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"ALMA LO Timing Considerations
Abstract:
For a connected-element interferometer, in order to retain coherence the total
signal propagation time, from the source to each antenna and then to the
correlation process, must to be equalized. However, for a fixed frequency of
operation, there is no need to equalize the local oscillator propagation time
from the central technical building to each antenna, provided the phase of all
local oscillators at the receiver can be kept constant, and can be calibrated.
At Chajnantor, there may be a difference of 10 km or more in the lengths of
fiber optic cables, carrying local oscillator and timing signals, to different
antennas. The consequence of this differential delay of ~50 microseconds, if
uncorrected, is that very small changes of frequency will result in significant
differential phase changes in some antenna pairs. If the delays are known, this
can be allowed for in software, either in real time or in post observation data
reduction. The precision to which the delays are known sets the limit on how
great a local oscillator frequency change can be tolerated without independent
calibration. Equivalently, the scientific requirement for magnitude of local
oscillator frequency changes, without carrying out an independent phase
calibration, sets an additional engineering requirement on stability, and
sets an operational requirement on the measurement precision of the LO
propagation delay within, and to, each antenna."