Lock-in amplifiers measure a singal amplitude and phase relative to a periodic reference. To do so they act as [homodyne detectors](Homodyne%20detectors.md#Electronic%20homodyne%20detectors) that use [low-pass filtering.](Low-pass%20filter) # Input and signal processing of Lock-in amplifiers A lock-in amplifier multiplies an input with a reference signal and then applies an adjustable low-pass filter in order to _demodulate_ the signal. # Outputs of lock-in amplifiers $R = \sqrt{X^2+Y^2}$ $\theta = \arctan{\bigg(\frac{Y}{X}\bigg)}$ %% see whitepaper by ZI. #ExperimentalTools/ExperimentalToolsinElectronics