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 1 MHz. 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 25 μV. 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 2.2 μW. 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.