 About Lock-In Amplifiers by Stanford Research Systems

Official application note: www.thinksrs.com/downloads/pdfs/applicationnotes/AboutLIAs.pdf

Application Note 3. No publication date is inferred. Physical pages 1–2 describe synchronized multiplication, low-pass rejection of sum frequencies, detection bandwidth and two quadrature outputs. Noise near the reference remains within the selected passband; coherent background at that frequency cannot be distinguished by synchronization alone.