In frequency-division multiplexing (FDM,) equipment used to provide system end-to-end carrier frequency synchronization and frequency accuracy of tones. Note: The following types of oscillators are used in the Defense Communications System FDM systems:Type 1 - A master carrier oscillator as an integral part of the multiplexer set. Type 2 - A submaster oscillator equipment or slave oscillator equipment as an integral part of the multiplexer set. Type 3 - An external master oscillator equipment that has extremely accurate and stable characteristics. Synonym master oscillator.