Incoherent clocking in coherent radio interferometers

Abstract

Radio interferometer arrays are instruments requiring real-time synchronisation of all array elements to capture and align the observed source wavefront prior to further processing such as correlation and/or beamforming. One method is to deliver a frequency, from a common central local oscillator (LO), using a round-trip phase-stable distribution system such that each element has its own phase-stable copy of the clock with required real-time fidelity for down-conversion and digitisation. The method described here involves equipping each array element with its own independent 'eLO' and sending a 'tracer' of it to a central site, where the difference between the elements' eLOs and the central 'cLO' is measured. The digitised data from each element is corrected at the central site according to the measurement, before correlation/beamforming. The advantage of this method is that only a 'trace' of eLO, needed to measure the rate of change of eLO relative to cLO integrated over a measurement interval, needs to be conveyed over this tracer link, rather than the frequency itself, thereby requiring lower link temporal fidelity and bandwidth.