Advanced Digital Spectrometers: Cutting-Edge Technology Overview

1. The Correlators( Spectrometers )Mopra Induction - May 2005

2. Frequency Conversion Mixer RF IF LO RF IF LSB LO USB Frequency

3. Sampling Filter Sampler Digitiser IF N Sampling Clock SCLK BB LSB USB Baseband Frequency IF Frequency

4. 01 00 10 11 Digitisation V=0

5. Coarse digitisers -> Quantisation noise • S/N compared to ideal: • 1-bit digitiser - 66% • 2-bit 3-level - 82% • 2-bit 4-level - 88%

6. Digital Spectrometer Types • Autocorrelation spectrometer (correlator) • well established • Polyphase Digital Filter Bank • new development

7. Autocorrelation spectrometer (Correlator) Power Spectrum -- Autocorrelation Function

8. C C C C C C C C C C C C C C C C C C C C C C = multiply and accumulate Digital Correlator INPUT 1 INPUT 2 ---- -9 -8 -7 -6 -5 -4 -3 -2 -1 0 +1 +2 +3 +4 +5 +6 +7 +8 +9 ----- Lag (delay) in samples

10. Synthesis Array Correlators • XF • For each baseline ( ~N2 ) : • Form cross correlation function • FT to form spectrum • FX • For each signal ( ~N ) : • Form spectrum • For each baseline ( ~N2 ) : • Multiply corresponding frequencies

11. FX correlator has advantages for large N arrays • Preferred scheme for SKA - very large N • Key SKA technology • Demonstrate in upgrade of ATCA - CABB • - increase max. bandwidth from 128MHz to 2GHz • - 2048 channels • Funded under MNRF2001

12. FX • For each signal ( ~N ) : • Form spectrum • For each baseline ( ~N2 ) : • Multiply corresponding frequencies

13. x (m) X (m) 0 0 0 x (m) X (m) 1 1 1 M-Point x n DFT X (m) x (m) via ρ k ρ FFT X (m) x (m) M-1 M-1 M-1 Fourier Transform Spectrometer

14. Polyphase Digital Filterbank

15. Implemented in FPGAs • Field Programmable Gate Arrays • Large commercially available chips containing many tens of thousands of re-configurable logic cells, plus memory and digital multipliers. • Clock rates of up to 300MHz.

16. Conventional Channelisation

17. Filterbank Channelisation

18. ZOOM • Standard • Zoom • n*Zoom • n*Zoom^m

19. Autocorrelator : • 2-bit bandwidths 4MHz to 128MHz • At 4MHz BW - up to 16,000 frequency channels • At 128MHz BW - up to 256 frequency channels • 1-bit bandwidth 256MHz - up to 512 channels • Digital Filterbank : • July 2004 - 256MHz BW - 1024 channels • July 2005 - 600MHz BW - 1024 (2048?) channels • Sep 2005(??) - 2GHz BW - 2048 channels • Next season - 8GHz - 8192 channels

20. 4-12 GHz IF from mm Receiver E/O Vertex Room Control Room IF UNIT SAMPLER SAMPLER SAMPLER SAMPLER DIGITAL FILTER BANK DIGITAL FILTER BANK DIGITAL FILTER BANK DIGITAL FILTER BANK 4 * ( 2GHz BW, 2048 ch. ) Mopra 8GHz DFB Spectrometer

21. Polyphase Digital Filter Bank Spectrometer- using commercial hardware

22. ATNF 2 x 2GHz DFB PCB - CABB Prototype PCB: 28,600 Pads 136,000 Track segments 18,800 holes 2,300 components 12 signal layers 9 power layers 17 Xilinx FPGAs Each FPGA: 21,000 Logic Cells 1.6Mbit RAM 88 18x18 multipliers 564 I/O signals 1152 pin BGA 12U