The next-generation Very Large Array (ngVLA) is an astronomical observatory planned to operate at centimeter wavelengths (25 to 0.26 centimeters, corresponding to a frequency range extending from 1.2 GHz to 116 GHz). The observatory will be a synthesis radio telescope constituted of approximately 214 reflector antennas each of 18 meters diameter, operating in a phased or interferometric mode.
We provide an overview of the current system design of the ngVLA. The concepts for major system elements such as the antenna, receiving electronics, and central signal processing are presented. We also describe the major development activities that are presently underway to advance the design.
The Expanded Very Large Array (EVLA) is an international project to improve the scientific capabilities of the Very
Large Array (VLA), an aperture synthesis radio telescope consisting of 27, 25-meter diameter antennas distributed in a
Y-shaped configuration on the Plains of San Augustin in west-central New Mexico. The EVLA's major science themes
include measuring the strength and topology of magnetic fields, enabling unbiased surveys and imaging of dust-shrouded
objects that are obscured at other wavelengths, enabling rapid response to and imaging of rapidly evolving transient
sources, and tracking the formation and evolution of objects in the universe. The EVLA's primary technical elements
include new or upgraded receivers for continuous frequency coverage from 1 to 50 GHz, new local oscillator,
intermediate frequency, and wide bandwidth data transmission systems to carry signals with 16 GHz total bandwidth
from each antenna, and a new digital correlator with the capability to process this bandwidth with an unprecedented
number of frequency channels for an imaging array. The project also includes a new monitor and control system and new
software that will provide telescope ease of use. The project was started in 2001 and is on schedule and within budget.
Scientific observations with the new correlator started in March 2010. The structural modifications that convert the VLA
antennas to the EVLA design were completed in May 2010. The project will be complete in December 2012 when the
last receiver will be installed on an antenna.
The design of the National Radio Astronomy Observatory's Expanded Very Large Array (EVLA) project is approaching completion. Four of the twenty-seven antennas have been upgraded into the final configuration. The 2200 miles of fiber optic cables have been installed underground and are functional. The master oscillator and the round trip phase hardware have been operating uninterrupted since November 2003. Hundreds of hours of test observations have been performed as we start the task of characterizing the upgraded system. This paper discusses the results of this testing and describes the techniques used to maintain phase coherence of the EVLA LO chain and of the new wideband receivers. The enhancements to the VLA system include a new local oscillator (LO) system, a fiber optic LO distribution system, and a digital round trip phase measurement system. The phase requirement for the LO system requires that the long term phase drift slope be less than 6.0 picoseconds per 30 minutes at 40 GHz and be maintained across the entire array. To accomplish this, a near real time continuous measurement is made of the phase delay in the fiber optic cable distributing the LO reference signals to each antenna. This information is used by the correlator to set the phase on each of the baselines in the array.
The Expanded Very Large Array (ELVA) uses fiber optic technologies for the Digital Data Transmission, the Local Oscillator and Reference distribution, and all Monitor/Control functions. These signals are sent on separate fibers to each of the twenty-seven EVLA antennas. The Data Transmission System (DTS) is used to transmit the four digitized IF signals from the antennas to the Central Electronics Building. A sustained data rate of 10.24 Gbits/s per channel and 122.88 Gbits formatted per antenna is supported. Each IF signal uses a parallel interface of three synchronized single bit high-speed serial optical fiber transmission channels. Each set of three channels, twelve channels in total, is wavelength division multiplexed onto a single fiber. The formatted data are received and de-formatted before the data are sent to the correlator. The system configuration includes a CW laser, an Erbium Doped Fiber Amplifier, passive optical multiplexers, up to 22 km of standard single mode fiber and an APD optical receiver. This paper presents a complete description of the EVLA fiber system including specific component specifications. The calculated performance of the IF system is compared to the actual performance and resulting "Lesson Learned" are presented.