The DBBC3 is a flexible VLBI backend and\ua0environment that supports a wide range of observational\ua0needs via a suite of FPGA firmware types. The\ua0hardware can sample up to eight 4 GHz-wide baseband\ua0signals and convert to digital streams over multiple\ua010GE links on fibre. The development team has an ongoing\ua0development programme that has enhanced existing\ua0modes and introduced new desired modes as user\ua0requirements evolve. Three dierent firmware types for\ua0observing have been implemented which will be briefly\ua0summarised: Direct Sampling Conversion (DSC), arbitrary\ua0selection of bands (OCT), Digital Down Conversion\ua0(DDC). These modes cover all the requirements\ua0of astronomical, VGOS and legacy geodetic VLBI of\ua0the present, but also of the near future. At the same\ua0time the DBBC3 is an important platform for additional\ua0new modes to be implemented for the BRAND\ua0receiver. This paper describes the use of the DBBC3\ua0for the receiver development, pointing out which element\ua0in the current DBBC3 structure will be part of\ua0the BRAND receiver in order to simplify its introduction\ua0into the existing VLBI environment at telescopes\ua0with a DBBC3 backend

Alef, Walter

Dornbusch, Sven

Haas, R\ufcdiger

Johansson, Karl-\uc5ke L

La Porta, Laura

Rottmann, Helge

Roy, Alan L

Tuccari, Gino

Wagner, Jan

Wunderlich, Michael

English

Tuccari, G.

Alef, W.

Dornbusch, S.

Haas, R.

Johansson, K.

La Porta, L.

Rottmann, H.

Roy, A.

Wagner, J.

Wunderlich, M.

MPG.PuRe

DBBC3 Towards the BRAND EVN Receiver

Chalmers Research

DBBC3 Towards the BRAND EVN ReceiverDownloaded from: https://research.chalmers.se, 2020-01-17 16:22 UTCCitation for the original published paper (version of record):Tuccari, G., Alef, W., Dornbusch, S. et al (2019)DBBC3 Towards the BRAND EVN ReceiverProceedings of the 24th European VLBI Group for Geodesy and Astrometry Working Meeting: 27-30http://dx.doi.org/10.7419/162.08.2019N.B. When citing this work, cite the original published paper.research.chalmers.se offers the possibility of retrieving research publications produced at Chalmers University of Technology.It covers all kind of research output: articles, dissertations, conference papers, reports etc. since 2004.research.chalmers.se is administrated and maintained by Chalmers Library(article starts on next page)DBBC3 Towards the BRAND EVN ReceiverG. Tuccari, W. Alef, S. Dornbusch, R. Haas, K.-Å Johansson, L. La Porta, H. Rottmann, A. L. Roy,J. Wagner, M. WunderlichAbstract The DBBC3 is a flexible VLBI backend andenvironment that supports a wide range of observa-tional needs via a suite of FPGA firmware types. Thehardware can sample up to eight 4 GHz-wide basebandsignals and convert to digital streams over multiple10GE links on fibre. The development team has an on-going development programme that has enhanced ex-isting modes and introduced new desired modes as userrequirements evolve. Three di↵erent firmware types forobserving have been implemented which will be brieflysummarised: Direct Sampling Conversion (DSC), arbi-trary selection of bands (OCT), Digital Down Conver-sion (DDC). These modes cover all the requirementsof astronomical, VGOS and legacy geodetic VLBI ofthe present, but also of the near future. At the sametime the DBBC3 is an important platform for addi-tional new modes to be implemented for the BRANDreceiver. This paper describes the use of the DBBC3for the receiver development, pointing out which ele-ment in the current DBBC3 structure will be part ofthe BRAND receiver in order to simplify its introduc-Gino TuccariINAF Istituto di Radioastronomia, Sezione di Noto, ContradaRenna, IT-96017 Noto (SR), ItalyGino Tuccari · Walter Alef · Sven Dornbusch · Laura La Porta ·Helge Rottmann · Alan L. Roy · Jan Wagner · Michael Wunder-lichMax Planck Institute for Radio Astronomy, Auf dem Hu¨gel 69,DE-53121 Bonn, GermanyRu¨diger Haas · K.-Å JohanssonOnsala Space Observatory, SE-439 92 Onsala, SwedenLaura La Porta also atReichert GmbH/BKG, Hittorfstr. 26, DE-53129 Bonn, Germany(Correspondence: tuccari@ira.inaf.it)tion into the existing VLBI environment at telescopeswith a DBBC3 backend.Keywords Digital Receiver · VLBI · VLBI Backends1 IntroductionThe Digital Base-Band Converter 3 (DBBC3) isthe third generation VLBI backend developed in aEuropean collaboration under the lead of G. Tuccari(see e.g. Tuccari et al., 2019). The system is mod-ular and, as of today, in a configuration with eightsampler/processor pairs is fully VGOS compliant.It takes as input eight 4GHz-wide IFs in the range0GHz to 15GHz. Di↵erent channel (sub-band) widthsof 1-2-4-8-16-32-64-128MHz can be selected fromeach 4 GHz input via Digital DownConverter (DDC)firmware for output. Alternatively OCT filters allowthe selection of arbitrary parts of the band using 32-tapFIR convolutional filters acting on the time series fromthe sampler. Filter tap weights have been designedto produce filter widths of 512MHz, 1024MHz and2048MHz and others are possible by simply loadingdi↵erent weights into the tap registers. When usingthe 2048MHz filter one can use two filters to coverthe full 4GHz, which makes this mode compatiblewith the R2DBE system used in the Event HorizonTelescope. The full 4096MHz band can be output alsowithout any channelisation. The resulting output datarates are 16/32/64/128Gbps.For the European VLBI Network (EVN) theDBBC3 shall replace the DBBC2 using its backward-compatible modes, and at the same time open the2728 Tuccari et al.Fig. 1: DBBC3 VGOS model: 8 IFs with 8 groups of ADB3samplers and CORE3 processing boards. This unit can deliver128Gbps with 2-bit samples (output limit is 512Gbps).avenue to observations with data-rates up to 32Gbps,possible with EVN’s widest IFs at higher frequencies.2 Modifying the DBBC3 for the BRANDprojectThe BRANDEVN project is a EU-sponsored engineer-ing research e↵ort to develop and build a prototypereceiver with a frequency coverage from 1.5GHz to15.5GHz (see Alef et al., 2019). The full band willbe sampled in the receiver box without any down-conversion. All usable parts of the band will be selectedon the same board as the sampler chip via four power-ful FPGA processors. Pieces of up to 4GHz width rep-resented as 8-bit samples will be sent to the backendvia optical fibres. The overall structure of the data flowcan be seen in Fig 2.The DBBC3 is used here as an ideal platformto handle 4GHz portions of the full 14GHz inputband for creating compatible VLBI channels with theobserving modes described in the introduction. Theactual bandwidth of the BRAND EVN channels willbe slightly di↵erent from VLBI standards due to thedi↵erent sampling frequencies compatible with thesampler chip. The sampler device includes its ownsynthesizer which operates in a restricted range fromwhich sampling clocks can be selected. The valueselected for BRAND, 28.732 Gsps, is not a power of2 but allows one to derive integer MHz bandwidthsFig. 2: Four bands (IFs) are fed to the four inputs of the samplerchip (blue). They are either one of 0GHz to 14GHz or 14GHzto 15.5GHz and one of the possible polarisations. In the FPGAs(green) the serial data streams are converted to parallel. Nextstep is band selection, then polarisation conversion or other pre-processing. Last the digitised data are streamed to ethernet. In theDBBC3 (red), standard VLBI channelisation and formatting areperformed with output of the VLBI data to internet or recorders.in the digital signal processing chain. This leads tochannelised bandwidths of 7/14/28/56/112/224 MHzout from the digital frontend, from which the OCTor DDC firmware in the DBBC3 selects bands of4/8/16/32/64/128 MHz (plus optional resampling),ensuring compatibility with the bandwidths currentlyused in VLBI.The BRAND EVN data flow involving samplingand digital processing can be functionally divided intotwo blocks, one placed in the receiver frontend in theantenna and one in the control roomwhere traditionallythe backends are located. For this reason we can definethe first part a digital frontend and the second a digi-tal backend. The distinction is useful because di↵erentfunctions are performed in the two separated locations.The main process in the digital frontend part will,in the first implementation, be the formation of 4GHzslices as input to the digital backend. The process inthe digital backend will be the normal VLBI processof further channelisation to select sub-bands from the4GHz slice, truncating to 2 bits, and producing VDIFstreams for recording.In more detail, the digital frontend is now the lo-cus directly following the analogue signal path whichconsists of antenna, feed, filters, low-noise amplifiersand other components, all of which bring the signal toa level suitable for the sampler to sample with a nomi-DBBC3 Towards BRAND 29nal 8-bit depth. The digitised data from the sampler arethen converted from serial to parallel streams, filteredwith a 4 GHz OCT filter, decimated, time-stamped, for-matted as VDIF with 8-bit depth, and prepared in IPpackets for transmission on 16 10 GE links per ana-logue input to the digital backend. Four such analogueinputs are sampled by one sampler chip, coming fromthe four channels carrying the ranges 0-14 GHz/14-15.5 GHz in both polarisations from the analogue sec-tion of the receiver. (Fig 2).The resulting data rate from the sampler is huge –more than 900 Gbps. It is transferred from the digitalfrontend to digital backend on a large number of op-tical fibers (64) carrying VLBI standard VDIF pack-ets, which contain also the sampling time informationalong with the data packed as the DBBC3 receivingsection requires.The timestamps inserted by the digital frontend arethe time reference used for later correlation of VLBIdata, therefore the UTC 1 PPS and frequency referencemust be available at the frontend digital receiver, whichincludes for instance connecting a GPS receiver to thedigital frontend.The digital backend DBBC3 will receive the datainto the CORE3H boards via the ethernet connectorsusing a dedicated firmware/software to be loaded forthis mode. Subsequent processing proceeds as usual forthe DBBC3 when doing VLBI.The digital frontend can in principle perform othermore complex operations which will be implementedin the future, like dynamic OCT filtering for RFI mit-igation, broadband polarisation conversion, and otherdesired tasks compatible with the DBBC3 system.As mentioned above the first version of thefirmware for the digital frontend will be to send 4GHzbands to the DBBC3, which requires the so-calledCORE3H2 configuration of the CORE3 boards. TheCORE3H2makes use of all eight transceivers on-boardin receiving mode. This architecture allows one toreceive 4GHz bands with 8-bit representation, whichis similar to feeding a CORE3H using the standardADB3L sampler boards as in standard DBBC3s.It is worth noting that if required, there could be fullcompatibility with a standard DBBC3 equipped withADB3L sampler boards to be compatible with tradi-tional receivers whose analogue signal is sampled inthe backend. Indeed switching between input data tothe CORE3H boards coming from the local ADB3L orFig. 3: The FPGA processing board CORE3H can be populatedwith up to eight optical transceivers. They can work as output(standard DBBC3 VDIF streams for recording) and as input tothe FPGA (application for “digital” receivers.via the digital frontend could be implemented by usingdi↵erent firmware.Another architecture could also be chosen whenadditional functionality is required. This is for exam-ple the case for the digital linear to circular polari-sation in FPGA. The conversion is required for theBRAND receiver, which is linearly polarised, to workwith circularly-polarised VLBI networks. The conver-sion can be performed in analogue with a hybrid junc-tion after the feed, or in the digital domain with aHilbert transform in FPGA to give accurate 90 degreephase shift over a wide fractional bandwidth. The ad-ditional FPGA resources for performing this processin the CORE3H are not available due to the great de-mands of the main function of down conversion andpacket forming. A clean solution can be provided byreplacing a number (or all) the ADB3L with additionalstandard CORE3H processing boards, if required. In-deed CORE3H boards can be connected in functionalsequence through their optical transceivers. The flexi-bility of the DBBC3 allows one to configure the sys-tem blocks in diverse ways to satisfy the changing andhighly demanding requirements.3 Conclusions and outlookWe have shown in this article that the DBBC3 is a veryversatile backend. The development to adapt it to the30 Tuccari et al.Fig. 4: The block diagram shows a DBBC3 whose analogue andsampling parts are by-passed by digital signals from the BRAND(or other) digital frontend.BRAND digital receiver opens a cost-e↵ective path tohandle the data flood which the BRAND receiver willproduce.At the same time the BRAND digitiser/processorunit in or close to the receiver box together with themodified DBBC3 backend can be adapted e ciently toother receivers with large bandwidths. One immediateapplication will be the triple-band receiver for the highfrequencies 22GHz, 43GHz, and 86GHz. Rather thanrenewing the limited IF cabling of, for instance, the Ef-felsberg antenna we are planning to sample as muchbandwidth as possible close to the triple-band receiverwith the BRAND sampler/processing unit and trans-port the IF as digital on fibre, which is much easier. Weuse one or two DBBC3s to form channels (sub-bands)suitable for VLBI correlation. Assuming 8GHz band-width per polarisation and receiver, we could see datarates of up to 192Gbps on output.It should also be noted that the control of theDBBC3 in this new mode via the Field System1 willbe an extension of the existing implementation, whichshould be cost e↵ective to realise.ReferencesTuccari G, Alef W, Dornbusch S, et al. (2019) DBBC3 the newwide-band backend for VLBI. In: Proceedings of the “14th1 See for instance http://www.metsahovi.fi/pub/evn-om/4fs-4.htmlEuropean VLBI Network Symposium & Users Meeting”,PoS(EVN2018)140Alef W, Tuccari G, Dornbusch S, et al. (2019) BRAND - Awideband receiver for astronomy and geodesy. In: R. Haas,S. Garcia-Espada, J. A. Lo´pez Ferna´ndez (eds.): Proc. 24thEVGA Working Meeting, 31–36

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DBBC3 Towards the BRAND EVN Receiver

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