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.