Stewing White Rabbits

© Paul Boven

As part of the JIVE contribution to the SADT consortium, I have been working on the design of absolute time (UTC) distribution for the SKA1 telescopes. White Rabbit is an open standard and open design for the distribution of time and frequency reference signals, and can achieve sub-ns accuracy on links of up to 10km. The links in the SKA will be much longer (up to 175km of fiber) and some of the equipment will be exposed to the harsh temperature changes of the semi-desert SKA locations. To still meet the required timing accuracy of 2ns requires the use of wavelength stabilized lasers (using DWDM optics) and external wavelength multiplexing filters.

Of course, such a design needs to be thoroughly tested. And so, in the days leading up to Christmas, several pieces of White Rabbit hardware got stewed, frozen, and stewed again. The ASTRON climate chamber was programmed to cycle from 0°C to 50°C and back in hourly steps of 10°C, while holding some of the parts that make up a White Rabbit link.

The image shows the complete distribution system as designed for the SKA. It consists of a White Rabbit distribution switch (in white, on the left), and underneath it, an enclosure holding the wavelength filters. Above are a time interval counter to measure the link performance and 2 spools of 10km fiber each. Inside the climate chamber (on the right) is the White Rabbit end-point, about to experience its first taste of what winter and summer will be like, in rapid succession.

The graph shows that the repeated temperature changes don't bother our White Rabbit system too much. The effects of the temperature steps are clearly visible, but are small compared to the required accuracy.

We gratefully acknowledge the ASTRON R&D group for their support and equipment.