Correlator design(s)Trade-offs

20 July 2009Christian Holler/Kris Zarb Adami

Oxford University

Bolometer vs. Interferometer

Only about 25 bolometer beams (blue dots) fit over the cluster area, with a low filling factor due to thediffraction-limited single-mode optics. None of the other bolometers in the array (of which there may be athousand) contribute to the image. In contrast all the interferometer antennas point directly at the cluster.Background image is an actual interferometric image of a rich cluster at 30 GHz, made with a telescope(CBI2) with filling factor f = 0.7, Tsys = 35 K

Complex lag-correlator layout

Amiba Correlator

GUBBINS Broad-band Correlator (2-20GHz) layout

3D-Correlator layout for large N correlators

Antenna1

Antenna2

Antenna3

timedelay

Correlator layout

Taking as a goal the requirement to correlate Nant = 90 antennas withB = 40GHz bandwidth and Nch = 8 frequency channels, the approximate number of operations required is:

Nop = 16B ln 2(Nch)Nant + 2Nant(Nant − 1)B = 815Top/s

assuming an FX correlator and a single polarization (four times larger for full polarization).

Digital FPGA/ASIC Correlators and Spectrometers

Multiplier replaced by digital logic, fourier transform replaced by complex multipliers

Largest advantage in F-X correlators specifically for spectroscopic science

Can be built quickly with Off-the-Shelf Products using standard FPGA(s) and network switches

Very stable, analogue and systematic effects (such as band-pass, differential mismatch) can be corrected

RFI Excision and other effects can be easily incorporated – an example of this could be a digital back-end to C-BASS which is possible with today’s technology

Easily programmable, possible use for on-the-fly systematic reduction, template matching

ALMA Correlator

Digital Correlator Design Limitations

ADC front-end (max. of 8-bit 56 GSa/s reported in literature)

Network switch, Back-plane and inter-chip communications for large-N correlators

Processing capability and power consumption (~1 Top/s; ~300W)

Not likely in the next 5 years

BUT, digital spectrometers are possible

Back up Slides

Wilkinson power dividerWPD2 0.1-30GHz

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WPD2 0.1-30GHz

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Andrew Harris‘ and Steve Maas‘ multiplier

Digital FPGA/ASIC Spectrometers