[time-nuts] 10 MHz Phase Coherent

Javier Serrano Javier.Serrano at cern.ch
Fri May 25 12:24:57 EDT 2007


Hi Martyn,

I work with other people in the timing system for CERN's particle
accelerator complex in Geneva (Switzerland), which includes the Large
Hadron Collider (LHC), hopefully to start operation in spring 2008. Our
timing system consists basically of a lot of complicated high level
software orchestrating the transfer of particles from one accelerator to
another, and the subsequent acceleration cycles. Hardware-wise, we have
home-made VME modules which receive the 10 MHz and PPS signals from a
commercial GPSDO (Symmetricom XLi) and use these two signals to encode
messages (as decided in advance by the high level software) on a serial
link. Using the 10 MHz, the second is broken in millisecond slots and
then further in 125 microsecond slots. Each 125 us slot can carry a
message signaling some event to occur in the accelerators. On the
receiving side, we have home-made PCI, PMC and VME boards which can
react to any of these messages by starting a counter, generating an
output pulse on their front panel or an interrupt on the bus. This
allows hardware and software synchronization all around the complex (the
LHC is 27 km in circumference).

Now, to come to your question: we rely heavily in the PPS containing
exactly 10 million ticks of the 10 MHz. All our UTC time-tagging logic
is based on that, so we definitely need a stable phase between the 10
MHz and the PPS. But as Tom says, I thought you got that by design in
every GPSDO. Maybe we have misunderstood the question?

Cheers,

Javier




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