Analog Lock-in Research Articles

Analog Lock-In Amplifier Design Using Subsampling for Accuracy Enhancement in GMI Sensor Applications.

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%.

Deep tissue multi-photon imaging using adaptive optics with direct focus sensing and shaping.

High-resolution optical imaging deep in tissues is challenging because of optical aberrations and scattering of light caused by the complex structure of living matter. Here we present an adaptive optics three-photon microscope based on analog lock-in phase detection for focus sensing and shaping (ALPHA-FSS). ALPHA-FSS accurately measures and effectively compensates for both aberrations and scattering induced by specimens and recovers subcellular resolution at depth. A conjugate adaptive optics configuration with remote focusing enables in vivo imaging of fine neuronal structures in the mouse cortex through the intact skull up to a depth of 750 µm below the pia, enabling near-non-invasive high-resolution microscopy in cortex. Functional calcium imaging with high sensitivity and high-precision laser-mediated microsurgery through the intact skull were also demonstrated. Moreover, we achieved in vivo high-resolution imaging of the deep cortex and subcortical hippocampus up to 1.1 mm below the pia within the intact brain.

Dual Phase Lock-In Amplifier with Photovoltaic Modules and Quasi-Invariant Common-Mode Signal

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.

Low-cost electronic circuitry for photoacoustic gas sensing

The circuitry comprises a sine wave generator based on direct digital synthesis, a laser diode driver module, a band-pass frequency filter, a synchronous detector with phase adjustment circuitry and a low pass filter to form an analog lock-in amplifier, and an analog-to-digital converter. A 32-bit ARM microcontroller programmed with the open source Mecrisp dialect of the Forth interpreter language is used to set the frequency, and read the data from the analog-to-digital converter. The circuitry is tethered via a serial interface to a personal computer. A graphical user interface written in Phython allows easy interaction with the microcontroller by sending the appropriate Forth commands. The data acquired is visualized and stored on the personal computer for further processing. The circuitry is easy to build as it is based on through-hole devices, except for two necessary surface mount items, which, however, still can be soldered with a fine tipped soldering iron. The performance of the circuitry was demonstrated by the photoacoustic detection of NO2 using a laser diode with a wavelength of 450 nm.

Palm-Sized Laser Spectrometer with High Robustness and Sensitivity for Trace Gas Detection Using a Novel Double-Layer Toroidal Cell.

A palm-sized laser spectrometer has been developed for detecting trace gases based on tunable diode laser absorption spectroscopy in combination with a novel double-layer toroidal cell. With the benefit of a homemade electronic system and compact optical design, the physical dimensions of the sensor are minimized to 24 × 15× 16 cm3. A toroidal absorption cell, with 84 reflections in 2 layers for an effective optical path length of 8.35 m, was used to enhance the absorption signals of gaseous species. A homemade electronic system was designed for implementing a distributed feedback (DFB) diode laser controller, an analog lock-in amplifier, data acquisition, and communication. Calibration-free scanned wavelength modulation spectroscopy was employed to determine the concentration of the gas and reduce the random fluctuations from electronical noise and mechanical vibration. The measurement of CH4 in ambient air was demonstrated using a DFB laser at 1.653 μm. The rise time and fall time for renewing the gas mixture are approximately 16 and 14 s, respectively. Vibration and temperature tests have been carried out for verifying the performance of the spectrometer, and standard deviations of 0.38 ppm and 0.11 ppm for 20 ppm CH4 at different vibration frequencies and temperatures, respectively, have been determined. According to the Allan deviation analysis, the minimum detection limit for CH4 can reach 22 ppb at an integration time of 57.8 s. The continuous measurement of atmospheric CH4 for 2 days validated the feasibility and robustness of our laser spectrometer, providing a promising laser spectral sensor for deploying in unmanned aerial vehicles or mobile robots.

Improved motional Stark effect signal processing using fast Fourier transform spectral analysis.

A Fast Fourier Transform (FFT) based method has been developed, which improves the frequency response of the Motional Stark Effect (MSE) system by about a factor of 10 over the conventional analog lock-in method. The method uses fits to rigorously derived analytic expressions for the FFT spectral components of the MSE signal to accurately obtain the amplitudes and phases of the 2f1 and 2f2 photo-elastic modulator (PEM) frequencies that encode the polarization angle. Since no frequency filtering is used in the FFT method, the frequency response is limited by fundamental measurement properties: the frequency response of the detector, photon statistics, sample rate, and the ability to resolve the spectral components. In contrast, the frequency response of the analog lock-in is limited by a low pass filter with a cutoff of around 500Hz. In the case of the DIII-D MSE system, the output of the photo-multiplier tube detector was sampled at 500 kHz and FFTs with as few as 100 points were used to obtain the amplitudes of the 2f1 and 2f2 PEM frequency components. This corresponds to a frequency response of 5 kHz, about ten times faster than the analog lock-in amplifier system. Details of the FFT method will be presented and compared to those of the analog lock-in system.

A Lossy Capacitance Measurement Circuit Based on Analog Lock-in Detection

This paper presents a lossy capacitance measuring circuit which is based on analog lock-in detection technique. Lossy capacitance can be modelled as a pure capacitor connected in parallel with a resistor. The measurement circuit mechanism consists of an excitation signal to drive the lossy capacitance, a transimpedance amplifier to produce a voltage, and a lock-in detection circuit to extract lossy values of capacitance. The lock-in detector multiplies its input with a square wave using switches and filters out high frequencies to give a DC output that is actually in proportional to the measured values. A field programmable gate array is employed to generate direct digital synthesis based sinusoidal excitation signal to generate reference signals required for demodulation and to measure the output of lock-in detection. The phase shift between the excitation signal and reference signals is controlled accurately in digital domain. Thus, due to the phase mismatch, errors are properly reduced. Also, analog phase shifter and analog switch-driving circuits are no longer required. Three different lossy capacitors realized using discrete components are simulated and tested. The maximum relative error is 1.62 % for the resistance measurement and 6.38 % for the capacitance measurement.

Multi-channel analog lock-in system for real-time motional Stark effect measurements

The motional Stark estark effect (MSE) diagnostic system developed to measure the plasma current density distribution in the Korea Superconducting Tokamak Advanced Research performs data analysis by applying Fourier transform algorithm using the Interactive Data Language (IDL) software after measurement and digital archiving. However, in order to realize advanced plasma control aiming at the development of high performance tokamak operations, there has been a demand for the MSE diagnostic system capable of real-time pitch-angle measurement associated with a plasma control system (PCS). For real-time, multi-channel, and high-speed MSE data processing with high precision, an analog lock-in technique has been proposed and its feasibility has been demonstrated through the table-top tests using the photoelastic-modulator-base polarimetry. The prototype multi-channel analog lock-in system is ready for the plasma commissioning and the integration with the PCS.

FPGA-Based Digital Lock-in Amplifier With High-Precision Automatic Frequency Tracking

Aiming to overcome the problem of the frequency error between the reference signal and the measured signal in an analog lock-in amplifier, this paper presented a digital lock-in amplifier (DLIA) with accurate frequency automatic tracking to improve its performance. In this paper, a modular design approach based on DSP Builder was employed to develop a quadrature vector type lock-in amplifier with automatic frequency tracking. A reference signal generator with high-frequency resolution and a digital filter with good performance were implemented. The performance of the design was tested by determining the linearity, the Q value, and the noise immunity of the DLIA. The experimental results showed that the proposed DLIA has good linearity and the Q value can reach up to 82. The relative errors of the signals with amplitudes greater than 10 mV were less than 1% when the equivalent input was a sine wave signal with a frequency of 1 kHz and an amplitude of 500 mV. When the superimposed noise was less than or equal to 400 mV, the relative error was less than 2% in the same condition. The proposed DLIA has higher precision and efficient noise immunity.

Low Cost Autonomous Lock-In Amplifier for Resistance/Capacitance Sensor Measurements

This paper presents the design and experimental characterization of a portable high-precision single-phase lock-in instrument with phase adjustment. The core consists of an analog lock-in amplifier IC prototype, integrated in 0.18 µm CMOS technology with 1.8 V supply, which features programmable gain and operating frequency, resulting in a versatile on-chip solution with power consumption below 834 µW. It incorporates automatic phase alignment of the input and reference signals, performed through both a fixed −90° and a 4-bit digitally programmable phase shifter, specifically designed using commercially available components to operate at 1 kHz frequency. The system is driven by an Arduino YUN board, thus overall conforming a low-cost autonomous signal recovery instrument to determine, in real time, the electrical equivalent of resistive and capacitive sensors with a sensitivity of 16.3 µV/Ω @ εrS < 3% and 37 kV/F @ εrS < 5%, respectively.

Ultralow-Power Synchronous Demodulation for Low-Level Sensor Signal Detection

The design and experimental characterization of a high-resolution analog lock-in amplifier (LIA)-based measurement system is presented in this paper. Different design strategies are used to attain a versatile solution which features programmable gain (72.6–82.6 dB) and operating frequency (5–115 kHz), preserving good recovery performance. The prototype, integrated into the United Microelectronics Corporation 0.18- $\mu\text{m}$ CMOS process with a single supply voltage of 1.8 V, achieves a resolution of 200 nV and a high dynamic reserve of 43.5 dB, showing significantly lower power consumption ( $885~\mu \text{W}$ ) than high-resolution state-of-the-art LIAs with minimum size (0.075-mm2 silicon area). To validate the amplitude recovery performance of the proposed single-phase LIA, it was used to measure the equivalent value of a passive resistive sensor, exhibiting a maximum error of 1.9% when $\text{R}_{\mathrm {S}}$ is varied from $100~\Omega $ to 10 $\text{k}\Omega $ with an input signal in the order of hundreds of microvolts. Thus, it constitutes a highly suitable choice for portable and lab-on-a-chip sensing applications.

Early detection system for coal spontaneous combustion by laser dual-species sensor of CO and CH4

As a common hazard, coal spontaneous combustion (CSC) causes the loss of coal in piles and mines. To rapidly and reliably forecast CSC, a portable and powerful detection system based on a laser spectroscopic technique is proposed, to precisely measure gaseous inhibitors; the technique uses a single, distributed-feedback diode laser emitting at 2.33 μm as a sensing light source for dual species. The detection system carries out wavelength-modulation spectroscopy to suppress detection noise by using a two-channel analogue lock-in amplifier (ALIA) and a complex programmable logic device (CPLD) controller. To improve detection sensitivity, a Herriott cell is adopted to increase the interaction length between the laser and the target species. The embedded system on a chip (SoC) of STM32F103 is the charge of the current driver and temperature control of the laser. Deployment of the SoC of STM32H743 acquires and digitally filters the original demodulated absorption signal, after which communication with a personal computer takes place. The second demodulation component, normalized by the first demodulation component, is used to improve the signal-to-noise ratio. The sensor-calibration experiment reveals the square of the correlation coefficients R2 to be 0.99718 and 0.99905 for carbon monoxide (CO) and methane (CH4), respectively, over the concentration range of 5–200 ppm. Using Allan-Werle deviation analysis, the detection sensitivity for CO and CH4 are 0.27 ppm and 0.20 ppm with a 2.5 s sampling time, and 0.05 ppm and 0.03 ppm with ~125 and 95 s integration times, respectively. Finally, the detection system is used for the online measurement and analysis of the growth pattern of gaseous products which are released during the CSC process, the temperature varying from 30 to 200 °C. Two critical temperatures were correctly determined to further forecast CSC status. On beginning a vigorous oxidation process, the exponential-evolution trends of the CH4 and CO gases agree with each other. Compared with CH4, CO may be more suitable for online, early detection of CSC.

CMOS-Based Multifrequency Impedance Analyzer for Biomedical Applications.

In this paper, we present a monolithic microsystem, which can perform bioimpedance analysis and electroimpedance tomography measurements as well as record electrocardiogram signals. In contrast to a full analog lock-in approach, a mixed analog/digital solution is adopted. The proposed solution has been designed, implemented, and tested using a commercial 0.35-μm CMOS technology. The tuning range of the signal generator and the detector is from 10kHz to 10MHz in 1kHz steps. The circuit ensures a CMRR of 81dB@10kHz, which increases to 84dB@10MHz. The measured equivalent input noise power spectral density is en = 2.57 nV/√Hz at 10kHz in the worst case, close to the 1/f corner frequency. It decreases until en = 1.8 nV/√Hz at 1MHz and en = 1.9 nV/√Hz at 10MHz. Measurements of a reference RC network performed with the proposed monolithic solution and compared with a Keysight E4980A Precision LCR Meter shows a maximal relative error of 0.8% over the whole operating frequency range.

Fully integrated wearable impedance cytometry platform on flexible circuit board with online smartphone readout

We present a wearable microfluidic impedance cytometer implemented on a flexible circuit wristband with on-line smartphone readout for portable biomarker counting and analysis. The platform contains a standard polydimethylsiloxane (PDMS) microfluidic channel integrated on a wristband, and the circuitry on the wristband is composed of a custom analog lock-in amplification system, a microcontroller with an 8-bit analog-to-digital converter (ADC), and a Bluetooth module wirelessly paired with a smartphone. The lock-in amplification (LIA) system is implemented with a novel architecture which consists of the lock-in amplifier followed by a high-pass filter stage with DC offset subtraction, and a post-subtraction high gain stage enabling detection of particles as small as 2.8 μm using the 8-bit ADC. The Android smartphone application was used to initiate the system and for offline data-plotting and peak counting, and supports online data readout, analysis, and file management. The data is exportable to researchers and medical professionals for in-depth analysis and remote health monitoring. The system, including the microfluidic sensor, microcontroller, and Bluetooth module all fit on the wristband with a footprint of less than 80 cm2. We demonstrate the ability of the system to obtain generalized blood cell counts; however the system can be applied to a wide variety of biomarkers by interchanging the standard microfluidic channel with microfluidic channels designed for biomarker isolation.

Low-Cost Portable 1 MHz Lock-In Amplifier for Fast Measurements of Pulsed Signals in Sensing Applications

We report on a low-cost, portable, analog lock-in amplifier (LIA) designed to perform fast measurements of amplitude variations of pulsed signals modulated up to 1MHz. The LIA architecture, based on the phase-sensitive synchronous demodulation technique, is simple and includes two amplification stages, together with filtering blocks that allow setting instrument gain, sensitivity, and resolution, as well as its response time. With respect to conventional and commercial LIAs typically working at lower frequencies, the presented solution provides a fast dc output with response times as low as only few milliseconds. As a case-example, the prototype PCB implemented with high-speed discrete off-the-shelf components allowed characterizing and validating the circuit through optoelectronic measurements reaching a minimum detection resolution of about 25V. Moreover, by employing 100-ns infrared laser pulses at 1-MHz repetition rate, LIA is capable of performing fast optical detection of very small variations of light power with a minimum detection resolution of about 2.2W. This envisages its use in chemical sensing applications to detect and measure small and fast variations of the concentration of substances, as well as to follow rapid changes of physical phenomena by employing sensor systems modulated at high frequencies.

Final design of the ITER outer vessel steady-state magnetic sensors

The outer vessel steady-state sensors (OVSS) – a subsystem of the ITER magnetic diagnostics – will contribute to the measurement of the plasma current, plasma-wall clearance, and local perturbations of the magnetic flux surfaces near the wall. The diagnostic consists of a poloidal array of sixty sensors welded to the vacuum vessel outer surface. Each OVSS contains a pair of bismuth Hall sensors with the measurement axes parallel (in the poloidal plane) and normal to the vacuum vessel. These Hall sensors were successfully tested in environmental conditions corresponding to those in ITER (neutron irradiation, vacuum vessel bakeout, etc.) and have proven to be capable of measuring the magnetic field up to at least ±7T. An AC analogue lock-in current spinning signal processing electronics has been developed for the OVSS to eliminate the spurious voltage due to Hall sensor manufacturing imperfections and the planar Hall effect. This paper presents the final design of the OVSS diagnostic, developed jointly by the ITER Organization and the Institute of Plasma Physics in Prague.

A simplified digital lock-in amplifier for the scanning grating spectrometer.

For the common measurement and control system of a scanning grating spectrometer, the use of an analog lock-in amplifier requires complex circuitry and sophisticated debugging, whereas the use of a digital lock-in amplifier places a high demand on the calculation capability and storage space. In this paper, a simplified digital lock-in amplifier based on averaging the absolute values within a complete period is presented and applied to a scanning grating spectrometer. The simplified digital lock-in amplifier was implemented on a low-cost microcontroller without multipliers, and got rid of the reference signal and specific configuration of the sampling frequency. Two positive zero-crossing detections were used to lock the phase of the measured signal. However, measurement method errors were introduced by the following factors: frequency fluctuation, sampling interval, and integer restriction of the sampling number. The theoretical calculation and experimental results of the signal-to-noise ratio of the proposed measurement method were 2055 and 2403, respectively.

A portable analog lock-in amplifier for accurate phase measurement and application in high-precision optical oxygen concentration detection

A portable analog lock-in amplifier capable of accurate phase detection is proposed in this paper. The proposed lock-in amplifier, which uses the dual-channel orthometric signals as the references to build the xy coordinate system, can detect the relative phase between the input and x-axis based on trigonometric function. The sensitivity of the phase measurement reaches 0.014 degree, and a detection precision of 0.1 degree is achieved. At the same time, the performance of the lock-in amplifier is verified in the high precision optical oxygen concentration detection. Experimental results reveal that the portable analog lock-in amplifier is accurate for phase detection applications. In the oxygen sensing experiments, 0.058% oxygen concentration resulted in 0.1 degree phase shift detected by the lock-in amplifier precisely. In addition, the lock-in amplifier is small and economical compared with the commercial lock-in equipments, so it can be easily integrated in many portable devices for industrial applications.

Micropower CMOS lock‐in amplifier for portable applications

A new fully integrated micropower CMOS analogue lock-in amplifier (LIA) is presented. To achieve a compact architecture able to operate under low voltage with low-power consumption, it is based on a current-mode amplifier with embedded rectification within the transconductance stage. Post-layout results for a 0.18 µm CMOS implementation show a dynamic reserve of 51.2 dB, a power consumption of 482 µW and an integration area of 0.03 mm2. The frequency operation ranges from 450 Hz to 1.1 MHz, whereas the gain is programmable from 39 to 59 dB, providing the LIA with flexibility to match a wide range of applications.

A CMOS lock-in amplifier for low-power biomedical applications

An analogue Lock-In Amplifier (LIA) is employed in an optical receiver front end to retrieve signals from an extremely noisy environment. The conventional LIA consists of a current source, Transimpedance Amplifier (TIA), set of active Gm-C filter and Phase Locked Loop (PLL). In this work, a high gain low power TIA making use of Operational Transconductance Amplifier (OTA) is proposed. This structure has a 54% improvement in gain and consumes 8% lesser power compared to replica biasing TIA. The gain boosting circuit in the NMOS current steering charge pump reduces the power consumption by 38% compared to conventional structure. In the modified Phase Frequency Detector (PFD), True Single Phase Clock (TSPC) logic is utilised which consumes 50% lesser power compared to conventional design. So, the low power LIA designed with proposed structures can be used for low power biomedical applications. Simulations are done using Cadence Spectre, GPDK 180 nm technology.