Australia Telescope Compact Array

Phase transfer system

Nomenclature and terminology

The Local Oscillator Distribution System (LODS) comprises the central reference oscillator (Rubidium Standard and its clean-up oscillator) and distribution equipment located in the screened room, the inter-station post 7/8" Superflex cables and couplers, and the antenna mounted oscillator with its reference generation and distribution chain.

The Phase Transfer System (PTS) incorporates much of the LODS as well as further equipment in the screened room and antenna (notably the Time Interval Counter and the Phase Stabilization PC).

The Phase Stabilization System (PSS) incorporates the PTS, software tasks LOMON and (a small component of) CAOBS, along with all the software and hardware necessary to control the UHF Phase Rotators in the antennas. The LOPLOT program sits as a monitor facility to allow visibility into the PSS operation. (It is fed with the product of LOMON).

The Phase Stabilization PC produces a measure of the two way phase variation incurred in transferring the screened room reference oscillator signal to each antenna mounted oscillator. This phase variation is due to weather induced / di-urnal variation in the performance characteristics of the distribution system / link between the screened room and antennas. The phase measurement made is referred to the phase change that would have been incurred if the measurement had been made using a 1 GHz signal (done presumably to simplify subsequent utilization of the data). This then is the LO 2-way round trip phase variation at 1 GHz as measured by the PTS.

The astronomical phase correction which needs to be made to accommodate this distribution system variation is the LO 1-way phase variation at 1 GHz scaled to the observing frequency (in GHz). CAOBS calculates this value from the PTS 2-way number and (when LO transfer is enabled in CAOBS) includes it in the starting phase of the UHF oscillator Phase Rotators. Note that it is a variation from some arbitrary starting value. This value whilst not actually visible anywhere could be loosely considered as the LO phase correction term incorporated into the starting phase of the UHF Phase Rotators.

The variation in round trip phase measured by the PTS can be considered as the LO phase stability. Whilst not specifically quantified anywhere it is most graphically portrayed in the LOPLOT or VIS display.

Original: Graham Baines (13-Feb-1995)
Modified: Mark Wieringa (3-Sep-1996)