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The Millimeter-Wave 1st Local Oscillator for ALMA

March 2005 Section Meeting

When: Tuesday, March 15, 2005


Bill Shillue, NRAO ALMA Project - Tucson, AZ. Click here for his biographical information. (Note that due to an illness, Bill Shillue didn't speak. Skip Thacker, NRAO Charlottesville, spoke on the same topic.)


The National Radio Astronomy Observatory (NRAO) is one of the world's premier research facilities for radio astronomy. NRAO operates powerful, advanced radio telescopes spanning the western hemisphere. Scientists from around the world use these instruments to probe fundamental questions in astronomy and physics.

The Atacama Large Millimeter Array (ALMA) is an international radio astronomical facility currently under construction in Chile [1]. It will detect and study the earliest and most distant galaxies, the epoch of the first light in the Universe. It will also look deep into the dust-obscured regions where stars are born to examine the details of star and planet formation. In addition to these two main science drivers the array will make major contributions to virtually all fields of astronomical research.

ALMA is an advanced radio interferometer array that will consist of an array of 64 12-meter parabolic antennas operated spanning 31-950 GHz. The antennas will be located on a plateau at 5500m elevation and will be separated by distances of up to 20 km. The receivers require a phase-stabilized 1st Local Oscillator (LO) that is coherent among all antennas.

The 1st Local Oscillator (LO) will be supplied by assemblies of electronic (YIG) oscillators with relatively low (12-25 GHz) fundamental frequencies that get multiplied up to the higher frequencies by multiplier/amplifier stages. Each antenna and receiver has dedicated LO assemblies mounted directly to the receiver front ends. These local oscillators must have very low phase noise, and the phase drift between any two such LOs must also be very low. The LOs must actually be coherent across the wavefront of the astronomical signal; this requires that their phases be continuously adjusted in real time to account for the differential Doppler shift induced by the rotation of the earth. We choose to accomplish this by distribution of a centrally-generated reference signal to all antennas. The reference will be transmitted over buried optical fiber whose routing requires fiber lengths of up to 15 km. The reference signal is used to phase-lock the electronic oscillators at the antennas. The talk will cover some background of the ALMA project and then detail the 1st Local Oscillator System.

Biographical Information:

Bill Shillue is affiliated with the National Radio Astronomy Observatory, Tucson, AZ. He received the BS degree in electrical engineering from Cornell University in 1985, and the MS degree in electrical engineering from Univ. of Massachusetts in 1990. In 1991 he joined the National Radio Astronomy Observatory (NRAO) in Green Bank, WV, and participated in the design and deployment of a 13.7-m satellite earth station in support of the NASA Orbiting-Very-Long-Baseline-Intereferometer Project. In 1994, he began work at the NRAO Tucson, AZ location, and developed millimeter-wave receivers and instrumentation for the 12-m telescope. In 1998, he began work on the Atacama Large Millimeter Array (ALMA), developing key photonic technologies for local oscillator generation and distribution. He is a member of IEEE Lasers and Electro-Optics Society, Antennas and Propagation Society, and Microwave Theory and Techniques Society.

For more details about the National Radio Astronomy Observatory, please visit their web site,

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