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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