MNRF Upgrade: Australia Telescope Local Oscillator Upgrade | |
| Project Personnel The System Related Information size=4> | Project PersonnelProject Leader
Project Scientist
The SystemA very stable local oscillator reference-frequency (LORF) is sent to each antenna of the Australia Telescope Compact Array (ATCA). The local oscillator upgrade will provide a reliable, low-noise, system for distributing master-reference signals to the Australia Telescope Compact Array antennas. Currently, the LORF is distributed linearly along a single 7/8 inch co-axial cable. Multiple connectors along the path, inherent losses in the co-axial cable and the susceptibility of the twisted pair control lines (associated with the distribution scheme) to lightning damage have contributed to unreliability of the system.Part of the MNRF grant is being used to improve reliability of the ATCA LORF distribution. The new system will use a "star topology" with the LORF being sent along single-mode fibre-optic cable to each antenna in parallel. Fibre-optic cable has the advantages that signals of higher frequency can be sent than with the older co-axial cable and signals can be sent simultaneoulsy in both directions. In addition, fibre-optic cable attenuates the signal less than co-axial cable. The network will use 40km of commercially available "cable TV" fibre-optic cable, purchased in Australia. Used in conjunction with upgraded reference generation equipment to be constructed in the course of the project, this arrangement will meet the demanding phase-noise requirements of a synthesis array operating at 100 GHz. The ATCA will be one of the first millimetre-wave arrays to use a fibre distributor, and the LO electronics will invoke a number of new design concepts, including the use of dielectric resonators in high-frequency reference oscillators. Each antenna will be cabled with 12 single-mode fibres. The LO reference system will use two fibres. Another pair of fibres have been dedicated to improved antenna control computer communications. Spare single-mode fibres will allow future expansion of antenna capabilities, for example to wider-band intermediate frequencies. An explanation of the first stage of the local oscillator reference upgrade, illustrating the fibre optic distribution of the 160.05 MHz reference signal to an interferometer pair, is available here (pdf file). For comparison, an explanation of the current 160.05 MHz local oscillator reference distribution scheme is available here (pdf file). Progress to date:In the past year an experimental 2 km fibre link has been set up at the ATNF Marsfield headquarters, allowing representative frequency generation, distribution and link equipment to be characterized. The test setup has also been invaluable in formulating a viable electrical line length measurement system, needed to account for minute changes in the fibre length caused by, for example, temperature fluctuations.At present, the total phase noise of the experimental link amounts to about 20 rms at 100 GHz. However, with the use of a wideband laser diode and associated refinements, it is expected that the upgrade specification of 5 rms will be attainable. ATCA antennas are re-positioned quite frequently by the standards of comparable arrays, and the implementation of robust, optically stable, intensive-use connectors for single-mode optical fibre is not trivial. Commercial solutions have recently become available but are prohibitively expensive; the ATNF is therefore designing and evaluating sturdy connectors based on standard fibre components. As well as the work on the LO distributor, project members are responsible for all signal distribution associated with new antenna stations on the E-W and north interferometer tracks, together with patching and related hardware at the the ATCA control building. It is expected that service cabling of the new stations will be completed by mid-1999 and that one antenna will be available for evaluation of the fibre LO distributor at that time. Changeover of the whole array to the new system is expected to take place in mid-2000. Related Information
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Last update by Michelle Storey. 18/3/99
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