The Event Horizon Telescope (EHT) is a very long baseline interferometry (VLBI) array that comprises millimeter- and submillimeter-wavelength telescopes separated by distances comparable to the diameter of the Earth. At a nominal operating wavelength of ~1.3 mm, EHT angular resolution (λ/D) is ~25 μas, which is sufficient to resolve nearby supermassive black hole candidates on spatial and temporal scales that correspond to their event horizons. With this capability, the EHT scientific goals are to probe general relativistic effects in the strong-field regime and to study accretion and relativistic jet formation near the black hole boundary. In this Letter we describe the system design of the EHT, detail the technology and instrumentation that enable observations, and provide measures of its performance. Meeting the EHT science objectives has required several key developments that have facilitated the robust extension of the VLBI technique to EHT observing wavelengths and the production of instrumentation that can be deployed on a heterogeneous array of existing telescopes and facilities. To meet sensitivity requirements, high-bandwidth digital systems were developed that process data at rates of 64 gigabit s^(−1), exceeding those of currently operating cm-wavelength VLBI arrays by more than an order of magnitude. Associated improvements include the development of phasing systems at array facilities, new receiver installation at several sites, and the deployment of hydrogen maser frequency standards to ensure coherent data capture across the array. These efforts led to the coordination and execution of the first Global EHT observations in 2017 April, and to event-horizon-scale imaging of the supermassive black hole candidate in M87.

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First M87 Event Horizon Telescope Results. II. Array and Instrumentation

The Collaboration for Astronomy Signal Processing and Electronics Research (CASPER) has been working for a decade to reduce the time and cost of designing, building and deploying new digital radio-astronomy instruments. Today, CASPER open-source technology powers over 45 scientific instruments worldwide, and is used by scientists and engineers at dozens of academic institutions. In this paper, we catalog the current offerings of the CASPER collaboration, and instruments past and present built by CASPER users and developers. We describe the ongoing state of software development, as CASPER looks to support a broader range of programming environments and hardware and ensure compatibility with the latest vendor tools.

A Decade of Developing Radio-Astronomy Instrumentation using CASPER Open-Source Technology

The Collaboration for Astronomy Signal Processing and Electronics Research (CASPER) has been working for a decade to reduce the time and cost of designing, building and deploying new digital radio-astronomy instruments. Today, CASPER open-source technology powers over 45 scientific instruments worldwide, and is used by scientists and engineers at dozens of academic institutions. In this paper we catalog the current offerings of the CASPER collaboration, and instruments past and present built by CASPER users and developers. We describe the ongoing state of software development, as CASPER looks to support a broader range of programming environments and hardware and ensure compatibility with the latest vendor tools.

A 32GHz bandwidth VLBI capable correlator and phased array has been designed and deployeda at the Smithsonian Astrophysical Observatory’s Submillimeter Array (SMA). The SMA Wideband Astronomical RO...

https://www.worldscientific.com/doi/pdf/10.1142/S2251171716410063

SWARM: A 32 GHz Correlator and VLBI Beamformer for the Submillimeter Array

A 32 GHz bandwidth VLBI capable correlator and phased array has been designed and deployed at the Smithsonian Astrophysical Observatory's Submillimeter Array (SMA). The SMA Wideband Astronomical ROACH2 Machine (SWARM) integrates two instruments: a correlator with 140 kHz spectral resolution across its full 32 GHz band, used for connected interferometric observations, and a phased array summer used when the SMA participates as a station in the Event Horizon Telescope (EHT) Very Long Baseline Interferometry (VLBI) array. For each SWARM quadrant, Reconfigurable Open Architecture Computing Hardware (ROACH2) units shared under open source from the Collaboration for Astronomy Signal Processing and Electronics Research (CASPER) are equipped with a pair of ultra-fast Analog-to- Digital Converters (ADCs), a Field Programmable Gate Array (FPGA) processor, and eight 10 Gigabit Ethernet ports. A VLBI data recorder interface designated the SWARM Digital Back End, or SDBE, is implemented with a ninth ROACH2 per quadrant, feeding four Mark6 VLBI recorders with an aggregate recording rate of 64 Gbps. This paper describes the design and implementation of SWARM, as well as its deployment at SMA with reference to verification and science data.

We have designed, manufactured, and characterized an 8-bit 5 Giga samples per second (Gsps) ADC printed circuit board assembly (PCBA). An e2v EV8AQ160 ADC chip was used in the design and the board is plug compatible with the field programmable gate array (FPGA) board developed by the Collaboration for Astronomy Signal Processing and Electronics Research (CASPER) community. Astronomical interference fringes were demonstrated across a single baseline pair of antennas using two ADC boards on the Yuan Tseh Lee Array for Microwave Background Anisotropy (AMiBA) telescope. Several radio interferometers are using this board for bandwidth expansion, such as Submillimeter Array; also, several experimental telescopes are building new spectrometers using the same board. The ADC boards were attached directly to the Reconfigurable Open Architecture Computing Hardware (ROACH-2) FPGA board for processing of the digital output signals. This ADC board provides the capability of digitizing radio frequency signals from DC to 2 GHz (3 dB bandwidth), and to an extended bandwidth of 2.5 GHz (5 dB) with derated performance. The following worst-case performance parameters were obtained over 2 GHz: spur free dynamic range (SFDR) of 44 dB, signal-to-noise and distortion (SINAD) of 35 dB, and effective number of bits (ENOB) of 5.5.

https://iopscience.iop.org/article/10.1086/677799/pdf

A 5 Giga Samples Per Second 8-Bit Analog to Digital Printed Circuit Board for Radio Astronomy

An 8 bits, 5 Giga samples per second Analog to Digital Printed Circuit Board has been designed by the authors for digitizing The Yuan Tseh Lee Array for Microwave Background Anisotropy(AMiBA) and other radio telescopes. Associated with the Field Programmable Gate Array platform developed by the Collaboration for Astronomy Signal Processing and Electronics Research community, AMiBA will have a digital correlator system in the future, that was fiscally difficult in 2001 when it was at design phase. The board can digitize input radio frequencies from DC up to 2.5GHz with a flatness of 5dB. The ENOB ranges from 6.1 to 6.5 bits across the 2.5GHz band, the SFDR from 41 dB to 48 dB, and the SINAD from 32 dB to 37.5 dB if the gain and offset tuning is applied.

Digitizing The Yuan Tseh Lee Array for Microwave Background Anisotropy by 5Gsps ADC boards

This paper reports the laboratory design and implementation of a four-antenna digital spectro-correlator for the Yuan Tseh Lee Array for Microwave Background Anisotropy (AMiBA). The system comprises four 5 Giga-samples/s (Gs/s), 8-bit analog to digital converter (ADC), and four Reconfigurable Open Architecture Computing Hardware 2 systems (ROACH2). The 5 Gs/s ADC board was designed by the authors, and the ROACH2 platform was manufactured by the Collaboration for Astronomy Signal Processing and Electronics Research (CASPER) community. The system has a bandwidth of 1.6 GHz, 128 spectral channels, an FX-type correlator, one polarization, and four antenna inputs. The correlation accumulators report the cross-correlation results of the six baselines, as well as the autocorrelation results of the four antennas. The system is planned to be characterized according to the results of various experiments before deployment: Allen deviation, optimal input power of the system, and confirmation of the correlator coefficient of the system. The new digital system is intended to replace the old analog system, which has no spectral information. This new system enables the AMiBA for new scientific missions, such as CO intensity mapping.

https://www.ursi.org/Proceedings/ProcGA14/papers/ursi_paper1012.pdf

Design of an interim digital spectro-correlator for the Yuan Tseh Lee array for microwave background anisotropy

A 4-bit, 10 gigasamples per second (Gsps) analog-to-digital converter (ADC) printed circuit board (PCB) was designed and manufactured for digitizing radio telescopes. Associated with the field-programmable gate array (FPGA) platform developed by the Collaboration for Astronomy Signal Processing and Electronics Research (CASPER) community, the developed PCB provides data acquisition systems with a wider bandwidth and simplifies the intermediate frequency (IF) section. The wider ADC bandwidth enables down converters in the IF section to be simplified, thereby saving resources. The current version of the PCB possesses an analog bandwidth of up to 5.9 GHz, whereas the chip possesses an analog bandwidth of to 18 GHz. This facilitates second and third Nyquist sampling using the revised PCB.

The design of a10-Gsps analog-to-digital converter board for the radio astronomy community

We have designed, manufactured and characterized a 5 Giga samples per second (5 Gsps), 4-bit, ADC printed circuit board (PCB). The board is compatible with the Field Programmable Gate Array(FPGA) platforms developed by the Collaboration for Astronomy Signal Processing and Electronics Research (CASPER) community. The board can digitalize input radio frequencies from DC up to 2.5GHz with a flatness of 5dB. The ENOB ranges from 3.8 to 2.5 bits across the 2.5GHz band, the SFDR from 35dB to 25 dB, and the SINAD from 20dB to 12 dB respectively. The board enables direct detection for observations in the radio frequency range, and will save tremendous resources in the Intermediate Frequency (IF) distribution for observations in the microwave frequency region. We have delivered the boards to several world class radio observatories for the detection of celestial objects.

Howard Liu

Papers

A 5 Giga Samples Per Second 8-Bit Analog to Digital Printed Circuit Board for Radio Astronomy

Digitizing The Yuan Tseh Lee Array for Microwave Background Anisotropy by 5Gsps ADC boards

Design of an interim digital spectro-correlator for the Yuan Tseh Lee array for microwave background anisotropy

The design of a10-Gsps analog-to-digital converter board for the radio astronomy community

A 4-Bit, 5Gsps ADC board for the radio astronomy community