  Any cavity controller for a distributed system needs a Phase Reference Line
(PRL) signal from which to define phases of a cavity field measurement. The
LCLS-II PRL system at SLAC provides bidirectional (forward and reverse) phase
references at 1300 MHz to each rack of the LLRF system. The PRL controller
embedded with the Master Oscillator (MO) locks the average phase of the two
directions to the MO itself. Phase-averaging tracking loop is applied in
firmware which supports the feature of cancelling the phase drift caused by
changes in PRL cable length. FPGA logic moves the phase of digital LO to get
zero average phase of the two PRL signals. This same LO is used for processing
cavity pickup signals, thus establishing a stable reference phase for critical
cavity RF measurements. At low frequencies, open-loop PRL noise relative to the
LO distribution includes a strong environment and 1/f components, but the
closed-loop noise approaches the noise floor of the DSP. The implication is
that the close-in phase noise of the cavities will be dominated by the chassis
DAQ noise. The final out-of-loop phase noise relevant to machine operations is
that of the cavity field relative to the beam.
