Project of Direct Sampling Digital Backend for Quasar VLBI-network Evgeny Nosov Institute of Applied Astronomy Russian Academy of Sciences (IAA RAS) www.iaaras.ru

Evolution of signal chain at VLBI radio telescopes K6/GALAS, OCTAD BRAND EVN DSDB (Direct Sampling Digital Backend) … ? DBBC, RDBE, ADS3000+, CDAS, BRAS, MDBE R1002М DAS Mark IV DAS, VLBA4 DAS, R1000 DAS, etc.

Direct sampling: pros and cons + Simple and short signal chain + Higher reliability, easier maintenance + Signal chain is perfectly stable and predictable after ADC + Identical channels with good linearity of phase response + Easy implementation of new observation modes Still very short list of available RF ADCs Very high data rate from ADC. Not easy to receive and process it in FPGA Data synchronization issues Required high end (i.e. expensive) electronic test equipment for hardware debugging

List of RF ADCs Part, Manufacturer Bits Bandwidth, GHz Data rate, Gsps ADC-30-NG, Micram 6 20 32 HMCAD5831, Analog Devices 3 26 CHAIS ADC, Fujitsu 8 18 56 ASNT7123, Adsantec 4 16 A6B25G, Alphacore 25 EV5AS210, e2v 5 Williamson ADC, Jariet Tech. 10 68

Signal chain based on DSDB Sampler and DSP units are separated: Digital signals are not affected by group delay variations in cables/fibers (temperature, bending), reflections, EMI, low dynamic range of E/O converter DSP unit has no strong restrictions on size, power consumption, EMI radiation level DSP unit can be based on COTS FPGA boards + specially designed FMC modules CDMS is strongly required to measure/eliminate reference clock variations Analog front-end is specific for a particular receiver. Sampler and DSP units are then compatible with any receiver.

Analog front-end for RT-32 Fs=20.48 GHz

Analog front-end for RT-13 with Tri-band receiver

Analog front-end for RT-32

Digital part of DSDB

First prototype of DSDB to try key technologies Tasks to achieve using the prototype: Errorless ADC data capture in FPGA Data synchronization with the input 1PPS signal Efficient firmware implementation of digital RF/IF converter Formulating the requirements for DSDB design

Summary Direct sampling technique gives valuable advantages over previous generation DBEs. It is desirable to locate the samplers in the antenna cabin, while keeping away the DSP units. CDMS is strongly required! Hardware design of DSP units is flexible. COTS devices can be widely used. DSDB development is in the very beginning. Hardware design scheduled for 2020.

Thank you! Questions?