================================ "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."