A lock-in amplifier (also known as a phase-sensitive detector) is a type of amplifier that can extract a signal with a known carrier wave from an extremely noisy environment (the signal-to-noise ratio can be -60 dB or even less). It is essentially a homodyne detector followed by a steep low pass filter, making it very narrow band. Practical lock-in amplifiers use mixing, through a frequency mixer, to convert the signal's phaseand amplitude to a DC — actually a time-varying low-frequency — voltage signal.
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