Fringes are created because the instrumental delay is applied at the intermediate frequency [IF] rather than the radio frequency [RF] (for which the center frequency is usually much less). The phase rotates as an object is tracked in the sky. In order to "stop" the phase rotation, adjustment of the phase of the local oscillator are made so that the fringe frequency is slowed down to nearly zero. This allows the visibility function to be measured at a much slower speed. This also allows the fringe pattern to be measured near the their maximum in the center. When an object is fringe stopped, a complex correlator must be used, since a real correlator will measure only a constant amplitude.
