Abstract
A lock-in amplifier is constructed using a synchronous filter instead of a phase sensitive detector. The synchronous filter based on a signal sampling method offers an extremely narrow bandpass characteristic whose centre frequency can be easily controlled by its drive frequency. Since the filter makes smaller responses for harmonics of the signal at the tuned frequency than a phase sensitive detector, the error due to the harmonics becomes less and required performances of a prefilter are relaxed. By this lock-in amplifier the magnitude of a low-level signal affected by noise can be measured accurately without annoying phase adjustments which are required in a conventional lock-in amplifier.