Dissemination of Reference Signals for a Next Generation Radio Telescope

A fully integrated system for transmitting atomic clock signals over optical fibre in a high branching ratio (64) directly to L-Band Digitiser units is demonstrated to achieve ultra low jitter, sub picosecond phasing and sub nanosecond timing. The Digitiser units are located on each MeerKatradio telescope antenna which is located 6 km to 12 km from the transmitter. A 1712 MHz sampling clock and a pulse per second signal are transmitted to the Digitisers by RF over Fibre techniques. The Digitisers demodulate the timing signals and use them internally to timestamp astrophysical signals which are sampled at a rate of 1712 MSPS. The integrated jitter of the sample clock transmitter is 70 fs and the RMS phase drift over 20 minutes is 140 fs RMS at the Digitiser units connected to short lengths of fibre. Sub sample period phase variation with a short term standard deviation of < 320 ps is demonstrated between the timing pulses and RF sample clock zero crossing cycles utilizing on board time interval counting on the Digitisers. An integrated time of flight measurement is also performed by the transmitter, to compensate for differential path delays between antennas, with an estimated uncertainty of <1 ns. The system is unique in that it supplies reference clock and timing signals directly to the point of sampling inside Digitisers placed after the receivers of the telescope. The final system is fully EMC qualified. Interferometry was utilized for initial verification of the full telescope phasing stability and drift of the order of 20 ps over 4 hours was achieved.