The ALMA project at Arcetri: Digital filtering

The second generation correlator

An improved correlator for ALMA has been proposed by an European consortium including Astron (NL), Bordeaux University (FR), IRAM (FR) and Arcetri.

The basic approach is that of a hybrid correlator. The input bandwidth is decoposed in a number (34 in the present design) of separate subbands, that can be independently correlated by a standard (non time-multiplexed) correlator. This approach is intermediate between a XF time multiplexed nstrument, like the first ALMA correlator, and a FX correlator, like the one proposed by the Japanese NRO.

The work in Arcetri has focused on the efficent design of a digital receiver, that exploits the peculiarities of the astronomical signal, i.e. the extremely coarse quantization and the stochastic nature of the input signal.

Three designs have been proposed, with decreasing resource usage.

All these designs have been extensvely simulated, including truncation effects and finite coefficient precision. Design implementation is currently in progress at the Bordeaux University using Altera Stratix FPGA. The design is used in a filtercard that will be included in the baseline ALMA correlator. See also the Digital filter activity page.


Simulations of the "analytic approach" filter. From top to bottom, left to right we have: a) the input signal, composed of stochastic noise plus three monochromatic lines; b) the undecimated output of the first (broad) filter; c) The same, after decimation; d) The output of the complex mixer; e) The signal after the second (sharp) filter; f) The real output. All vertical scales are in dB. Click on the images to display them full scale.

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