We developed analytical technique for extracting the phase, visibility and amplitude information as needed for interferometric astrometry with the Space Interferometry Mission (SIM). Our model accounts for a number of physical and instrumental effects, and is valid for a general case of bandpass filter. We were able to obtain general solution for polychromatic phasors and address properties of unbiased fringe estimators in the presence of noise. For demonstration purposes we studied the case of

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... rectangular bandpass filter with two different methods of optical path difference (OPD) modulation -- stepping and ramping OPD modulations. A number of areas of further studies relevant to instrument design and simulations are outlined and discussed.