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The ALMA Local Oscillator

31 December, 2009 / Read time: 2 minutes

In 2009, former North American ALMA Project Director Dr. Adrian Russell spoke with Jean-François Cliche about the precise timing systems that coordinate all of ALMA's telescopes into one array. ALMA's telescopes are spread across miles of the Atacama Desert. Making their signals come together requires perfect timing. The Local Oscillator helps us achieve this.

Complex electronics accurately stitch ALMA’s individual wave detections together into one dataset. The first step in this process is to have exact measurements of where and when the antenna picked up its waves. On each antenna is a clock that timestamps the data using a kind of atomic metronome, or rhythm-keeping device, kept near the supercomputer. The timekeeping waves from this central oscillator beam out to each of ALMA’s antennas. Onboard the antennas, a local oscillator injects this timekeeping beat into a microscopic mixer with the waves coming through the receiver, and a mixed-down signal is digitized and sent back along the fiber into the supercomputer.

The timekeeping signals generated by the central local oscillator are sent through optical fiber, which presents an additional challenge. The length of the optical fiber can vary with temperature, but in order to achieve the incredible level of accuracy that ALMA requires, the fiber length must not change by more than one millionth of a millimeter over the entire 15-kilometer-distance to the farthest antenna. Stabilizing the length of the optical fiber is another job of the central local oscillator. 

Credits: National Radio Astronomy Observatory (NRAO)