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 (https://doi.org/10.1515/pac-2019-0203)