Optic Data Link for Tevatron Synchronization System

A. Mason and V. Rytchenkov
Fermi National Accelerator Laboratory


The Fermilab Accelerator Timing and Synchronization System is comprised of four clock signals. Three of these are beam synchronous clocks (BS CLK) which are synchronized to the actual beam revolution frequencies (or equivalently, to the accelerating RF) of the Main Ring, Tevatron protons, or Tevatron antiprotons. The serial clock signal carries timing markers, or "events", encoded in a biphase (or modified Manchester) code onto the 7.5 MHz (53 MHz RF divided by 7) carrier.

An optic data link (ODL) has been designed which is capable of delivering a 7.5 MHz biphase encoded BS clock signal and the 53 MHz RF signal simultaneously on one fiber, to 30 service buildings throughout the 6.28 km Tevatron ring. In order to do this, the biphase encoded BS clock signal along with the RF are used to produce a special 53Mbps encoded data signal.

The ODL uses multimode graded index fiber (62.5/125 um) and has 30 repeaters with inexpensive optic transmitters and receivers operating at 1300 nm wavelength. To decrease accumulated time jitter and pulse width distortion, PLL clock recovery and data retiming circuits are installed in a few of the repeaters. Decoded biphase output electrical signals of the repeaters are compatible with the
hardware of the existing Control System.

This optic data link was tested at the Tevatron with the real RF and beam synchronous clock signals. The test results have shown that time jitter is less than +/- 2 ns at the far end of the link, while allowing for the variation in the RF of up to 300 kHz during the acceleration process.