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Design of a Multi-Channel Phased-Array Receiver for Radio Astronomy Applications

Parent Category: 2020 HFE

By Scalambra Alessandro and Navarrini Alessandro

Introduction

The INAF (National Institute for Astrophysics)-Institute of Radioastronomy (IRA) in Bologna, Italy, and the Astronomical Observatory of Cagliari (OACa) in Cagliari, Italy, conduct research into the physics of radio sources such as active galactic nucleus (AGN) and galaxies, clusters of galaxies, the galaxy stars and star formation, and cosmology. INAF scientists and staff are involved in the development and management of the radio telescopes distributed in the national territory and operate three radio telescope facilities for use by the scientific community. The Institute was extensively involved in the design, construction, and testing of the Sardinia radio telescope (SRT), a new 64m antenna that is part of the very long baseline interferometry (VLBI) global network. This application note presents the development of a room temperature, multi-channel warm-section (WS) heterodyne receiver operating across the 2.3 - 8.2 GHz RF band for radio astronomy applications. The developers used Cadence® AWR Design Environment® software for the receiver design.

PHAROS Phased-Array Feed

High-sensitivity, large-scale surveys are an essential tool for new discoveries in radio astronomy, and a phased-array feed (PAF) placed at the focal plane of an antenna can increase the field of view (FoV) and mapping efficiency by fully sampling the sky [1-3]. A typical PAF consists of few hundred closely packed antenna elements with about half wavelength element separation that, by spatially sampling the focal plane, can synthesize multiple independent beams and be set to sample the sky using the Nyquist-Shannon sampling theorem. Through a beam-forming process, PAFs are capable of electronic steering and optimization of the antenna illumination to provide high-quality far-field patterns for each of the beams.

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Figure 1 • PHAROS2 focal plane array and dome-shaped vacuum window attached to the cryostat.

 

A phased array for reflector observing systems (PHAROS) is a cryogenically cooled PAF demonstrator with analog beamformer based on an array of dual polarization 10x11 Vivaldi antennas (Figure 1) designed for radio astronomy observations. The Vivaldi array utilizes high-performance Taconic printed circuit boards (PCBs) based on a three-layer laminated board structure.

The latest upgraded version, PHAROS 2, utilizes new components to reduce the system noise temperature, enhance the aperture efficiency, and digitize the signals from a subarray of 24 single-polarization PAF antenna elements that synthesize four independent single polarization beams.

PHAROS 2 Architecture

The block diagram in Figure 2 illustrates the PHAROS2 architecture, including the WS signal chain section located between the antenna array and the digital beamformer that enables the first-ever radio astronomy observation 2020 10 HFE receivers 02A 4-8GHz cryogenic section comprising an array of Vivaldi antennas cascaded with low-noise amplifiers (LNAs) with state-of-the-art performance (noise temperature Tn = 2.3K typical);

A 2.3-8.2GHz room-temperature WS multi-channel receiver (described in this article);

An Italian tile processing module (ITPM) digital backend based on a field programmable gate array (FPGA) that is capable of digitizing and forming four independent beams across a ≈275MHz intermediate frequency (IF) band [4].

WS Multi-Channel Receiver Design

The WS receiver architecture (shown in Figure 3) includes four eight-channel WS RF/IF modules, one local oscillator (LO) distribution module, and one WS monitor and control module. The receiver performs signal filtering by using a switched filter bank, signal conditioning, and single frequency down-conversion of a section of the 2.3 - 8.2GHz RF band down to the 375-650MHz IF band (27MHz instantaneous bandwidth). The down-conversion scheme utilizes sideband separating mixers in the lower side band (LSB), with the upper side band (USB) being internally terminated. The IF signals are converted to optical signals by an analog wavelength division multiplexing (WDM) IF over fiber (IFoF) fiber-optic transmitter (OTX) that transports two IFs over a single optical fiber.

The switched filter bank on the WS receiver input section is used to mitigate the effects of the radio frequency interference (RFI) signals across the RF band that could otherwise saturate the receiver chain and the optical links. One of the four BPF filters, BPF-A, is specified to cover the broad 2.3-8.2GHz RF frequency band, while the other three filters (BPF-B, -C and -D) have ≈275MHz “narrowband” features, with bands centered around astronomical lines. The selection of one of the narrowband filters allows it to achieve an image sideband rejection greater than 50dB, as a result of the combined effects of the filter rejection (≥ 30dB for USB to LSB frequency separation of 2 × fIFmin = 750MHz) and of the sideband separating mixer rejection (≥20dB).

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Figure 3 • PHAROS2 WS receiver.

Eight-Channel WS RF/IF Module

Each of the RF/IF modules (Figure 4) includes a PCB circuit and four WDM fiber-optic transmitters (OTXs) attached on opposite sides of a mechanical support. The RF/IF module has eight SubMiniature version A (SMA) RF input connectors, one SMA local oscillator (LO) input connector centered on the front panel, and one quad Lucent Connector/Angle Polished Connector (LC/APC) output connector to extract the four WDM G652D IFoF optical fiber outputs provided by the four OTXs.

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Figure 4 • RF/IF module showing the eight RF inputs, LO input, (LC/APC) fiber-optics connector, and PCB and its mechanical housing.

 

Figure 5 shows a schematic generated by AWR® Microwave Office® circuit design software of the receiver chain implemented on the board. The module employs a single four-layer PCB based on the RG4003C substrate with a thickness of 0.508mm and commercial surface-mounted components soldered in place (no bonding required).

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The receiver specifications of wide bandwidth and high-performance required precise electrical modeling of the individual components and RF characterization of the multi-layer PCB substrate in order to achieve the accurate simulation results necessary for successful design. Microwave Office software was used to design the PCB circuitry of the receiver chains. The S-parameters of the SMT components were imported from the manufacturer’s website into the circuit schematic for network analysis, directly incorporating the results of the AWR AXIEM® planar electromagnetic (EM) co-simulation and optimization of the interconnecting transmission lines and matching circuitries. The Microwave Office PCB import wizard was used to streamline the exchange of design information from the PCB layout tool into the simulator, which was required for the EM verification of the entire layout, including the integrated distributed filters and power splitters.

Figure 6 provides plots of the simulated RF transmissions that were obtained by selecting the four BPFs, shown on the WS RF/IF board design in Figure 6. The simulated response of the filters closely matches the specified ones in terms of passband (3dB bandwidth), insertion loss, return loss, in-band ripple, stop-band attenuations, and size.

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Figure 6 • WS RF/IF board design with view of two different PCB layers. (a) Top metallization layer with components showing the BPF-A layer; (b) Metallization layer n. 3, showing the BPF-B, BPF-C and BPF-D.

 

One of the four BPF filters, BPF-A, was specified to cover the broad 2.3-8.2GHz RF frequency band, while the other three filters, BPF-B, C, and D, had ≈275MHz “narrowband” features. The BPF-A was based on an inductor/capacitor (L-C) cell high-pass filter cascaded with a microstrip eight-pole Chebyshev low-pass filter, while BPF-B, C, and D were based on six-pole Chebyshev bandpass filters fully implemented on microstrip. The plot in Figure 7 shows the simulated RF transmissions that were obtained by selecting the four BPFs. The simulated response of the filters closely matches the specified ones in terms of passband (bandwidth at half-power points), insertion loss, return loss, in-band ripple, stop-band attenuations, and size.

Conclusion

Test results confirmed that the two WS 32-channel receivers, one of which is used in PHAROS 2, performed well and according to specification. A switched filter bank located at the input of each of the channel, enabled selection of one out of four possible bandpass filters: either a wide-band filter (BPF-A) or one of three narrow-band filters (BPFB, C and D) centered around astronomical lines. The 2.3–8.2 GHz signal frequency band can be covered with multiple local oscillator (LO) tuning by selecting filter BPF-A. EM analysis of the entire board and accurate manufacturer models for the surface mount passive components ensured accurate simulation of the switched filter bank, as confirmed by measurements.

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Figure 7 • AXIEM EM simulation of the S21 for one of the WS RF/IF board receiver chains from SMA board input to mixer, including the RF LNA, BPF, and mixer conversion loss, obtained by selecting the four BPFs.

 

About the Authors

Scalambra Alessandro serves as Designer, PCBs, INAF; Navarrini Alessandro is the Project Manager, INAF.

References

Fisher, J.R.; Bradely, R.F. Full-sampling array feeds for radio telescopes. In Proceedings of the SPIE Astronomical Telescopes and Instrumentation, Munich, Germany, 3 July 2000.

Warnick, K.; Maaskant, R.; Ivashina, M.V.; Davidson, D.B.; Jeffs, B.D. High-Sensitivity Phased Array Receivers for Radio Astronomy. Proc. IEEE 2016, 104, 607–622.

Anish Roshi, D.; Shillue, W.; Simon, B.; Warnick, K.F.; Jeffs, B.; Pisano, D.J.; Prestage, R.; White, S.; Fisher, J.R.; Morgan, M.; et al. Performance of a highly sensitive, 19-element, dual-polarization, cryogenic L-band phased array feed on the Green Bank Telescope. Astron. J. 2018, 155, 18.

Naldi, G.; Comoretto, G.; Chiello, R.; Pastore, S.; Pupillo, G.; Mattana, A.; Melis, A.; Concu, R.; Alderighi, M.; Aminaei, A.; et al. Development of a new digital signal processing platform for the Square Kilometer Array. In Proceedings of 2nd URSI Atlantic Radio Science Meeting (AT-RASC), Gran Canaria, Spain, 28 May–1 June 2018.

November 2020

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