lock-in amplifier 

https://doi.org/10.1351/goldbook.08267
Amplifier that amplifies only those signals that have, within a specified spectral bandwidth, the same frequency as, and a specified phase relation with, the selected reference signal.
Source:
PAC, 2021, 93, 647. (Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)) on page 666 [Terms] [Paper]