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Software-Based Lock-in Amplifiers-Myth or Reality?
The development of software-driven instrumentation is revolutionizing the extraction of small AC signals that are either partially or completely buried in noise.
by Armando Valim, Aljosa Vrancic, Philip Kromer, & Roger D. Bengtson
We are all familiar with examples of new PC technology that changed traditional ways of doing things, such as listening to MP3s, watching DVDs, and the multiple advantages of the Internet. PC performance is improving so much that desktop PCs can run applications that once required proprietary hardware such as stand-alone equipment, workstations, etc. Common examples of software-based applications are music compression (MP3), video compression (MPEG), and software-based modems (modems with fewer ICs and more software decoding). This same technology is revolutionizing the world of instrumentation. One example of this rapid change is in lock-in amplifiers.
The traditional way to measure small signals buried in noise was to use a stand-alone lock-in amplifier instrument. With software-based lock-in amplifiers, engineers reduce development time and deliver applications that can run more than 30 times faster than traditional methods. The paradigm shift occurred because all functions of a traditional lock-in amplifier can be performed by a PC using off-the-shelf data acquisition devices and free software such as the NI Lock-in amplifier start-up kit and UTiLIA (A PC-Based Lock-in Amplifier from www.mrflip.com)1. In fact, PC processing power is increasing so fast that it is hard for DSP (digital signal processor) technology to keep up with the latest advances. Therefore, using software-based instruments, you can easily customize them to your particular needs and leverage new developments from the PC industry. Actually, thousands of engineers and scientists are going beyond what traditional black-box DSP implementations can do by defining the hardware functionality in software and using low-cost computer-based technologies to create virtual instrumentation.
Lock-in applications are used across several industries for spectroscopy, optics, and other applications. For example, Philip Kromer and Roger Bengtson, from the Department of Physics at the University of Texas at Austin, used a software implementation of the lock-in amplifier technique (UTiLIA) to measure the resistance of a high-temperature superconductor in an undergraduate laboratory directly by applying a known current across the sample and measuring its resistive voltage drop. The problem is particularly difficult because the resistive voltage vary from ~1mV while normally conducting to <10 nV during superconducting.
Another software lock-in implementation was presented by Aljosa Vrancic during NIWeek 2002, which extracted a 1.0 microV signal buried 160 dB in the input signal noise. The experiment goal was to use a photodetector to measure a weak light signal from a flash light and extract it from 9V noise coming from the ambient light (several reflectors and a spot light). To add complexity to the demo, the presenter utilized a general purpose data acquisition board used earlier for vibration measurements.