zi_whitepaper_principles_of_lock in_detection.pdf

Lock-in amplifiers were invented in the 1930’s [1, 2, 3] and commercialized [4] in the mid 20th century as electrical instruments capable of extracting signal amplitudes and phases in extremely noisy environments (see Figure 1). They employ a homodyne detection scheme and low-pass filtering to measure a signal’s amplitude and phase relative to a periodic reference. A lock-in measurement extracts signals in a defined frequency band around the reference frequency, efficiently rejecting all other frequency components. The best instruments on the market today have a dynamic reserve of 120 dB [5], which means they are capable of accurately measuring a signal in the presence of noise up to a million times higher in amplitude than the signal of interest.