Extraction of weak signals using virtual instrumentation based lock in detection (VLID) technique

Several experiments in the advanced physics laboratory require an accurate measurement of an extremely small signal voltage. Due to presence of strong noisy signals it is challenging to detect such signals. Lock-in detection is a powerful technique to recover such a signal, even in the presence of broadband noise whose magnitude is several times greater than the signal itself. Lock in based signal detection is an expensive technique which requires a dedicated lock in amplifier. The aim of present work is to develop and demonstrate a novel technique involving development of software based lock in amplifier, which works on data acquired through a digitizer. For validating the developed technique, signal acquisition was done using a USB digitizer (sampling 10 KSps) and oscilloscope (sampling 100 MSps). The software module for lock in detection using Lab View based virtual instrument is used to extract the signal of a desired frequency. This method though offline but offers a cost effective method of recovering signals embedded in noise. Several tests were conducted in order to verify this technique and signals as weak as 100 mV of low frequency embedded in high frequency noise levels of 10 V were easily recovered. Weak laser signal captured from photo diodes were also recovered using data acquired from oscilloscope. This paper describes the methodology used in acquiring and extracting the signals with simulation and actual results. (author)