In ultrasonic gas flow metering the ultrasonic wave is complex modulated by natural or artificial structures in the fluid. The determination of the shifted phase angle makes problems if the limited range of the arctan-function is exceeded. It results in phase jumps and ambiguities of the phase angle . This paper presents a new method of phase angle determination which is called differential procedure . The differential procedure has been proved good to determine undersampled ultrasonic data in a wider range of phase shift. The expanded range of phase angle is advantageous for vortex measurement as well as for correlation measurement. Phase and amplitude modulation are physically mutually connected. Phase modulation corresponds to frequency modulation and results in connection with the receiving response of the ultrasound sensor in an additional nonlinear amplitude modulation.