Commercial Lock-in Amplifier Research Articles

Flexible, low-cost phase-sensitive detection for the undergraduate laboratory with a Teensy microcontroller

We perform phase-sensitive detection with an inexpensive microcontroller, the Teensy 3.5. The programming and pricing of this microcontroller is similar to that of commonly used Arduino microcontrollers, but the Teensy 3.5 offers superior hardware performance. Our Teensy-based phase-sensitive detector can operate either with an external reference signal or by internally generating a reference signal. Additionally, we have developed an open-source graphical user interface for controlling the instrument. We demonstrate that our phase-sensitive detector exhibits good linearity in amplitude and phase, even with 25 mV signals dominated by larger-amplitude noise. We also use our phase-sensitive detector in a simple laboratory measurement: determining the distance dependence of the intensity of a light-emitting diode (LED) with the room lights on. Our instrument is a useful tool for teaching students about phase-sensitive detection and can be a viable low-cost alternative to commercial lock-in amplifiers.

High-Performance Digital Lock-In Amplifier Module Based on an Open-Source Red Pitaya Platform: Implementation and Applications

A lock-in amplifier (LIA) is an instrument that can extract weak signals from extremely noisy environments, and it is widely used in various precision measurement fields. However, there are still challenges in lock-in techniques in experiments that require high-frequency or multiple lock-in channels, such as high-frequency vibration detection and experiments with quantum sensors. The direct use of commercial LIAs will result in high experimental costs. Herein, a high-performance digital LIA module based on an open-source Red Pitaya (RP) platform is constructed. The quantitative description of the demodulation results, efficient data transfer method, and the development of the automated measurement functions in experiments are carried out in this module. As demonstrations, the phase difference and thermal noise measurements are conducted using the module. The research on temperature sensing with the frequency shift of quartz crystals and semiconductor doping concentration measurements has also been implemented. These demonstrations show that the digital lock-in module has good application prospects.

Oxygen detection based on Faraday modulation spectroscopy and wavelength modulation spectroscopy: A comparison

AbstractAn oxygen sensor system was developed based on Faraday modulation spectroscopy (FAMOS). In the system, a distributed feedback laser near 760.9 nm was used as the light source and a 30‐cm long gas cell was employed for gas absorption. The first harmonic was extracted by a commercial lock in amplifier for the derivation of concentration. Oxygen measurement experiments were conducted the performance of the sensor system was obtained. In addition, oxygen sensor system with wavelength modulation spectroscopy (WMS) scheme was developed with the same light source and gas cell for a direct comparison. Based on Allan deviation analysis, a detection limit of 0.186% for the WMS scheme and a detection limit of 0.108% for the FAMOS scheme were acquired at an averaging time of 1 s. The precisions can be further improved to 0.032% and 0.03% at an averaging time of 15 s, respectively.

A flexible and scalable, fully software-based lock-in amplifier for nonlinear spectroscopy

We demonstrate a cost-effective, fully software-based lock-in amplifier (LIA) implemented on a commercial computer. The device is designed for application in nonlinear spectroscopy, such as transient absorption and coherent multidimensional spectroscopy, but may also be used in any other application. The performance of our device is compared to a state-of-the-art commercial LIA with nearly identical results for both devices. Advantages of our device over commercial hardwired electronic LIAs are the improved flexibility in the data analysis and the possibility of arbitrary up-scaling of the number of LIA channels.

DR-RINS: Digital real-time relative intensity noise suppressor for pump-probe spectroscopy and microscopy.

Relative intensity noise (RIN) inherent in fiber lasers poses a serious obstacle to their use in pump-probe spectroscopy and imaging. RIN can be removed through an analog balanced detector, or, as we have previously shown, software adaptive noise cancellation (ANC) on digitized signals. One major drawback to software ANC is the added time required for digitizing and post-processing. In this article, we describe a design for ANC on a field-programmable gate array (FPGA), making use of high-level synthesis tools and fixed-point arithmetic to achieve real-time laser RIN suppression at 25MHz sample rates. Unlike the software-ANC approach, the FPGA-ANC device can serve as a dedicated drop-in denoiser, placed between the detectors and a commercial lock-in amplifier. We demonstrate its application to transient absorption spectroscopy and microscopy, lowering the noise floor to ∼17 dB above the shot noise limit. Furthermore, we demonstrate a dramatic improvement in data acquisition time from ∼6 h to ∼5min in a real-time imaging scenario.

A Thin-Film Piezoelectric-on-Silicon MEMS Oscillator for Mass Sensing Applications

In this study, a Thin-film Piezoelectric-on-Silicon (TPoS) MEMS oscillator is successfully demonstrated that uses lead zirconate titanate (PZT) thin-film piezoelectric layer deposited on a single-crystal silicon substrate to realize a length-extensional mode resonator, having a resonant frequency of 5.12 MHz and a quality factor (Q) of 311. For closed-loop implementation, a commercial Lock-in amplifier with a Phase Locked Loop (PLL) facility was used to fulfill the Barkhausen criteria for oscillation. The mass resolution and sensitivity of the proposed mass sensor are 0.54 pg and 4.96 Hz/pg, respectively. Finally, a board-level sustaining circuit, which includes a transimpedance amplifier (TIA) and an attenuator is integrated with the TPoS MEMS resonator for oscillation to achieve a portable sensor. Phase noise of -104.11 dBc/Hz at 1 kHz offset was recorded under ambient pressure and room temperature for a carrier frequency of 5.12 MHz. The mass resolution for the board-level mass sensor implemented in this work is 0.15 pg.

A low-cost vibrating sample magnetometry based on audio components

In this paper, we developed a low-cost, highly sensitive magnetization characterization instrument using a sound card and compared its sensitivity with commercially available measurement techniques. Firstly, we use an RLC circuit to measure the frequency-dependent response of the sound card and compare it with a lock-in amplifier. Then, we construct a simple but versatile measuring instrument, vibrating sample magnetometer (VSM) based on audio components, where data acquisition is performed by the sound card and the commercial lock-in amplifier. We test our magnetometry by measuring the magnetic hysteresis of three distinct samples: Nickel bulk piece, perm-alloy thin film with an easy-plane anisotropy, and Co/Pt multilayer with perpendicular magnetic anisotropy. We also analyze the magnetizations of three samples for different vibration frequencies and compare the sensitivities of two data acquisition methods. Although the sound card performs slightly worse than the lock-in amplifier at the vibration frequencies lower than 20 Hz due to the frequency cut-off, the sound card provides approximately seven times better sensitivity than the lock-in amplifier does at higher frequencies up to 60 Hz. We found that the sound card based VSM can have a sensitivity as high as 2 micro-emu at 34.7 Hz vibration frequency, which can be suitable for various laboratory and industrial applications.

\xc5-Indentation for non-destructive elastic moduli measurements of supported ultra-hard ultra-thin films and nanostructures

During conventional nanoindentation measurements, the indentation depths are usually larger than 1–10 nm, which hinders the ability to study ultra-thin films (<10 nm) and supported atomically thin two-dimensional (2D) materials. Here, we discuss the development of modulated Å-indentation to achieve sub-Å indentations depths during force-indentation measurements while also imaging materials with nanoscale resolution. Modulated nanoindentation (MoNI) was originally invented to measure the radial elasticity of multi-walled nanotubes. Now, by using extremely small amplitude oscillations (<<1 Å) at high frequency, and stiff cantilevers, we show how modulated nano/Å-indentation (MoNI/ÅI) enables non-destructive measurements of the contact stiffness and indentation modulus of ultra-thin ultra-stiff films, including CVD diamond films (~1000 GPa stiffness), as well as the transverse modulus of 2D materials. Our analysis demonstrates that in presence of a standard laboratory noise floor, the signal to noise ratio of MoNI/ÅI implemented with a commercial atomic force microscope (AFM) is such that a dynamic range of 80 dB –– achievable with commercial Lock-in amplifiers –– is sufficient to observe superior indentation curves, having indentation depths as small as 0.3 Å, resolution in indentation <0.05 Å, and in normal load <0.5 nN. Being implemented on a standard AFM, this method has the potential for a broad applicability.

An auxiliary frequency tracking system for general purpose lock-in amplifiers

Lock-in amplifiers (LIAs) are designed to measure weak signals submerged by noise. This is achieved with a signal modulator to avoid low-frequency noise and a narrow-band filter to suppress out-of-band noise. In asynchronous measurement, even a slight frequency deviation between the modulator and the reference may lead to measurement error because the filter’s passband is not flat. Because many commercial LIAs are unable to track frequency deviations, in this paper we propose an auxiliary frequency tracking system. We analyze the measurement error caused by the frequency deviation and propose both a tracking method and an auto-tracking system. This approach requires only three basic parameters, which can be obtained from any general purpose LIA via its communications interface, to calculate the frequency deviation from the phase difference. The proposed auxiliary tracking system is designed as a peripheral connected to the LIA’s serial port, removing the need for an additional power supply. The test results verified the effectiveness of the proposed system; the modified commercial LIA (model SR-850) was able to track the frequency deviation and continuous drift. For step frequency deviations, a steady tracking error of less than 0.001% was achieved within three adjustments, and the worst tracking accuracy was still better than 0.1% for a continuous frequency drift. The tracking system can be used to expand the application scope of commercial LIAs, especially for remote measurements in which the modulation clock and the local reference are separated.

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.

Flow-through sensor based on piezoelectric MEMS resonator for the in-line monitoring of wine fermentation

The traditional procedure followed by winemakers for monitoring grape must fermentation is not automated, has not enough accuracy or has only been tested in discrete must samples. In order to contribute to the automation and improvement of the wine fermentation process, we have designed an AlN-based piezoelectric microresonator, serving as a density sensor, resonantly excited in the 4th-order roof tile-shaped vibration mode. Furthermore, conditioning circuits were designed to convert the one-port impedance of the resonator into a resonant two-port transfer function. This allowed us to design a Phase Locked Loop-based oscillator circuit, implemented with a commercial lock-in amplifier with an oscillation frequency determined by the resonance mode. We measured the fermentation kinetics by simultaneously tracking the resonance frequency and the quality factor of the microresonator. The device was first calibrated with an artificial model solution of grape must and then applied for the in-line monitoring of real grape must fermentation. Our results demonstrate the high potential of MEMS resonators to detect the decrease in sugar and the increase in ethanol concentrations during the grape must fermentation with a resolution of 1mg/ml and 20μPas as upper limits for the density and viscosity, respectively.

Signal conditioning and processing for metallic Hall sensors

Two electronics prototypes for the ITER outer vessel steady-state magnetic field Hall sensors based on a lock-in signal processing and current spinning technique were developed. The prototypes, the first using a rectangular lock-in signal processing with sensor offset cancellation and the second employing advanced lock-in signal processing with sensor and electronics offset cancellation, were compared with a DC technique and commercial lock-in amplifier. The influence of long interconnection cables, white noise, and interfering planar Hall effect to a measurement accuracy was evaluated. The planar Hall voltage suppression by the current spinning technique was investigated. Whilst the rectangular lock-in amplifier prototype failed to meet the ITER accuracy requirement, the advanced lock-in amplifier prototype demonstrated required functionality and accuracy.

Piezoelectric MEMS resonators for monitoring grape must fermentation

The traditional procedure followed by winemakers for monitoring grape must fermentation is not automated, has not enough accuracy or has only been tested in discrete must samples. In order to contribute to the automation and improvement of the wine fermentation process, we have designed an AlN-based piezoelectric microresonator, serving as a density sensor and being excited in the 4th-order roof tile-shaped vibration mode. Furthermore, conditioning circuits were designed to convert the one-port impedance of the resonator into a resonant two-port transfer function. This allowed us to design a Phase Locked Loop-based oscillator circuit, implemented with a commercial lock-in amplifier with an oscillation frequency determined by the vibrating mode. We were capable of measuring the fermentation kinetics by both tracking the resonance frequency and by determining the quality factor measurements of the microresonator. Moreover, the resonator was calibrated with an artificial model solution of grape must and then applied for the monitoring of real grape must fermentation. Our results demonstrate the high potential of MEMS resonators to detect the decrease in sugar and the increase in ethanol concentrations during the grape must fermentation with a resolution of 100 μg/ml and a sensitivity of 0.16 Hz/μg/ml as upper limits.

Optical fiber oxygen sensor utilizing a robust phase demodulator

Abstract Measuring the phase shift of luminescence is always used to achieve oxygen concentration detection. A robust phase demodulator is proposed in this paper to measure luminescent phase, overcoming common faults of bad SNR in optical fiber oxygen sensor. Its phase detection linearity of 0.99999 R-square is calibrated by a commercial lock-in amplifier (Model 7230). Modulation frequency of 16 kHz is used in the oxygen measurements and a phase resolution of 0.02° is achieved by the demodulator. Phase noises of 0.03, 0.06, 0.08 and 0.1° were measured in different oxygen levels of 0.99%, 3%, 5% and 8% in experiments. A resolution of better than ±0.005% in 0.99% O2 is realized. In addition, the demodulator which is simple but robust can be used in many luminescence phase based applications.

Development of a magneto-optical sensor prototype to measure current by means of the induced magnetic field

The design and construction of the working prototype of a device to measure the magnetic field induced by a conducting wire as well as the current that induces this field, based on the magneto-optical Faraday effect, is described in this work. The transducer element is a nanocomposite material made up of maghemite nanoparticles embedded in a porous silica matrix, with a high low-field Verdet constant and a figure of merit, relating Faraday specific rotation to the absorption coefficient, of 5. The device has been mostly developed with low-cost parts, using the open-source prototyping platform Arduino plus a compact commercial lock-in amplifier. It is shown that the sensing material shows a linear response up to 30G, which would allow measuring AC currents of 1300A at ∼10cm from the wire. This setup allows avoiding the use of any magnetic field concentrator.

Piezoelectric resonators and oscillator circuit based on higher-order out-of-plane modes for density-viscosity measurements of liquids

We report the use of two AlN-based piezoelectric microresonators for the monitoring of density and viscosity of liquids and its application to detect lubricant oil dilution with diesel fuel. Two devices designed to resonate in the 4th-order roof tile-shaped vibration mode, but with two different anchor schemes, were fabricated and characterized. Interface circuits were designed to convert the one-port impedance into a resonant two-port transfer function. This allowed us to implement a phase locked loop (PLL)-based oscillator circuit based on the resonators, the interface circuit and a commercial lock-in amplifier. Our results demonstrate the performance of the resonators in fluids having viscosities up to 500 mPa · s. The performance of the sensors in terms of sensitivity and resolution are compared for both anchor configurations.

New sensor and non-contact geometrical survey for the vibrating wire technique

The tolerances for the alignment of the magnets in the girders of the next machine of the Brazilian Synchrotron Light Laboratory (LNLS), Sirius, are as small as 40µm for translations and 0.2mrad for rotations. Therefore, a novel approach to the well-known vibrating wire technique has been developed and tested for the precise fiducialization of magnets. The alignment bench consists of four commercial linear stages, a stretched wire, a commercial lock-in amplifier working with phase-locked loop (PLL), a coordinate measuring machine (CMM) and a vibration sensor for the wire. This novel sensor has been designed for a larger linear region of operation. For the mechanical metrology step of the fiducialization of quadrupoles an innovative technique, using the vision system of the CMM, is presented. While the work with pitch and yaw orientations is still ongoing with promising partial results, the system already presents an uncertainty level below 10µm for translational alignment.

High-Sensitivity High-Resolution Optical Phase Shift Detection Technique Using a Si Photodiode Operating in Photovoltaic Mode

In this paper, we report the optimization of the operating conditions and the maximization of the performances of the phase shift detection technique allowing to approach the best sensitivity and resolution in terms of light power variations. The system is based on the synchronous demodulation technique and uses a Si photodiode operating in photovoltaic mode biased through a small amplitude sinusoidal modulating signal. Employing a commercial lock-in amplifier for phase measurements and varying the signal modulating frequency, we prove that it is possible to obtain the maximum theoretical phase shift variation range equal to 90° with a resulting sensitivity and resolution equal to 3100°/ $\mu \text{W}$ and 3 pW, respectively. We demonstrate that these results are more than two orders of magnitude greater than those ones achievable by employing a Si photodiode in photoconductive mode. As a case-example for chemical applications, we apply the proposed technique to detect the variations of the molar concentration of a methylene blue solution by measuring the variations of the transmittance of a laser beam passing through the substance. The experimental findings are compared with those ones achieved by using a commercial spectrophotometer and the conventional amplitude detection technique employing the lock-in amplifier. Despite the achieved results are limited by the fixed phase resolution of the employed lock-in amplifier, here, the proposed approach allows to measure molar concentration variations with a resolution of 79 pM resulting 1354 times higher than that one obtained using the spectrophotometer and 33 times better than the value achieved with the amplitude detection technique.

A High Performance LIA-Based Interface for Battery Powered Sensing Devices.

This paper proposes a battery-compatible electronic interface based on a general purpose lock-in amplifier (LIA) capable of recovering input signals up to the MHz range. The core is a novel ASIC fabricated in 1.8 V 0.18 µm CMOS technology, which contains a dual-phase analog lock-in amplifier consisting of carefully designed building blocks to allow configurability over a wide frequency range while maintaining low power consumption. It operates using square input signals. Hence, for battery-operated microcontrolled systems, where square reference and exciting signals can be generated by the embedded microcontroller, the system benefits from intrinsic advantages such as simplicity, versatility and reduction in power and size. Experimental results confirm the signal recovery capability with signal-to-noise power ratios down to −39 dB with relative errors below 0.07% up to 1 MHz. Furthermore, the system has been successfully tested measuring the response of a microcantilever-based resonant sensor, achieving similar results with better power-bandwidth trade-off compared to other LIAs based on commercial off-the-shelf (COTS) components and commercial LIA equipment.

A Fully Analog High Performances Automatic System for Phase Measurement of Electrical and Optical Signals

In this paper, we present a high performance, automatic fully analog phase-to-voltage converter (PVC) circuit for phase shift measurements of periodic waveforms and repeated pulsed signals with zero and nonzero mean values. The PVC operation is fully automatic because an internal closed feedback loop guarantees the self-phase alignment (self-locking) between the reference and input signals. The system has been tested by measuring the phase shifts between the electrical signals generated by digital waveform generators and by photodiodes. The PVC output voltage dynamic range is ±10 V, independently from the phase-shift full-scales equal to ±10°, ±1°, and ±0.1°. The increase of the PVC sensitivity, expressed in volts/degree, which can be easily set to about 1, 10, and 100 V/°, is achieved without any amplification of the signal so avoiding the increasing and/or introduction of additional electronic noise to the measurements. As a consequence, for the smallest phase-shift full-scale equal to ±0.1°, the proposed PVC circuit is capable to achieve phase resolutions in the order of 0.0001°, a value one order of magnitude better than that one of commercial lock-in amplifiers. In the case of optical applications, the PVC is able to detect laser power variations with sensitivity values settable to about 6, 60, and 600 V/ $\mu \!$ W so achieving the best resolution of 16 pW.