Methods of implementing fringe tracking, sideband separation, and phase switching in the ALMA telescope are described and analyzed. All required features make use of direct digital synthesizers for precise phase control of the first and second local oscillators. Sideband suppression is achieved via frequency offsets. Phase modulation of 180d is cancelled by bit inversion in the digitizers; it requires no action by the correlator, and a complete period of 64 mutually-orthogonal waveforms can be made to occur within one correlator dump time. Post-correlation sideband separation is not supported; it could be added by 90d phase switching of the first LO, but 64 correlator dumps would be needed for a complete period. Synchronization requirements among antennas and between each antenna and the correlator are also considered. A novel choice of orthogonal switching waveforms (linear-frequency square waves, rather than Walsh or Rademaker functions) may make synchronization unnecessary. Finally, some detailed design considerations for DDS control are discussed in an appendix.
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Last modified: February 22, 2000
kweather@nrao.edu