The ALMA project at Arcetri: Digital filtering

The second generation correlator

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.

• Arcetri technical report n. 5/2002. A complex mixer followed by a two stage bandpass filter. The SSB rejection of the undesired band is performed in the first filter. The second filter operates on a bandpass decimated real signal.
• Ver. 2 of report 5/2002 The complex mixer is followed by a two stage complex low pass filter. A low pass filter uses much less resources for comparable sharpness, and this compensates for the use of a complex signal processing. The signal is translated to real before the output stage.
• Analytic design approach. A complex band pass filter is followed by a decimation stage and a complex mixer. The mixer rotates the filter oputput in order to place the desired subband around frequency zero. A second low pass complex filter produces the desired bandpass response, followed by a complex-to-real conversion, as in the previous design.
• Phase effects in a hybrid correlator. It is shown that overlapping the sub-bands in a hybrid real correlator is necessary to prevent large phase errors at the band edge.
• Sub-band calibraton errors and alignment in a hybrid correlator Simulations and theorical analysis of quantization effects and their effects in alignin sub-bands. (draft)
• Quantization of the FIR coefficents for the two-stage ALMA digital filter Analysis of the bandshape of the digital filter proposed for ALMA as a function of the number of bits used in the tap weight representation.

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.

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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.

• G. Comoretto: A Digital BBC for the ALMA Interferometer - ALMA memo n. 305. This report deals with general quantization effects for a digital LO with a few-bit representation.
• G. Comoretto: Possible architectures for an hybrid correlator - Arcetri internal report n.8/2000. Some alternative architectures for a hybrid correlator are presented and compared in term of resource usage.
• B. Quertier, G. Comoretto, A. Baudry, A. Gunst, A. Bos: Enhancing the Baseline ALMA Corrrelator Performance with the Second Generation Correlator Digital Filtercard - ALMA memo n. 476. A filtercard with 32 digital filters is described.
• P.Camino, B. Quertier, A.Baudry, G.Comoretto, D.Dallet: The new 3-stage, low dissipation digital filter of the ALMA Correlator - ALMA memo n. 579. An alternative architecture for the ALMA filterboard, with lower power consumption, is described.
• G. Comoretto: Algorithms and formulas for hybrid correlator data reduction - Alma memo 583