The Correlators( Spectrometers )

Mopra Induction - May 2005

Frequency Conversion

Mixer

RF

LO

IF

Frequency

USBLSBIF LO

RF

Sampling

Sampler

IF

Sampling Clock

IF Frequency

USBLSBBaseband

SCLK

Digitiser

N

BB

Frequency

Filter

00

10

01

11

V=0

Digitisation

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

AfP

tEtEA

Power Spectrum -- Autocorrelation Function

Autocorrelation spectrometer

(Correlator)

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

Polyphase Digital Filterbank

• 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.

Conventional Channelisation

Filterbank Channelisation

• Standard

• Zoom

• n*Zoom

• n*Zoom^m

ZOOM

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. )

Polyphase Digital Filter Bank Spectrometer- using commercial hardware

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