2475-1472 (c) 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/LSENS.2017.2713449, IEEE Sensors Letters Volume 2(3) (2017) 1949-307X © 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications standards/publications/rights/index.html for more information. (Inserted by IEEE) Analogue Lock-In Amplifier ____________________________________________________ Low-Cost Portable 1MHz Lock-In Amplifier for Fast Measurements of Pulsed Signals in Sensing Applications Andrea De Marcellis 1 *, Elia Palange 1 , Nicola Liberatore 2 , and Sandro Mengali 2 1 Dept. of Industrial and Information Engineering and Economics, University of L’Aquila, 67100 L’Aquila, Italy 2 Electro-Optics Research Centre, CREO Consortium, 67100 L’Aquila, Italy * Member, IEEE Received 1 Nov 2016, revised 25 Nov 2016, accepted 30 Nov 2016, published 5 Dec 2016, current version 15 Dec 2016. Abstract—We report on a low-cost, portable, analogue 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 25μV. Moreover, by employing 100ns infrared laser pulses at 1MHz repetition rate, LIA is capable to perform fast optical detection of very small variations of light power with a minimum detection resolution of about 2.2μW. 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. Index Terms—Analogue Lock-In Amplifier, Fast Signal Detection, Low-Cost Sensor Interface, IR Pulsed Laser Measurement. I. INTRODUCTION Sensors are devices able to reveal physical and chemical phenomena converting them into “electrical” signals whose amplitude can be very small and also lower than the noise level. High-precision and high-speed measurements of fast dynamics of chemical and physical phenomena is very important for several different industrial and laboratory applications, such as the control of encoder angular speed and apparatus energy efficiency, as well as the continuous monitoring of chemical and biomedical systems and for the detection of dangerous gases and toxic substances for workplace safety and environmental issues [1-7]. For these reasons, accurate measurements should be done through suitable high sensitivity and high resolution instrumentations. Nevertheless, especially in the case of very small and noisy signals, they cannot perform simple linear filtering operations and/or amplifications, in Voltage-Mode and/or in Current-Mode approaches, to recover the input signal information [8-13]. In this regards, the detection through optical sensing of vibration/displacement/position as well as of the presence of chemical/biological substances are generally performed by accurate measurement systems based on phase-sensitive synchronous demodulation technique, as in Lock-In Amplifiers (LIAs) [7,14-16]. These instruments permit to enhance the detection sensitivity and resolution and improve the Signal-to-Noise Ratio (SNR) allowing measuring small AC signal amplitudes (and/or their Corresponding author: Andrea De Marcellis (andrea.demarcellis@univaq.it). Digital Object Identifier: 10.1109/LSEN.XXXX.XXXXXXX (inserted by IEEE). variations) modulated at an operating frequency f 0 by providing a proportional DC output voltage level. Only few commercial analogue and digital LIAs (e.g., DSP-based) are able to work at high operating frequencies up to hundreds of MHz but at very high costs (i.e., from 5000 to 14000$), sizes and weights so resulting not appropriate for portable/integrated sensor systems [17-21]. On the other hand, ad-hoc solutions for sensor applications reported in literature, suitable also for noisy industrial environments (e.g., motor and turbine fault control), can be cheap, compact and lightweight but are mainly designed for low frequency phase-sensitive detection of conventional periodic signals (i.e., sinusoidal waveforms) with, typically, very high response times [22-37]. To overcome these limitations, in this Letter we propose a simple, portable, low-cost, analogue LIA capable to measure amplitude variations of pulsed signals (i.e., 100ns voltage pulses) at repetition frequencies up to 1MHz with response times of few milliseconds for fast detection in indoor and outdoor sensing applications. II. THE PROPOSED LIA ARCHITECTURE AND ITS ELECTRONIC CIRCUIT IMPLEMENTATION The proposed LIA is based on the phase-sensitive synchronous demodulation and its internal architecture is schematically reported in Fig. 1. As shown, it is composed of six main blocks: the Low Noise Amplifier, the Band-Pass Filter, the Tunable Phase Shifter, the Mixer, the Low Offset Amplifier and the Low-Pass Filter. The two different amplification stages, together with the final filter, allow setting the total LIA gain, its sensitivity and resolution as well as the measurement system response time (i.e., the LIA response