Multiple-channel digital lock-in amplifier with PPM resolution

A digital lock-in amplifier built with a couple of input/output boards in a personal computer is described. The use of direct memory access allows the generation of a reference sinewave and the sampling of up to eight channels synchronously with it, leaving enough time for the processor to calculate the in phase and in quadrature responses at and only at selected harmonics (h1,h2,...) of the reference frequency. This digital lock-in is drift free and has the gain stability of the analog-to-digital converter, that is within ±1 ppm for a few minutes, increasing to ±20 ppm for 24 h. Even a simple Mac IIci can monitor in real time h1 at 24 ksamples/s for two channels, corresponding to an upper frequency of 3 kHz, with no limitations on the low-frequency side. Higher sampling rates and processing power are available with more recent hardware.