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In LCLS-I, the 476MHz MO provides an RF source to the timing system.  This signal is combined with a 360Hz signal derived from a sequencer chassis (i.e. derives a 360Hz signal coincident with the zero crossings from the 3-phase AC power line).  The 476MHz and the 360Hz signals are routed to the inputs of the VME-based Master Trigger Generator (VMTG).  The VMTG samples a 71.4kHz signal (a sub-harmonic of LCLS-I & LCLS-II MO frequencies) and generates a 360Hz fiducial that is aligned with the 71.4KHz signal.  

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(Borrowed from Timing for LCLS slide deck - see References section below)

The VMTG output is thus synchronized with the 476MHz MO --via the 71.4kHz RF signal-- and it triggers the fiducial generator at 360Hz.  The 360Hz output of the fiducial generator is amplitude-modulated onto the original 476MHz signal (hence 2.78ms between peaks and with a 476MHz frequency) and is propagated down the Main Drive Line (MDL).  The aforementioned trigger generation occurs in Sector-10 in its entirety and then it gets propagated to other sectors and buildings.

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(Borrowed from Timing for LCLS slide deck - see References section below)

Linac Locking Description

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