Synchronization through the RF backplane
Introduction
The eRTM modules have been designed to deliver high quality clocks and
RF signals to the RF backplane to be used by AMCs and RTMs. A clock (or
carrier) signal doesn't transport information by definition, therefore
unsynchronized AMCs require a side channel to perform synchronization.
Notably, the RF backplane doesn't provide a deterministic data channel
to provide PPS and time-code information to the AMCs. The only
communication channel from eRTMs to the AMCs is through a (LVDS) SPI
connection, which requires an MCH-RTM equipped with an FPGA. Moreover,
the MCH-RTM's FPGA must re-route the data to the front MCH and then to
the AMC backplane. This solution has been discarded due to unnecessary
complexity.
Deterministic data channel for synchronization
The requirement of a deterministic data channel originates from the
problem of PPS marking.
The clocks synthesized by the eRTM modules can have a frequency as high
as 500 MHz, or 2 ns clock period. The most challenging aspect is to
correctly mark the beginning of a second (i.e. 1-PPS) accordingly to the
related rising edge of the 500 MHz clock. This task requires the use of
a high speed (i.e. rise time less than 1 ns) and deterministic data
channel (i.e. no jitter due to data processing).
Notably, the requirements for UTC/TAI synchronization are very relaxed,
once 1-PPS synchronization is done. Therefore, the time-code can be
retrieved using the same side channel used by 1-PPS or using
non-deterministic channels (e.g. NTP).
The proposed solution
The proposed solution is to modulate the clocks distributed by the RF backplane to perform 1-PPS synchronization. The additional UTC/TAI information can be retrieved using the same channel or using NTP (suggested approach). Notably, the AMCs board are connected to a Ethernet network using the AMC backplane, therefore no additional cabling is required. The UTC/TAI alignment is a one-off procedure, during cold start-up.