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High Speed Samplers and Digital Filters for VLBI

High Speed Samplers and Digital Filters for VLBI. Dick Ferris AT Electronics Development Group June 2003. Topics. Digital Filters can provide the identical, stable and linear-phase channel passbands required for an ideal interferometer.... Current Usage in VLBI

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High Speed Samplers and Digital Filters for VLBI

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  1. High Speed Samplers and Digital Filters for VLBI Dick Ferris AT Electronics Development Group June 2003

  2. Topics • Digital Filters can provide the identical, stable and linear-phase channel passbands required for an ideal interferometer.... • Current Usage in VLBI • Polyphase Digital Filterbanks • The Geocentric DAS • The slow march of ADC technology High Speed Samplers and Digital Filters for VLBI

  3. Eg. 1 K4 System (CRL ~1993) High Speed Samplers and Digital Filters for VLBI

  4. Eg. 2 X_FILT (ATNF 1994) High Speed Samplers and Digital Filters for VLBI

  5. Eg. 3 LBA DAS BandSplitter(ATNF 1997) B=16MHz,... 1MHz High Speed Samplers and Digital Filters for VLBI

  6. 0..192MHz IF F =128Msps s - F F B=64MHz o o F =32, 96, 160MHz o n(+1) - 1 HT LPF M/2 - n z - 1 D T F o I+jQ F Q I o Analytic ¯ M ¯ M - F F F o o o B=32, 16..., 1MHz Eg. 3 LBA DAS BandSplitter(ATNF 1997) High Speed Samplers and Digital Filters for VLBI

  7. IF 0..16MHz IF 0..16MHz - - jsin jsin w. w. t t cos cos w. w. t t D T 0..16MHz 0..16MHz - j p/2 p/2 ~ ~ M/2 - z ¯ M ¯ M Q I F F F F o o o o ¯ M ¯ M + + + - F LSB USB F F o o o B=16MHz,... 62.5kHz B=16MHz,... 62.5kHz F o Eg. 3 LBA DAS FineTuner (ATNF 1997) LBA DAS Summary Input 'virtual' LO at 32, 96 & 160MHz, 64MHz bandwidth Fine tuner 1Hz resolution over 16MHz LO-centred single passbands (64MHz), 32,…62.5kHz LO-contiguous dual passbands 16MHz,…62.5kHz Flip all passbands, separate duals 40dB sideband separation, 40dB stopbands 8-bit ADC, 10-28 bits internal 2-bit requantiser servos, 12dB range + 12dB AGC High Speed Samplers and Digital Filters for VLBI

  8. Eg. 4 VERA (NAO 2001) High Speed Samplers and Digital Filters for VLBI

  9. 1 0 1 X0(m) x(n) -jwkn j2p/M X1(m) x(n) e -j2pk(mM+r)/M WM= e = e wk=2pk/M B IF n=mM+r M M M M Xk(m) x(n) Fs=B 0 M-1 -j2pkr/M eg. B=1GHz M=1000 = e 2 1/M XM-1(m) x(n) =WM-kr -jwM-1n -jwkn -jw1n -jw0n e e e e The Filterbank Alternative High Speed Samplers and Digital Filters for VLBI

  10. Xk(m) x'(n) x(n) h(n) WM-kr r=n MOD M y(n)= h(i) x'(n-i) x0(m) x0(m) y0(m) y0(m) x0(m) x0(m) 0 p0(m) p0(m) p1(m) p0(m) p0(m) p1(m) p1(m) p1(m) WM-0 WM-0 x1(m) x1(m) y1(m) y1(m) x1(m) x1(m) 1 DFT as FFT WM-k pr(m) pM-1(m) pM-1(m) pr(m) pr(m) pr(m) pM-1(m) pM-1(m) WM-k Xk(m)= yr(m)WM-kr WM-kr M x(n) x(n) x(n) x(n) xr(m) xr(m) yr(m) yr(m) xk(m) xr(m) r WM-kr WM-kr yM-1(m) xM-1(m) xM-1(m) yM-1(m) xM-1(m) xM-1(m) M-1 WM-k(M-1) WM-k(M-1) Transforming the Filter High Speed Samplers and Digital Filters for VLBI

  11. 1GHz 1GHz 4k channels 4k channels Subband Analysis • Standard • Zoom • n*Zoom • n*Zoom^n High Speed Samplers and Digital Filters for VLBI

  12. Conventional Channelisation High Speed Samplers and Digital Filters for VLBI

  13. Filterbank Channelisation High Speed Samplers and Digital Filters for VLBI

  14. Schedule+DataBase+Clock PCFS Pointing Tuning (Digitisation) Channelisation 2-bit quantisation Data Transport >>>>>>>> Schedule+DataBase+Clock PCFS Pointing Tuning Digitisation Delay Tracking Channelisation 2-bit quantisation Data Transport >>>>>>>> Schedule+DataBase+Clock PCFS Pointing Tuning Digitisation Delay Tracking Channelisation Phase Tracking 2-bit quantisation Data Transport >>>>>>>> Schedule+DataBase+Clock PCFS Pointing Tuning Digitisation Delay Tracking Channelisation I Channelisation II Phase Tracking 2-bit quantisation Data Transport >>>>>>>> Schedule+DataBase+Clock CorrCon Data Transport Delay Tracking Phase Tracking Channelisation Correlation (Complex XMULT and Int) Schedule+DataBase+Clock CorrCon Data Transport Phase Tracking Channelisation Correlation (Complex XMULT and Int) Schedule+DataBase+Clock CorrCon Data Transport Channelisation Correlation (Complex XMULT and Int) Schedule+DataBase+Clock CorrControl Data Transport Correlation (Complex XMULT and Int) DAS<->Correlator Tasking Schedule+DataBase+Clock Control Process Pointing Tuning (Digitisation) Delay Tracking Channelisation Phase Tracking (Re) quantisation Correlation High Speed Samplers and Digital Filters for VLBI

  15. Basic Configuration Simple Zoom Multiple Zoom (>2 possible) FIR FFT FIR FFT FringeRotators Correlators DMUX Filterbank Compound Zoom A Wideband Upgrade for the Australia Telescope Compact Array Luneburg Lens Phased Array Luneburg Lens Phased Array 6 * 22m antenna AT Compact Array FPGA hardware may be completely reprogrammed to produce many different filterbank configurations, as different observations may require. A 2GHz bandwidth polyphase digital filterbank with 4096 channels ... ... will fit into four XC2V6000 FPGAs Filterbank Architecture RF, IF and Baseband Signal Path

  16. commercial experimental/military Analog-to-Digital Converters: COTS / non-COTS from Bob Walden HRL 1999

  17. High Speed Samplers and Digital Filters for VLBI from Paul Roberts ATEDG 2002

  18. Summary • Digital Filters can provide the identical, stable and linear-phase channel passbands required for an ideal interferometer.... • Current Usage in VLBI • Polyphase Digital Filterbanks • The Geocentric DAS • The slow march of ADC technology High Speed Samplers and Digital Filters for VLBI

  19. High Speed Samplers and Digital Filters for VLBI

  20. Digital Filter Basics • Two-point running mean: y(n)=(x(n)+x(n-1))/2 >> LPF • First order differences: y(n)=(x(n)-x(n-1))/2 >> HPF • FIR filter as generalised N-point running mean (convolution) y(n)= h(i) x(n-i)0<=i<N • h(n) is impulse response  h(n)  H(f) • Even part: he(n)=(h(n)+h(N-1-n))/2; a(i)=2he((N-1)/2-i) • Odd part: ho(n)=(h(n)- h(N-1-n))/2; b(i)=2ho((N-1)/2-i) •  H(f)=a(0)/2+(a(i) cos2if + j b(i) sin2if) Fs=1, N odd • he(n)  He(f); zero/linear phase, zero/constant group delay • ho(n)  Ho(f); quadrature phase Why High Speed Samplers and Digital Filters for VLBI

  21. Why choose Digital? • Completely deterministic; no component selection or tweaking • ‘Easy’ design, exact modelling, precise performance • Amplitude & phase characteristics as stable as sampling clock • Wider, flatter passbands, perfectly matched between systems • Nil dispersion/group delay distortion across the passband • Reduced closure errors >> better calibration • One sampler, multiple passbands; avoids ‘platforming’ • One hardware platform; many modes, functions • Cost effective for high performance multichannel systems • Ex High Speed Samplers and Digital Filters for VLBI

  22. Generic VC/BBC High Speed Samplers and Digital Filters for VLBI

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