Broadband digital lock‐in amplifier techniques

Using only commercially available equipment, we have developed simple digital lock‐in techniques that surpass the capabilities of analog lock‐in amplifiers. We describe both dual‐channel and single‐channel digital lock‐ins that are capable of measurement at any harmonic or subharmonic of the reference frequency. The best of these digital lock‐ins operates over a frequency range from dc to 400 kHz with lower phase noise and better flexibility, stability, and orthogonality than any analog lock‐in amplifier.