= Lock-in amplifier
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A <lock-in amplifier> measures the component of an input <signal> synchronized with a reference at a chosen <frequency>. It multiplies the input by a reference <sinusoid>, then uses a <low-pass filter> to retain the slowly varying product. Two references in <phase quadrature> recover both <amplitude> and <phase difference>. A shared timing source keeps the relative <wave phase> stable: this is coherent detection.<Model SR830 DSP Lock-In Amplifier manual by Stanford Research Systems>[Model SR830 DSP Lock-In Amplifier manual, revision 2.5, printed pp. 3-1–3-2: multiplication, filtering, reference locking and dual-phase detection]{ref}
The useful trick is to move the wanted <frequency> to zero before averaging. An <electronic amplifier> alone enlarges both the wanted <signal> and unwanted <noise>; a <lock-in amplifier> instead selects a component by its timing. A <photodiode>'s <electric current> can first be converted to an <electric potential difference>, so the same calculation can measure weak modulated <light>. The <chopped-light lock-in experiment> makes this selection tangible.
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