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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.

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

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With Linac Locking, the above 476MHz LCLS-I Master Oscillator gets replaced by a new RF reference that is phase-locked to the LCLS-II Master Oscillator (located in Sector-2) and transmitted to Sector-10 via an RF-over-Fiber (RFoF) connection.  Lastly, the alignment of the triggers between SC and NC timing is done in Sector-20.

RF Distribution Overview

Linac locking is a portion of the RF distribution network that aims to support LCLS-I timing generation and distribution.  See diagram below for the big picture.

(RF Distribution Network - Borrowed from Linac Locking Review slide deck - , see References section below)

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