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Developing Digital Receiver for Radio Astronomy by Using RFSoC
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Dr Chao Liu, Prof Michael Jones, Prof Angela TaylorExperimental Radio Cosmology, Astrophysics, Oxford
2020 RadioNet Workshop
Chao Liu, Oxford, Astrophysics
Background
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Large Millimeter Telescope Alfonso Serrano
3Reference: http://www.lmtgtm.org
Key Features
> 50 m diameter > 4600 m altitude > Ultra-wideband 75GHz – 111 GHz
Redshift Receiver and Down Conversion Blocks
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Reference: Robert E. Goeller, A Wideband Analog Correlating Spectrometer for Millimeter Astronomy, 2008
Redshift Receiver and first stage down conversion Second stage down conversion
Overview of RFSoC
5Reference: https://www.xilinx.com/content/dam/xilinx/imgs/block-diagrams/8x8-sd-fec-diagram.JPG
Key Features
> Data Converters
> 8x 4 GSPS 12-bit ADCs
> 8x 6.4GSPS 14-bit DACs
> Standard AXI interface between DCs and PL
> PL
> 930 logic cells
> 4271 DSP slices
> 60.5 MB Memory
> 16x33G transceivers
> PS
> A53
> R5
System Planning and Implementation
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Chao Liu, Oxford, Astrophysics
Firmware and Software Design and Implementation for a Spectrometer
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Spectrometer System Summary
> Spectrometer consist of IO ring and DSP, are both operated primarily in PL but configured and supported by software application in Processing System
> IO ring handles the interfaces to external devices and it remains the same for all the radio astronomy spectrometer> Interface to ADCs
> Interface to clocks
> Data movement from programmable logic to DDR (DMA)
> Data transmission from the device to host PC
> Handle control Command
> DSP performs the algorithms for radio astronomy> handles up to 8 x 400 billion samples to be operated to obtain a frame of final data
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System Architecture of a Spectrometer on RFSoC
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ADC
4GSPS
RFin
RFdc
AXIS FIFO AXISPFB &
FFTAXIS
Scaling &
IntegrationDMAAXIS
Programmable logic A
XI4
-Lite
AX
I4-L
ite
AX
I4-L
ite
AX
I4-L
ite
Memory
Interface
Generator
AXI AXIDDR4
4G
AXI
Processing System
AX
I
SoftwareApplications
I2C
Driver
RFdc
Driver
GEM
Driver
AX
IGEM
I2C
Host PCGigabit Ethernet
Clock
Device
Software Design and Implementation
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> Custom C and C++ applications running on ARM cortex A53
> Custom Python/ Matlabapplications for readout and control
RFSoC
I2C Driver
RFdcDriver
GEM Driver
Clock device
Gigabit Ethernet
RFdc
FPGA
FreeRTOS/Linux
DMA Driver
DMA data movement application
RFdc configuration and health check
application
GbE data transmit and receive application
Clock device configuration
application
Host PC
I2C
AXI
AXISDDR4
Python/Matlabreadout and
control application
ARM A53
Chao Liu, Oxford, Astrophysics
Spectrometer Tests
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Spectrometer Flatness and Spur Test
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High power noise source test circuit - shorter integration time Low power noise source test circuit - long integration time
Real-time FFT realised - all samples used for final result within the integration time, so it can expose any issue occurs at lower frequencies to provide solid confidence in ADC performance and reliability
High Power Noise Source Test
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> Noise Source: NC1109 100Hz-1GHz with output power of +10dBm and flatness of +/-2.0 dB
> Integration times: 1.024 ms and 1.024 s
> Direct comparison between flatness of noise source and spectrometer
Low Power Noise Source TestSubtraction between spectrums at different integration time lengths
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Frequency (GHz) 0.05 0.5 1 1.5
F(1.024s-10.24ms) (dB) 19.98 20.03 20.05 20.04
F(10.24s-1.024s)(dB) 10.11 10.09 10.1 10.11
F(102.4s-10.24s)(dB) 9.634 9.702 9.682 9.673
> Noise source with less output power but wider frequency range of 10MHz -12 GHz (limited by LPF) with flatness of +/-2.5dB
> Slope higher than noise source spec – LPF + amplifiers + baluns
> Interleaved spurs inspection with longer integration time – 102.4 s ~419.43x10^9 sample use for FFT
> Interleave spurs from spectrometer is negligible
Spectrums at different integration time lengths – 1.024s , 10.24s and 102.4s
Spectrometer CW Test Circuit
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> CW test of the spectrometer aimed to determine SFDR
> Tones generated by function generators
> Hittite HMC-T2240 – better ranges and less accuracy
> ANRITSU MG3692B – less ranges and higher accuracy
-1 dBFS Tone Test CW
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> ADC samples captured and full precision FFT performed: 80 dB SFDR at 1.5 GHz
> 1.5 GHz is selected as the harmonics will be depressed by the low pass filter
> Best SFDR achieved in this case is 70 dB
> 10 dB lower than the specification and offline floating point FFT
> Lower SFDR – fixed point precision in PFB and FFT implemented in FPGA or other reasons
> 70 dB SFDR is sufficient for the targeted radio astronomy receivers
Spectrum with 1.5 GHz tone generated by ANRITSU MG3692B
Power Sweep / Linearity Test
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Hittite HMC-T2240 ANRITSU MG3692B
1.5 GHz tones with 1dB steps in power generated by two different function generator – excellent linearity between input and output
CW Frequency Sweep
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Tones with power of 8 dBm generated at frequencies in 100MHz steps
Slope of the LPF – S12 parameter:
3 dB decrease from 100 MHz to 1.6 GHz
> Verify flatness with CW tones
> Output power from 100MHz to 1.6 GHz dropped by 5.5 dB
> Reasonable flatness with individual tones in the bandwidth
Insertion lose of the balun:
1 dB increase from 100 MHz to 1.6 GHz
Conclusion for Spectrometer Tests
> ADCs integrated in RFSoC demonstrated > 80 dB SFDR when ADC samples are captured and FFT with floating-point numbers in double precision is perform offline
> Noise source tests verified the flatness of the spectrometer and eliminated the concerns in interleaved spurs
> CW tests verified the flatness with tones at individual frequencies and the SFDR achieve has 10 dB gap with the offline FFT results (further investigation and characterisation required). 70 dB SFDR is more than enough for most of radio astronomy receivers.
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On-going and Future Works
> Scaling up the spectrometer to 8 channels per RFSoC and to multiple RFSoC boards
> Porting RFSoC to CASPER toolflow
> Merge the design to HTG-ZRF8 board from Hitech Global
> Control and management software implementation
> Down conversion circuit test and system integration
> Developments for other telescopes: CBASS, Goonhilly and etc.
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Chao Liu, Oxford, Astrophysics
Q&A
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