefore, developing techniques to extract valuable signals from noisy ones is crucial for ensuring accurate detection. Carbon fiber composites not only possess significantly lower electrical conductivity compared to conventional metallic materials but also exhibit notable anisotropy. To address this issue, we designed an '8' coil probe set where the excitation coil does not electromagnetically interfere with the detection coil. However, practical applications that require portability and miniaturization pose challenges when utilizing this coil probe set to identify carbon content or defects due to the typically weak output signal. To address this issue, this paper proposes a design that combines the '8' structure of the planar coil probe with the principle of phase-locked amplification to create a dual-phase sensitive phase-locked amplification detection circuit. These specific design ideas were tested using a weak signal, which passed through the preamplifier, secondary amplifier, and band-pass filter comprising the target channel for signal amplification and noise filtering. The effective signal amplitude is proportional to the inverse phase difference between the direct current (DC) signal and inversely proportional to the amplitude of the signal. Finally, the DC signal was passed through an analog-to-digital converter (ADC). The analog-to-digital converter (A/D) is used to collect and calculate the DC signal, enabling the detection of weak electrical signals. Simulation experiments demonstrated that the signal detection circuit has an amplitude error below 0.2% and a phase error below 0.5%. The phase-locked amplification circuit designed in this paper can effectively extract the tiny impedance change signals of the planar coil sensor probe with high sensitivity and good robustness. [ABSTRACT FROM AUTHOR]

The autofluorescence of erythrocyte porphyrins has emerged as a potential method for multi-cancer early detection (MCED). With this method’s dependence on research-grade spectrofluorometers, significant improvements in instrumentation are necessary to translate its potential into clinical practice, as with any promising medical technology. To fill this gap, in this paper, we present an automated ratio porphyrin analyzer for cancer screening (ARPA-CS), a low-cost, portable, and automated instrument for MCED via the ratio fluorometry of porphyrins. The ARPA-CS aims to facilitate cancer screening in an inexpensive, rapid, non-invasive, and reasonably accurate manner for use in primary clinics or at point of care. To accomplish this, the ARPA-CS uses an ultraviolet-excited optical apparatus for ratio fluorometry that features two photodetectors for detection at 590 and 630 nm. Additionally, it incorporates a synchronous detector for the precision measurement of signals based on the Walsh-ordered Walsh–Hadamard transform (WHT)w and circular shift. To estimate its single-photodetector capability, we established a linear calibration curve for the ARBA-CS exceeding four orders of magnitude with a linearity of up to 0.992 and a low detection limit of 0.296 µg/mL for riboflavin. The ARPA-CS also exhibited excellent repeatability (0.21%) and stability (0.60%). Moreover, the ratio fluorometry of three serially diluted standard solutions of riboflavin yielded a ratio of 0.4, which agrees with that expected based on the known emission spectra of riboflavin. Additionally, the ratio fluorometry of the porphyrin solution yielded a ratio of 49.82, which was ascribed to the predominant concentration of protoporphyrin IX in the brown eggshells, as confirmed in several studies. This study validates this instrument for the ratio fluorometry of porphyrins as a biomarker for MCED. Nevertheless, large and well-designed clinical trials are necessary to further elaborate more on this matter.

On‐site measurement for generating photovoltaic (PV) modules is important for detecting many PV cell failures; however, measuring the working cell voltage in a PV module is impractical. We improve a noncontact estimation method (modulated light–irradiation method) that irradiates modulating laser light to detect the synchronized signal in the PV module output current using an alternating current (AC) clamp sensor and a lock‐in amplifier to realize real‐time observation of the working cell voltage. We used a preamplifier with a high‐pass filter to enhance the sensitivity of the AC clamp sensor and lock‐in amplifier with an electronically modulated semiconductor laser while considering the conditions for the lock‐in amplifier and the preamplifier. Experiments under a solar simulator with weakly vibrating light at 100 Hz and results of time‐fluctuating voltages of all the cells under noise‐reduction‐optimized conditions are performed for validation. This procedure also reduces the time required to scan all cell voltages in PV modules. © 2023 Institute of Electrical Engineer of Japan and Wiley Periodicals LLC. [ABSTRACT FROM AUTHOR]

Neutral voltage (NV) reflects the unbalanced condition of a five-phase permanent magnet synchronous motor (PMSM). Thus, it can be used to screen out the open-switch, open-phase, and inter-turn faults of a single phase, with a low rate of misdiagnosis. However, because there is no neutral point lead for most electric drives, the measurement of NV is difficult. To this end, an NV observation method is proposed in this paper by exploiting phase currents and reference pole voltages. This method can be regarded as a kind of multi-sensor information fusion technology. On this basis, a lock-in amplifier technique is further presented to calculate the frequency spectrum from the re-constructed NV, which achieves accurate classification of the stated single-phase faults. Finally, experimental results confirm the validity of the presented method in terms of fast and accurate location determination of open-switch, open-phase, and inter-turn short-circuit faults. [ABSTRACT FROM AUTHOR]

Impedance measurements are crucial in a variety of applications, from the characterization of lithium batteries, microbial fuel cells, and biosensors to the study of polymers and material degradation, where strict requirements have to be met in terms of frequency bandwidth and current level. Here, we present a cost-effective compact solution for ultra-low-frequency impedance measurements, operating in a wide range, from 1 mHz to 250 kHz. Coupled to a lock-in amplifier, the designed circuit is based on a Howland current pump cascaded by a precision current divider in order to set the conversion factor at 100 nA/V, 1 μA/V, or 100 μA/V. Therefore, it is possible to generate very low-current signals to measure resistive impedances up to 100 MΩ. In addition, a feedback network is inserted to null the voltage drift induced by leakage currents and offset voltages, thus allowing the measurement of low-capacitance loads, experimentally tested down to 10 nF. Remarkably, the feedback network allows to perform measurements also in the presence of high voltage bias of the load and experimental results performed up to 60 V demonstrate the excellent stability of the designed system, thus a high voltage compliance. The proposed circuit is particularly interesting for the conditioning of both resistive and capacitive sensors and it is likely to be an effective solution for the implementation of a portable instrument for measuring signals from biosensors. [ABSTRACT FROM AUTHOR]

nowadays, the modern microelectronics digital communication systems are using low power and efficient design for wide variety of applications in real world. Recent researchers focusing on amplifier circuits for increasing applications with novel designs; conventional method uses FinFET CMOS design, and other reconfigurable designs, which consumes much power on designing and to overcome the design complexity of existing work, the proposed optimal power reduction strategy is employed on the ADC lock-in amplifier. Here the two stage lock-in amplifier is constructed with gated clocking of DFF switching strategy to reduce the power consumption and to obtain better results than previous works. Virtext FPGA model of amplifier is designed with Xilinx ISE 14.2 tool. The design of lock-in Amplifier has both sensing stage and latch stage with better sampling periods of edge triggering state, which improves the performance. High speed FPGA with mixed signal processing application is capable for this design. Experimental results analyzed with the power, area and delay as major concern and switching state with clocking model determines the high speed design. By comparing with the previous work, the proposed work is designed to achieve better results. [ABSTRACT FROM AUTHOR]

Development of compact and low-cost electronic circuits in the biosensor field is an essential demand. Especially, the design of noise reduction sections plays an important role in the measurement of small biosensing signals. This study develops an electrical impedance measurement circuit board to apply in the protein pre-concentration and detection microfluidic chips with integrated two-electrode impedance micro-sensor. A lock-in amplifier is applied in the circuit board to measure the impedance profile in the frequency range 10–200 kHz. An investigation on several ratios of signal to noise (SNRs) is conducted to indicate the ability of the proposed design in eliminating noise signals. Subsequently, a total impedance and series capacitance-based analysis approach from the recorded impedance data is demonstrated to be suitable for this protein chip in the frequencies range from 10 to 200 kHz. The obtained results enable a potential method in point-of-care diagnostic applications. [ABSTRACT FROM AUTHOR]

Eddy current inspection has been extensively employed in non-destructive testing of various conductive materials. The coil probe, as a mainstream sensor in the eddy current detection system, inevitably encounters interference from external signals while transmitting its own signal. Therefore, developing techniques to extract valuable signals from noisy ones is crucial for ensuring accurate detection. Carbon fiber composites not only possess significantly lower electrical conductivity compared to conventional metallic materials but also exhibit notable anisotropy. To address this issue, we designed an ‘8’ coil probe set where the excitation coil does not electromagnetically interfere with the detection coil. However, practical applications that require portability and miniaturization pose challenges when utilizing this coil probe set to identify carbon content or defects due to the typically weak output signal. To address this issue, this paper proposes a design that combines the ‘8’ structure of the planar coil probe with the principle of phase-locked amplification to create a dual-phase sensitive phase-locked amplification detection circuit. These specific design ideas were tested using a weak signal, which passed through the preamplifier, secondary amplifier, and band-pass filter comprising the target channel for signal amplification and noise filtering. The effective signal amplitude is proportional to the inverse phase difference between the direct current (DC) signal and inversely proportional to the amplitude of the signal. Finally, the DC signal was passed through an analog-to-digital converter (ADC). The analog-to-digital converter (A/D) is used to collect and calculate the DC signal, enabling the detection of weak electrical signals. Simulation experiments demonstrated that the signal detection circuit has an amplitude error below 0.2% and a phase error below 0.5%. The phase-locked amplification circuit designed in this paper can effectively extract the tiny impedance change signals of the planar coil sensor probe with high sensitivity and good robustness.

Lock-in amplifiers, used in several experimental physics applications, are instruments performing quadrature demodulation, which is useful when signals are affected by much noise. Generally, commercially-available lock-in amplifiers are very accurate, but expensive, especially if their operating range includes radiofrequencies. In many applications, high precision is not necessary for the measurements, but it is preferable to have low-cost, low-weight, compactness and a user-friendly graphical unit interface. In this paper, we describe a new fully-digital low-frequency lock-in amplifier developed at ENEA C.R. Frascati Laboratories for photoluminescence experiments based on an innovative low-cost architecture and processing algorithms. The hardware, firmware and software developed for the whole photoluminescence measurement set-up is presented. The present lock-in was first characterized with synthetic electrical sine wave signals and white noise. A dynamic reserve of 43 dB and a noise figure in the range of 25–44 dB were estimated. These results show compatibility with several measurement applications, such as photoluminescence, and the adequacy of the resolutions with respect to the hardware costs. Finally, preliminary results of photoluminescence measurements are presented. [ABSTRACT FROM AUTHOR]

Laser interferometer technology is used in the precision positioning stage as an encoder. For better resolution, laser interferometers usually work with interpolation devices. According to the interpolation factor, these devices can convert an orthogonal sinusoidal signal into several square-wave signals via digital processing. The bandwidth of the processing will be the limitation of the moving speed of the positioning stage. Therefore, the user needs to make a trade-off between the interpolation factor and the moving speed. In this investigation, a novel analog interpolation method for a heterodyne laser interferometer has been proposed. This method is based on the principle of the lock-in amplifier (LIA). By using the proposed interpolation method, the bandwidth of the laser encoder system can be independent of the interpolation factor. This will be a significant benefit for the ultra-high resolution encoder system and the laser interferometers. The concept, design, and experiment are revealed in this manuscript. The experimental results show that the proposed interpolation method can reach nanometer resolution with a heterodyne laser interferometer, and the bandwidth of the signal is independent of the resolution. [ABSTRACT FROM AUTHOR]

A key milestone for the pervasive diffusion of wireless sensing nodes for smart monitoring of water quality and quantity in distribution networks is the simplification of the installation of sensors. To address this aspect, we demonstrate how two basic contactless sensors, such as piezoelectric transducers and strip electrodes (in a longitudinal interdigitated configuration to sense impedance inside and outside of the pipe with potential for impedimetric leak detection), can be easily clamped on plastic pipes to enable the measurement of multiple parameters without contact with the fluid and, thus, preserving the integrity of the pipe. Here we report the measurement of water flow rate (up to 24 m3/s) and temperature with ultrasounds and of the pipe filling fraction (capacitance at 1 MHz with ~cm3 resolution) and ionic conductivity (resistance at 20 MHz from 700 to 1400 μS/cm) by means of impedance. The equivalent impedance model of the sensor is discussed in detail. Numerical finite-element simulations, carried out to optimize the sensing parameters such as the sensing frequency, confirm the lumped models and are matched by experimental results. In fact, a 6 m long, 30 L demonstration hydraulic loop was built to validate the sensors in realistic conditions (water speed of 1 m/s) monitoring a pipe segment of 0.45 m length and 90 mm diameter (one of the largest ever reported in the literature). Tradeoffs in sensors accuracy, deployment, and fabrication, for instance, adopting single-sided flexible PCBs as electrodes protected by Kapton on the external side and experimentally validated, are discussed as well. [ABSTRACT FROM AUTHOR]

A frequency downscaling technique for enhancing the accuracy of analog lock-in amplifier (LIA) architectures in giant magneto-impedance (GMI) sensor applications is presented in this paper. As a proof of concept, the proposed method is applied to two different LIA topologies using, respectively, analog and switching-based multiplication for phase-sensitive detection. Specifically, the operation frequency of both the input and the reference signals of the phase-sensitive detector (PSD) block of the LIA is reduced through a subsampling process using sample-and-hold (SH) circuits. A frequency downscaling from 200 kHz, which is the optimal operating frequency of the employed GMI sensor, to 1 kHz has been performed. In this way, the proposed technique exploits the inherent advantages of analog signal multiplication at low frequencies, while the principle of operation of the PSD remains unaltered. The circuits were assembled using discrete components, and the frequency downscaling proposal was experimentally validated by comparing the measurement accuracy with the equivalent conventional circuits. The experimental results revealed that the error in the signal magnitude measurements was reduced by a factor of 8 in the case of the analog multipliers and by a factor of 21 when a PSD based on switched multipliers was used. The error in-phase detection using a two-phase LIA was also reduced by more than 25%. [ABSTRACT FROM AUTHOR]

In order to eliminate the harmonics of the grid connected inverters accurate harmonic detection is essential. Though there are many kinds of different harmonic detection methods are available, it can hardly be achieved without the help of grid frequency detection of the fundamental component. In this paper a novel Lock-In Amplifier based Frequency Locked Loop (LIA-FLL) is proposed for the improved harmonic elimination of the grid-connected three phase inverters. Due to the outstanding performance of the LIA-FLL, the harmonics can be detected and compensated accurately under the grid frequency variation. To demonstrate the superiority of the proposed method, it is applied to a 10 kW three phase grid connected inverters. The results obtained by the proposed method are compared with those obtained by the harmonic elimination method with no FLL. The principle of LIA-FLL is discussed and its performance for the harmonic elimination is proved by the simulation and experiments with a 10KW three phase grid connected inverter under the highly polluted grid condition.

Electrical stimulation is widely used in nerve regulation and treatment. The detection of the distribution of stimulation current in tissues is of great significance to improve the accuracy of electrical stimulation, but the current technical means are still difficult to achieve the non-invasive detection of stimulation current. This study proposes a non-invasive detection method of electrical stimulation current based on the magneto-acoustic (MA) effect, which has the advantages of high spatial resolution and high spatial contrast. In this study, continuous sine waves with the frequency of 20kHz are used to stimulate samples. The MA signal generated by the stimulation current in the samples in a stable magnetic field is detected by lock-in amplifier, and the two-dimensional distribution of sound source is extracted. The current density distribution of samples is simulated by the method of finite element analysis, and the simulation results are verified by experiments. The results show that under the electrical stimulation of the order of 0.01A, the location measurement of two-dimensional surface sound source with millimeter accuracy can be achieved non-invasively in isolated pork and pig brain, and the measurement accuracy of weak MA signal can reach 10−7Pa. In short, the stimulation current detection method based on MA effect can achieve the non-invasive and high-precision detection of electrical stimulation current distribution, which is of great significance to improve the stimulation accuracy and study the neural regulation mechanism of electrical stimulation.

Nowadays, the modern microelectronics digital communication systems are using low power and efficient design for wide variety of applications in real world. Recent researchers focusing on amplifier circuits for increasing applications with novel designs; conventional method uses FinFET CMOS design, and other reconfigurable designs, which consumes much power on designing and to overcome the design complexity of existing work, the proposed optimal power reduction strategy is employed on the ADC lock-in amplifier. Here the two stage lock-in amplifier is constructed with gated clocking of DFF switching strategy to reduce the power consumption and to obtain better results than previous works. Virtext FPGA model of amplifier is designed with Xilinx ISE 14.2 tool. The design of lock-in Amplifier has both sensing stage and latch stage with better sampling periods of edge triggering state, which improves the performance. High speed FPGA with mixed signal processing application is capable for this design. Experimental results analyzed with the power, area and delay as major concern and switching state with clocking model determines the high speed design. By comparing with the previous work, the proposed work is designed to achieve better results. [ABSTRACT FROM AUTHOR]

Pulse rate variability is a physiological parameter that has been extensively studied and correlated with many physical ailments. However, the phase relationship between inter-beat interval, IBI, and breathing has very rarely been studied. Develop a technique by which the phase relationship between IBI and breathing can be accurately and efficiently extracted from photoplethysmography (PPG) data. A program based on Lock-in Amplifier technology was written in Python to implement a novel technique, Dynamic Phase Extraction. It was tested using a breath pacer and a PPG sensor on 6 subjects who followed a breath pacer at varied breathing rates. The data were then analyzed using both traditional methods and the novel technique (Dynamic Phase Extraction) utilizing a breath pacer. Pulse data was extracted using a PPG sensor. Dynamic Phase Extraction (DPE) gave the magnitudes of the variation in IBI associated with breathing (Δ I B I) measured with photoplethysmography during paced breathing (with premature ventricular contractions, abnormal arrhythmias, and other artifacts edited out). Δ I B I correlated well with two standard measures of pulse rate variability: the Standard Deviation of the inter-beat interval (SDNN) (ρ = 0.911) and with the integrated value of the Power Spectral Density between 0.04 and 0.15 Hz (Low Frequency Power or LF Power) (ρ = 0.885). These correlations were comparable to the correlation between the SDNN and the LF Power (ρ = 0.877). In addition to the magnitude Δ I B I , Dynamic Phase Extraction also gave the phase between the breath pacer and the changes in the inter-beat interval (IBI) due to respiratory sinus arrythmia (RSA), and correlated well with the phase extracted using a Fourier transform (ρ = 0.857). Dynamic Phase Extraction can extract both the phase between the breath pacer and the changes in IBI due to the respiratory sinus arrhythmia component of pulse rate variability ( Δ I B I) , but is limited by needing a breath pacer. [ABSTRACT FROM AUTHOR]

The need for affordable low-power devices has led MEMS-based thermal emitters to become an interesting option for optical gas sensors. Since these emitters have a low thermal mass, they can be easily modulated and combined with a lock-in amplifier for detection. In this paper, we show that the signal measured by a lock-in amplifier from a thermal emitter that varies its temperature periodically can have different spectral profiles, depending on the reference signal used. These virtual emitters appear because the Fourier series expansion of the emitted radiance, as a function of time, has different coefficients for each wavelength, and this spectral signature, which is different for each harmonic, can be retrieved using a reference signal that corresponds to its frequency. In this study, the effect is first proved theoretically and then is measured experimentally. For this purpose, we performed measurements with an IR camera provided with six different spectral filters of a modulated emitter, in combination with lock-in amplification via software. Finally, we show a potential application of this effect using multiple virtual emitters to gain spectral selectivity and distinguish between two gases, CO 2 and CH 4 . [ABSTRACT FROM AUTHOR]

Electrical stimulation is widely used in nerve regulation and treatment. The detection of the distribution of stimulation current in tissues is of great significance to improve the accuracy of electrical stimulation, but the current technical means are still difficult to achieve the non-invasive detection of stimulation current. This study proposes a non-invasive detection method of electrical stimulation current based on the magneto-acoustic (MA) effect, which has the advantages of high spatial resolution and high spatial contrast. In this study, continuous sine waves with the frequency of 20kHz are used to stimulate samples. The MA signal generated by the stimulation current in the samples in a stable magnetic field is detected by lock-in amplifier, and the two-dimensional distribution of sound source is extracted. The current density distribution of samples is simulated by the method of finite element analysis, and the simulation results are verified by experiments. The results show that under the electrical stimulation of the order of 0.01A, the location measurement of two-dimensional surface sound source with millimeter accuracy can be achieved non-invasively in isolated pork and pig brain, and the measurement accuracy of weak MA signal can reach 10−7Pa. In short, the stimulation current detection method based on MA effect can achieve the non-invasive and high-precision detection of electrical stimulation current distribution, which is of great significance to improve the stimulation accuracy and study the neural regulation mechanism of electrical stimulation. [ABSTRACT FROM AUTHOR]

High-frequency (HF) radio-frequency identification (RFIDs) tags can be combined to electrodes coated with semiconducting thin-films to produce low-cost and disposable sensors with wireless capabilities. Mobile phone applications generally use near-field connection (NFC) to read chip digital information, providing discrete information from the device. To obtain a continuous analogic reading, more sophisticated equipment such as spectrum analyzers are needed. In the present article, we describe a method to obtain the frequency response spectrum of RFID tags using synchronous detection via a lock-in amplifier. The tags were coupled to interdigitated electrodes coated with semiconducting metal oxides (SMOs) to produce UV-sensors and with semiconducting polymer polyaniline (Pani) as chemically responsive wireless devices for the proof-of-concept of the developed method. [ABSTRACT FROM AUTHOR]

In measuring small voltage deviations of about 1 µV and lower, it is important to separate useful signals from noise. The measurement of small voltage deviations between the amplitudes of two AC signals in wide frequency and voltage ranges, is performed by using lock-in amplifiers with the differential input as a comparator (null-indicator). The resolution and measurement accuracy of lock-in amplifiers is largely determined by the common-mode rejection ratio in their measuring channel. This work presents a developed differential signal recovery circuit with embedded photovoltaic modules, which allows implementing the dual phase lock-in amplifier with the differential input and quasi-invariant common-mode signal. The obtained metrological parameters of the proposed dual phase analog lock-in amplifier prove its applicability in comparing two signal amplitudes of 10√2 µV to 10√2 V in the frequency range of 20 Hz to 100 kHz with a 10 nV resolution. The proposed dual phase analog lock-in amplifier was characterized by a 130 to 185 dB CMRR in the frequency range up to 100 kHz with 20 nV/√Hz white noise. [ABSTRACT FROM AUTHOR]

Optical dissolved oxygen (DO) concentration measurement based on phase fluorometric theory is one of the most promising DO concentration measurement methods with advantages of insensitivity to the solution, short response time, and satisfying maintainability. However, most phase fluorometric DO concentration measurement systems have limitations such as strong dependence with the chemiluminescent films, low level integration and large noise interference. In this paper, a fully integrated lock-in based phase fluorometric DO sensor interface is proposed with a wide range tunable excitation frequency to be compatible with various chemiluminescent films. The lock-in technique is adopted to improve the sensitivity with large background noise. The proposed interface chip consists of driver for excitation light, transimpedance amplifier (TIA) to read out the weak current of the optical detector, and lock-in amplifier (LIA) for phase-shift detection. A tunable sinusoidal current generator (SCG) based on digital recursive oscillator (RDO) is adopted to generate excitation signal, meeting the requirements of different chemiluminescent films, while a counter-based orthogonal signals generator (OSG) is applied to generate orthogonal demodulating signals. The frequency error between the demodulating signals and excitation signal, which would cause phase-shift detection error, is cancelled by digital calibration method, which relaxes the challenge to match the demodulation signals with the tunable excitation signal. The TIA with DC cancellation is utilized to suppress the DC current of the photodiode (PD) and thus to improve the accuracy of phase-shift detection. The customized interface chip is implemented using a $0.18~\mu \text{m}$ CMOS process with a core area of $660~\mu \text{m}^\ast 464~\mu \text{m}$ . The test results show that the developed system has an accuracy of 0.06 degrees and a precision of 0.2 degrees in terms of the excitation frequency range from 4 kHz to 150 kHz.

Superconducting magnets with stable and strong magnetic fields play a pivotal role in fields such as biology, medical science, and physics. Given the difficult measurement conditions and low measurement accuracy of the strong magnetic field, a measurement method based on magnetic flux modulation is proposed in this paper. Additionally, a high-stability coil-driving device is established. A circuit for coil speed acquisition and measurement is developed. The effective values of induced electromotive forces corresponding to the measured strong magnetic field are obtained, following the de-noising principle of the lock-in amplifier. Particularly, a traceable transmission standard is formed when the measurement coil is calibrated by the national standard magnetic field. The experimental system results demonstrated that the uncertainty of the proposed method reaches the order of E-4 for the measurement of magnetic induction intensity below 7T. This significantly contributes to the measurement of superconducting strong magnetic fields. [ABSTRACT FROM AUTHOR]

Proper soil nutrients content plays an important role in agricultural production—undernutrition would reduce crop yield and quality and overnutrition would cause environmental pollution. Though the traditional approaches based on sampling and chemical analysis can comprehensively and accurately measure soil nutrients, but the soil sampling and pretreatment process are cumbersome, complicated, time-consuming, and costly. Therefore, rapid and accurate measurement of soil nutrients is of great significance for precise fertilizer application, which can increase yield, improve crop quality, and alleviate environmental pollution. Toward this objective, a rapid soil nutrients detection method based on modulated near infrared spectroscopy for active near-field telemetry was proposed, which could effectively minimize effect of sunlight during the measuring process. Eight channels narrow-band laser diodes with wavelengths of 1260, 1310, 1350, 1410, 1450, 1510, 1550 and 1610 nm were selected as active lighting sources for measuring the reflectance of soil samples. Eight channels narrow-band laser diodes were symmetrically placed on a concentric circle. A photodetector with a circular photosensitive area of 5 mm in diameter was placed at the center of the concentric circle to maximize the reception of laser beam reflected by soil. A focusing lens was placed in front of the photodetector to collect the laser beam reflected from the soil sample to increase the sensitivity. The sensing area of the photodetector was located at the focus of the lens. seventy four groups of soil samples with known N content were divided into training set (54 groups) and prediction set (20 groups) for data analysis. The spectral reflectance significantly correlated with soil N content was screened by analyzing the training set based on a general linear model and a quantitative measurement model with R2 of 0.97 between the screened spectral reflectance and soil N content was achieve. The predicted soil N content obtained from prediction set based on the established model and the referenced soil N content of the prediction set had a R2 of 0.9, indicating that this method has an ability to quickly, as well as accurately detect soil nutrients.

Laser interferometer technology is used in the precision positioning stage as an encoder. For better resolution, laser interferometers usually work with interpolation devices. According to the interpolation factor, these devices can convert an orthogonal sinusoidal signal into several square-wave signals via digital processing. The bandwidth of the processing will be the limitation of the moving speed of the positioning stage. Therefore, the user needs to make a trade-off between the interpolation factor and the moving speed. In this investigation, a novel analog interpolation method for a heterodyne laser interferometer has been proposed. This method is based on the principle of the lock-in amplifier (LIA). By using the proposed interpolation method, the bandwidth of the laser encoder system can be independent of the interpolation factor. This will be a significant benefit for the ultra-high resolution encoder system and the laser interferometers. The concept, design, and experiment are revealed in this manuscript. The experimental results show that the proposed interpolation method can reach nanometer resolution with a heterodyne laser interferometer, and the bandwidth of the signal is independent of the resolution.

During the last years, visible light communications (VLC) have been proposed for providing connectivity while ensuring satisfactory illumination in both indoor environments and also specific outdoor scenarios without the need of deploying complex infrastructures for that purpose. Transmission for VLC is carried out through light-emitting diodes (LEDs), which correspond to semiconductors based on PN-junction materials with a direct gap. In this sense, the current flow plays a major role in the behavior and performance of these devices for VLC. Therefore, characterizing the electrical response of high-power white LED results mandatory for the successful implementation of VLC. At this point, it is worth noting that the electrical characterization is usually not available for high-power LEDs since, in fact, determining these characterization results challenging. In this sense, there exist some measurement instruments such as LCRs or impedance analyzers typically employed for characterizing materials and passive electrical components. However, these kinds of instruments are subject to a limited input impedance and a maximum value of forwarding current. In this work, the electrical characterization of the LED LXHL-BW02 of Luxeon is analyzed to show that typically commercial instrumentation for characterizing these devices is limited for high-power LEDs, which may provide polluted results when these limitations are not considered. After that, the characterization of the LXHL-BW02 based on a lock-in amplifier is proposed.

Capacitance detection is a universal transduction mechanism used in a wide variety of sensors and applications. It requires an electronic front-end converting the capacitance variation into another more convenient physical variable, ultimately determining the performance of the whole sensor. In this paper we present a comprehensive review of the different signal conditioning front-end topologies targeted in particular at sub-femtofarad resolution. Main design equations and analysis of the limits due to noise are reported in order to provide the designer with guidelines for choosing the most suitable topology according to the main design specifications, namely energy consumption, area occupation, measuring time and resolution. A data-driven comparison of the different solutions in literature is also carried out revealing that resolution, measuring time, area occupation and energy/conversion lower than 100 aF, 1 ms 0.1 mm2, and 100 pJ/conv. can be obtained by capacitance to digital topologies, which therefore allow to get the best compromise among all design specifications.

The demodulation of mechanical oscillations is essential for the operation of micro- and nanometer-scale transducer systems, such as resonant MEMS mirrors and cantilever-based sensors. The use of external demodulators, such as lock-in amplifiers and spectrum analyzers is not always desired due to their large size, complexity, and cost. The method presented in this article enables simplified demodulation of the oscillation of mechanical oscillators with integrated deflection sensors. By configuring two deflection sensors in separate bridge circuits, which are supplied with in-phase and quadrature sinusoidal signals, the amplitude, and phase of the mechanical oscillation can directly be measured without additional demodulation. The method is implemented and experimentally verified by demodulating the oscillation of a self-sensing Atomic Force Microscopy cantilever with integrated piezoresistive elements. [ABSTRACT FROM AUTHOR]

Two simulation programs of a stimulated Raman scattering microscopy (SRS) imaging system with lock-in amplifier (LIA) detection were developed. SRS is an imaging technique based on the vibrational Raman cross-section as the contrast mechanism and enables fast, label-free imaging. Most SRS implementations are based on LIA detection of a modulated signal. However, building and operating such SRS set-ups still poses a challenge when selecting the LIA parameter settings for optimized acquisition speed or image quality. Moreover, the type of sample, e.g. a sparse sample vs. a densely packed sample, the required resolution as well as the Raman cross-section and the laser powers affect the parameter choice. A simulation program was used to find these optimal parameters. The focal spot diameters of the individual lasers (pump and Stokes) were used to estimate the effective SRS signal focal spot and the (optical) spatial resolution. By calibrating the signal and noise propagation through an SRS system for a known molecule, we estimated the signal and noise input to the LIA. We used a low pass filter model to simulate the LIA behavior in order to find the optimal parameters (i.e. filter order and time constant). Optimization was done for either image quality (expressed as contrast to noise ratio) or acquisition time. The targeted object size was first determined as a measure for the required resolution. The simulation output consisted of the LIA parameters, pixel dwell time and contrast to noise ratio. In a second simulation we evaluated SRS imaging based on the same principles as the optimal setting simulation, i.e. the signals were propagated through an imaging system and LIA detection. The simulated images were compared to experimental SRS images of polystyrene beads. Finally, the same software was used to simulate multiplexed SRS imaging. In this study we modeled a six-channel frequency-encoded multiplexed SRS system demodulated with six LIA channels. We evaluated the inter-channel crosstalk as a function of chosen LIA parameters, which in multiplex SRS imaging also needs to be considered. These programs to optimize the contrast to noise ratio, acquisition speed, resolution and crosstalk will be useful for operating stimulated Raman scattering imaging setup, as well as for designing novel setups. [ABSTRACT FROM AUTHOR]

We introduce a multi-channel integrated circuit for fast stimulated Raman scattering (SRS) microscopy on multiple simultaneous frequencies at a frame rate higher than 1 frame/s. The chip is a four-channel differential readout system, based on the lock-in technique. It is able to measure down to 10 ppm SRS signal, over a wide range of input optical powers (50–600 μW per channel), with a pixel dwell time of only 30 μs. Each acquisition channel includes two low-noise preamplifiers, two variable-gain amplifiers, a fully differential voltage subtractor, and a lock-in demodulator. The differential readout electronics rejects the power fluctuations of the laser, and it is automatically balanced by an analog feedback loop over ±30% input power mismatches. Due to the autobalancing network, the pixel dwell time is reduced by a factor up to 225 with a settling time of only 10 μs. The chip is fabricated in AMS 0.35 μm CMOS technology, and it is included in a combined electronics and optical system. Both single-pixel spectral measurements and multi-spectral imaging measurements are presented to validate the full SRS microscope. [ABSTRACT FROM AUTHOR]

A kind of signal acquisition circuit and the related signal processing method of the capacitance electromagnetic flowmeter were introduced. The circuit can eliminate the influence of distributed capacitance on the input impedance of the operational amplifier, and greatly improve the input impedance of the detection circuit to overcome the disadvantage of high signal source impedance. The rotating capacitor filter is a signal processing method based on the phase-sensitive detection technology. It can extract the weak signal from the strong and wide-band background noise, so it is very suitable for the processing of capacitive electromagnetic flow signals. Through the comparison of the signal amplitude obtained at different flow rates and the comparison of signal spectrum components before and after the filter, the effectiveness of the bootstrap signal acquisition circuit and rotating capacitor filtering method is verified.

This paper presents a simple scheme for enhancing the resolution of giant magnetoresistance (GMR) sensors by making use of an AC-biased method. In doing so, an alternating voltage was used to power the GMR sensor. As in the case of high-frequency bias, significant suppression of low-frequency (1/f) noise was experimentally observed. With the bare GMR sensor, the 1/f noise value at 1 Hz was recorded to be 15 nT/√Hz, whereas applying the AC bias, the noise was reduced significantly by a factor of 30, with a minimum noise level of 0.5 nT/√Hz@1 Hz. Furthermore, the hysteresis (Hc) of the sensor was also collapsed from 0.1 to 0.015 mT with a negligible offset. The performance of the AC-biased scheme was verified by measuring the Earth's magnetic field with the measured error being minimized from 2.5 μT down to 0.4 μT. The favorable results indicate that the proposed scheme is a pertinent way to improve the resolution of pinning GMR sensors in ultra-weak magnetic fields and static magnetic field measurements. [ABSTRACT FROM AUTHOR]

Size and absolute concentration of suspensions of nanoparticles are important information for the study and development of new materials and products in different industrial applications spanning from biotechnology and pharmaceutics to food preparation and conservation. Laser Transmission Spectroscopy (LTS) is the only methodology able to measure nanoparticle size and concentration by performing a single measurement. In this paper we report on a new variable gain calibration procedure for LTS-based instruments allowing to decrease of an order of magnitude the experimental indetermination of the particle size respect to the conventional LTS based on the double ratio technique. The variable gain calibration procedure makes use of a specifically designed tunable-gain, dual-channel, dual-phase Lock-In Amplifier (LIA) whose input voltage signals are those ones generated by two Si photodiodes that measure the laser beam intensities passing through the sample containing the nanoparticles and a reference optical path. The LTS variable gain calibration procedure has been validated by firstly using a suspension of NIST standard polystyrene nanoparticles even 36 hours after the calibration procedure was accomplished. The paper reports in detail the LIA implementation describing the design methodologies and the electronic circuits. As a case example of the characterization of biological nanostructures, we demonstrate that a single LTS measurement allowed to determine size density distribution of a population of extracellular vesicles extracted from orange juice (25 nm in size) with the presence of their aggregates having a size of 340 nm and a concentration smaller than 3 orders of magnitude. [ABSTRACT FROM AUTHOR]

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The phase-sensitive detection (PSD), popularly referred to as lock-in amplifier (LIA) has been around for quite a long time. Lock-in amplifier has been quite a versatile measurement instrument for extracting low-level signals buried in noise. In the recent past, the importance of LIA has been realized as a sensor interfacing platform and a number of portable implementations targeting sensing applications have been reported. This review surveys LIA design, implementation and application as portable sensor interfaces. The paper put forward a brief history of phase-sensitive detection leading up to the state-of-the-art implementation in various fields of application. Different designs and implementation techniques for LIA have been categorized and introduced as per the design topologies. The main category being analog, digital, FPGA and CMOS integrated design. Finally, the application of LIA in resistive, capacitive, impedance, optical and Nano-bio sensors is categorized and investigated in detail. [ABSTRACT FROM AUTHOR]

Stimulating current distribution in the tissue is unknown due to the complex distribution. A preliminary in vivo measurement of the magneto-acoustic (MA) signal of the human finger is performed in this study. The approach for locating the magneto-acoustic source of the stimulating current is studied. We use a lock-in amplifier to measure the MA signal under continuous wave electrical stimulation. The phase of the MA signal is used to extract the location of the sonic source. The experimental system is designed to measure the MA signal under electrical stimulation. Preliminary experiments results show that the amplitude precision is improved to less than 1 μPa. The sonic source is located with millimetre precision. We propose a new MA source-locating method with high measurement and location precision. This method will be significant to the study of the imaging and monitoring of the current distribution of electrical stimulation with high precision. [ABSTRACT FROM AUTHOR]

Laser interferometers can achieve a nanometer-order uncertainty of measurements when their frequencies are locked to the reference frequencies of the atom or molecule transitions. There are three types of displacement-measuring interferometers: homodyne, heterodyne, and frequency modulation (FM) interferometers. Among these types of interferometer, the FM interferometer has many advantageous features. The interference signal is a series of time-dependent harmonics of modulation frequency, so the phase shift can be detected accurately using the synchronous detection method. Moreover, the FM interferometer is the most suitable for combination with a frequency-locked laser because both require frequency modulation. In previous research, low modulation frequencies at some tens of kHz have been used to lock the frequency of laser diodes (LDs). The low modulation frequency for the laser source means that the maximum measurement speed of the FM interferometers is limited. This paper proposes a novel contribution regarding the application of a high-frequency modulation for an LD to improve both the frequency stability of the laser source and the measurement speed of the FM interferometer. The frequency of the LD was locked to an I2 hyperfine component at 1 MHz modulation frequency. A high bandwidth lock-in amplifier was utilized to detect the saturated absorption signals of the I2 hyperfine structure and induce the signal to lock the frequency of the LD. The locked LD was then used for an FM displacement measuring interferometer. Moreover, a suitable modulation amplitude that affected the signal-to-noise ratio of both the I2 absorption signal and the harmonic intensity of the interference signal was determined. In order to verify the measurement resolution of the proposed interferometer, the displacement induced by a piezo electric actuator was concurrently measured by the interferometer and a capacitive sensor. The difference of the displacement results was less than 20 nm. To evaluate the measurement speed, the interferometer was used to measure the axial error of a high-speed spindle at 500 rpm. The main conclusion of this study is that a stable displacement interferometer with high accuracy and a high measurement speed can be achieved using an LD frequency locked to an I2 hyperfine transition at a high modulation frequency. [ABSTRACT FROM AUTHOR]

A near-infrared trace CO2 detection system was experimentally demonstrated based on an 1,580 nm distributed feedback (DFB) laser using a cascaded integrator comb (CIC) filter-assisted wavelength modulation spectroscopy (WMS) technique and a digital lock-in amplifier (DLIA). An 1,580 nm DFB laser emitting at 1,579.1 nm was selected as the light source. A multi-pass cell (MPGC with a 30 m- long path) was adopted as the absorption cell. A DLIA was developed to extract the second harmonic WMS (WMS-2f) signal. The cascade-integrated comb filter (CICF) and the finite impulse response (FIR) low-pass filter (LPF) were combined in the DLIA to improve the performance of DLIA. Three methods of PD (Peak detection), LIA, CICF-assisted WMS, and DLIA were adopted to detect the CO2 concentration levels and the anti-interference performance of each detection method was analyzed. The experimental results indicated that compared with the peak detection and the lock-in amplifier method of the CICF-assisted WMS and DLIA developed in this manuscript had the highest linearity and precision. When the integration time is 3s, the LOD of the system was predicted to be 0.0812 ppm. Due to the advantages of high sensitivity, a low detection limit and a rapid response, the CO2 detection system reported in this manuscript has a good application prospect in the area of oilfield associated gases detection.

• A new method for the rotational speed estimation was proposed. • The method was applied to simulation and actual vibration signals. • The effectiveness of the algorithm was validated. • The instantaneous phase can be accurately estimated. • The mean value of the rotational speed can to be accurately estimated. • More precise results can be achieved than using a tachometer with low resolution. • The algorithm can be integrated with fault detection techniques. The condition monitoring of mechanical systems by vibration analysis becomes a problem whenever the rotational speed varies and is unknown. This work proposes an algorithm to automatically estimate the rotational speed from the vibration generated by a gear pair. The algorithm is based on the decomposition of the vibration signal into mono-component signals and the detection of amplitude modulations. Singular Spectrum Decomposition was used for decomposing the vibration signal into the mono-component signals, and a non-quadratic phase coupling analysis was applied in order to detect the amplitude modulations. The algorithm is validated by working with simulated and true signals resulting from the application of different speed profiles. Finally, two practical cases are presented: (1) the implementation of order tracking through the rotational angle estimated by the new algorithm; and (2) the identification of the first harmonic of gear-mesh frequency from a time-frequency representation of the vibration signal. [ABSTRACT FROM AUTHOR]

This paper presents the use of an improved laser shadowgraphy method for measuring the dynamic elastic moduli of tungsten, from room temperature to near melting point. The method is based on measuring the frequency of impulse induced longitudinal vibration of a tungsten filament, subject to joule heating. The natural resonance frequency of the filament is obtained by lock-in synchronous photo-detection of the lasergenerated filament shadowgraph. The correlation between the filament natural frequency and its temperature lead to the calculation of shear modulus at variable temperatures, the tensile and bulk moduli of tungsten. The results were compared to those obtained by laser Doppler vibrometry (LDV) of a tungsten wire induced by pulsed current technique. Our method permitted a wider measuring temperature range and an easier calculation procedure. The used setup was simple, non-destructive, non-contacting and accurate, enabling low cost vibrometry measurements that help in future synthesis and test of new grades of refractory materials, deployed as plasma facing material in the latest fusion reactor or super alloys in critical applications. [ABSTRACT FROM AUTHOR]