FPGA-based lock-in amplifier with frequency feedback for microcalorimetry

by Rickard Nilsson (Condensed Matter Physics, SU)

Tuesday 11 Dec 2007, 14:00 → 16:00 Europe/Stockholm

A2:1041

The implementation of digital lock-in technology has been present in various fields of science for some time and provides the ability to extract faint signals from heavy background noise. However, the inherent sensitivity to shifts in frequency make these instruments sub-optimal for certain applications and measuring methods. In this work an FPGA-based real time digital lock-in amplifier has been realized for measurements using the steady state ac method for applications within the field of microcalorimetry. By using a per-sample demodulation synchronized with an internally generated arbitrary waveform signal the system architecture allows for rapid changes in frequency without significant signal attenuation. Additional features include eight input channels performing synchronized lock-in calculations while simultaneously measuring their dc offset value, external triggering for locking on to other instruments and signal sources, as well as the option to integrate over a time constant that can be adjusted during a measurement without interference. Another important feature is the ability to constantly and automatically adjust the frequency so that a set control value is achieved. In some aspects it compares to commercially available units and provides a cost efficient alternative for specific applications. The system was designed using primarily the National Instruments PXI-8196 chassis and a PXI-7831 reconfigurable I/O card. The code was written in LabVIEW 8. Because of the reconfigurability of the FPGA and the generality of the system it can also be adapted to other fields of science outside of microcalorimetry with small adjustments.