The Correlators ( Spectrometers ) Mopra Induction - May 2005
description
Transcript of The Correlators ( Spectrometers ) Mopra Induction - May 2005
Coarse digitisers Quantisation noise
S/N compared to ideal:
1-bit digitiser - 66%
2-bit 3-level - 82%
2-bit 4-level - 88%
• Autocorrelation spectrometer (correlator)- well established
• Polyphase Digital Filter Bank- new development
Digital Spectrometer Types
Digital Correlator
CC C C CC C C CC C C CC C C CC C C C
0 +1 +3+2-1-3 -2-4-5-6 +5+4 +7+6 +8 +9 ------7-9 -8----
Lag (delay) in samples
INPUT 1 INPUT 2
C = multiply and accumulate
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
• 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
• FX– For each signal ( ~N ) :
• Form spectrum
– For each baseline ( ~N2 ) : • Multiply corresponding frequencies
x0(m)
x1(m)
xρ(m)
xM-1(m)
xn
(m)
M-PointDFT via FFT
0
1
ρ
M-1
X0(m)
X1(m)
Xk(m)
XM-1
Fourier Transform Spectrometer
• 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.
Autocorrelator :
2-bit bandwidths 4MHz to 128MHz
At 4MHz BW - up to 16,000 frequency channelsAt 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
Mopra 8GHz DFB Spectrometer
DIGITALFILTERBANK
DIGITALFILTERBANK
SAMPLER
DIGITALFILTERBANK
DIGITALFILTERBANK
SAMPLERSAMPLERSAMPLER
4-12 GHz IF from mm Receiver
Vertex Room
Control Room
IF UNIT
E/O
4 * ( 2GHz BW, 2048 ch. )
PCB:28,600 Pads136,000 Track segments18,800 holes 2,300 components12 signal layers 9 power layers
17 Xilinx FPGAs
Each FPGA:21,000 Logic Cells1.6Mbit RAM88 18x18 multipliers564 I/O signals1152 pin BGA
ATNF 2 x 2GHz DFB PCB - CABB Prototype
12U