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

2. 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.

3. 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

4. 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

5. 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.

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

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

8. Analog front-end for RT-32

9. Digital part of DSDB

10. 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

11. 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.

12. Thank you!
Questions?