Prototyping Algorithms for Next-Generation Radio Astronomy Receivers Using PXI-Based Instruments and High-Speed Streaming

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"Using National Instruments data acquisition and data streaming hardware, we developed calibration and correction algorithms for the DSSM and DOMT more efficiently and cost effectively than we could have using a real-time hardware signal processing implementation from the outset. "

- J. Richard Fisher, National Radio Astronomy Observatory

The Challenge:
Developing the next generation of high-performance, compact, integrated radio astronomy receivers using the latest advances in modern digital computing to digitize the signal as close to the antenna feed as possible.

The Solution:
Using National Instruments sampling, data acquisition (DAQ), and data-streaming hardware to capture the outputs from custom-designed microwave front ends and test new algorithms for numerically calibrated sideband separation and polarization isolation with high precision and stability.

Author(s):
J. Richard Fisher - National Radio Astronomy Observatory
Matthew A.. Morgan - National Radio Astronomy Observatory

The National Radio Astronomy Observatory (NRAO) is a facility of the National Science Foundation (NSF) charged with the construction, maintenance, and operation of radio astronomy facilities used by astronomers in the United States and around the world. The Central Development Laboratory (CDL) is the primary research and development arm of the NRAO.

Ground-breaking radio astronomy research depends on low-noise receivers followed by wide-bandwidth data transport systems. These systems are lower in cost, weight, and size, but more reliable, reproducible, and stable than the current best-in-class systems, without compromising sensitivity.

Digital Sideband Separation and Polarization Isolation

The next generation of radio instrumentation needs to digitize as close to the antenna feed as possible and integrate conversions from RF to baseband, from analog to digital, and from copper to fiber in a single compact housing. This involves transferring some functionality from the analog domain into the digital domain where signal processing can be conducted with the greatest fidelity.

Nature determines the frequency, bandwidth, and temporal characteristics of the signal studied by radio astronomers, requiring receivers with broader tuning ranges and larger instantaneous bandwidths than most commercial applications. Also, the cosmic signals are extremely weak by communications standards, so isolation from out-of-band signals is very demanding. Until recently, this led to complex downconversion systems with multiple local oscillators and intermediate filtering, which let low-level spurious mixing products corrupt the spectrum, especially in tightly integrated receivers. Simpler, single-downconversion, sideband-separating solutions have usually been ruled out by the difficulty in realizing ultrawideband hybrid couplers for the intermediate frequency (IF) and the relatively limited sideband isolation that results – typically less than 20 dB over wide bandwidths. To avoid this problem, we used a digital sideband-separating mixer (DSSM) to eliminate the analog IF hybrid. The DSSM digitizes the in-phase and quadrature mixer outputs separately and performs the final reconstruction of upper and lower sidebands numerically so we can create a mathematically perfect IF hybrid and correct any amplitude and phase imbalances in the preceding analog electronics.

Also, fairly unique to radio astronomy is the need to measure fractional polarizations of a randomly polarized signal, typically less than 1 percent polarization. In conventional systems, a passive electromagnetic device called an orthomode transducer (OMT) is inserted between the antenna and the first low-noise amplifiers to divide the orthogonal components of the signal onto two separate outputs. Although the performance of these devices is good, they are bulky and difficult to fabricate, lowering the yield and precluding their use in the most highly integrated, compact receivers. The digital orthomode transducer (DOMT) avoids this problem similarly to a DSSM.

Algorithm Development Using NI PXI-Based Data Acquisition and Streaming

Ultimately, the signal processing algorithms required for sideband and polarization reconstruction are programmed into field-programmable gate array (FPGA) firmware for real-time implementation. However, the calibration and processing algorithms require extensive development and testing. Therefore, we needed a system flexible enough to rapidly prototype several receiver concepts and repeat comparative postprocessing of the same data using different algorithms, while still capturing large amounts of data at high speed on up to eight channels synchronously. The NI HDD-8263 paired with PXI data acquisition modules met these needs.

For the initial tests of the DSSM, we captured the in-phase and quadrature outputs of a 1,250 to 1,650 MHz front end using an NI PXI-5152 dual-channel sampler operating at 500 MS/s. We buffered and stored the data using the NI HDD-8263 streaming RAID system with 1 TB storage capacity. The 128 MB maximum buffer size recorded data in 128 ms bursts. This yielded sufficient signal-to-noise ratio for calibration of the digital correction coefficients and measurement of sideband isolation in excess of 60 dB.

The follow-up tests on the 8 to 12 GHz DOMT followed by four DSSM receivers used the same NI HDD-8263 system to store the data. In this setup, we used an NI PXIe-8105 8-channel sampler operating at 60 MS/s. Each channel captured the in-phase or quadrature-phase component from one of up to four polarization vectors delivered by the analog hardware. In this case, data was recorded in 1.08 s bursts.

By streaming the data to disk and postprocessing the results in software, we fine-tuned our algorithms to get the best possible performance prior to committing to a complex and expensive FPGA implementation.

Results

Using National Instruments data acquisition and data streaming hardware, we developed calibration and correction algorithms for the DSSM and DOMT more efficiently and cost effectively than we could have using a real-time hardware signal processing implementation from the outset. The algorithms and correction parameters developed were powerful, accurate, and stable with temperature. The prototype DSSM achieved better than 50 dB sideband isolation over a 12 °C temperature range with a single calibration while capturing the entire L-Band spectrum (1,250 to 1,650 MHz) in a single snapshot. Two prototype DOMTs, a three-probe and a four-probe version, achieved polarization isolation greater than 50 dB over a 10 °C temperature range with single calibration while capturing a 60 MHz wide band around 9 GHz.

With these results, we can implement our algorithms in FPGA firmware with confidence in real-time operation over broader bandwidths.

Author Information:
J. Richard Fisher
National Radio Astronomy Observatory
rfisher@nrao.edu

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