In this memo, a technique for signal coherence loss estimation for Interferometer is introduced. We discuss the coherence loss caused by the phase noise in order to find out the cause of dominant coherence-loss source, and introduce a concept of coherent integration time. The coherence loss can be calculated from the measured phase stability in Allan standard deviation. The key of an interferometer is to maintain the signal coherence. In a connected interferometer, the instability of the distributed common reference signal is compensated for as a common noise. On the other hand, the independent instability of each element decreases the signal coherence. For ALMA, the instability of LLC (Line length corrector), WMA (Warm multiplier assembly) and CMA (Cold multiplier assembly) are independent and/or mounted on independent antennas. The frequency standard (including signal transmission and multiplier chain) of ALMA must be stable over the long-time period (up to observation time) and the short-time period (coherent integration time for fringe detection) to maintain the coherence. We will discuss the required phase stability and the coherence loss in the Allan standard deviation. This coherence estimation is essential for the VLBI application. As a result, the ALMA specification is good enough to keep the coherence, the estimated instrumental coherence losses are 5 % in LO, and 10 % in total (exclude atmospheric scintillation) at 938 GHz with over 260 sec coherent integration time.
View a pdf version of ALMA Memo #530.
Last modified: 2005-10-30
alma-memos@nrao.edu