Cross correlators," Lecture 4 in Synthesis Imaging
- L. D'Addario
- In Radio Astronomy, Astron. Soc. Pac. Conf. Ser.,
Introduction This report describes the conceptual design of certain parts of the local oscillator subsystem of the ALMA telescope, namely those parts concerned with ne control of the phase of the LO signals, along with related considerations for the correlator and the real-time software. Precise control of the LO phase is needed for fringe tracking, which is simply a matter of causing the LO to follow the variation in delay from the source to the receiver as the earth rotates. The same hardware can be used to e ect suppression of the interferometer response from the unwanted sideband of the rst mixer, which is expected to be incompletely suppressed in the front end; and to suppress certain spurious signals that might get introduced into the signal path after the rst mixer. It can also be used to allow a small amount of ne adjustment of the receiver's center frequency. Quantitative parameters mentioned here are consistent with the ALMA system design as of this writing. Some of these may be changed for practical reasons during the course of development, but the overall design is expected to remain as described here.