Simple synchronic detection at audio frequencies through a PC sound card

We present a simple personal computer based, synchronic detection system that emulates a lock-in amplifier at audio frequencies. The inputs of the sound card are used to acquire two sets of samples: the one to be measured, previously preamplified, and the reference. From the last one, two "quasiorthogonal" signals are derived to perform dual-phase detection. The procedure is fast and compares favorably with a benchtop lock-in amplifier. In the band from 100 Hz to 20 kHz we obtained average amplitude and phase errors below 1% and 0.1 degrees, respectively.