ALMA Memo #305

A Digital BBC for the ALMA Interferometer

A digital baseband converter is proposed for the ALMA correlator. The digital local oscillator/mixer can be implemented with lookup tables, and increases only slightly the complexity of the digital filter board.

Sideband rejection can be implemented either in the filter section, at the expense of a roughly doubled number of filter taps, or adopting a complex correlator. Several approaches are possible, that impact to a various extent the existing correlator architecture. In the simplest approach, only the memory board needs to be modified.

The digital nature of the mixer causes a loss in signal to noise ratio of a few percent. More important is the presence of LO harmonics, that create ghost images of the input lines. Spur level depends mainly on the number of bits in the data path between the mixer and the filters. For a 3-bit mixer, a spur level around -31 dBc (decibel with respect to the fundamental) can be obtained. To achieve a -38 dBc spur level, a 4-bit mixer is required.

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Last modified: April 26, 2000