A lock-in amplifier uses phase-sensitive detection to improve the signal-to-noise ratio in cw experiments. Using phase-sensitive detection requires that the analytical signal be modulated at some reference frequency. The lock-in-amplifier then amplifies only the component of the input signal at the reference signal, and filters out all other frequencies, i.e., noise. In the fluorescence detection example illustrated below, the analytical signal (the fluorescence) is modulated by chopping the optical excitation source (the laser) at the reference frequency.
Use of a lock-in amplifier in fluorescence detection
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