abstract an ultrasonic flow sensor in particular for determining the flow velocity of a medium flowing through a pipeline has at least one ultrasonic converter for transmitting and or receiving ultrasonic signals an oscillator which is connected with the ultrasonic converter and an evaluation unit which determines a phase shift of the ultrasonic signal that was received with respect to a reference signal the evaluation unit includes a plurality of quadrature demodulators which operate with different demodulation frequencies and each determine phase information about the ultrasonic signal that was received and when a vernier unit is provided which determines u2014 based on the individual pieces of phase information u2014 a phase angle or a quantity proportional thereto application_number US 4236908 A description fig1 through 3 are explained in the introduction to the description fig4 shows a schematic overview of a system for measuring the phase and or transit time of an ultrasonic signal 15 the system includes from left to right a sensor 26 such as a converter a or b a demodulator unit 27 with several quadrature demodulators see fig5 an optional unit 28 for phase correction of signals delivered by demodulator unit 27 a cordic algorithm which performs an arctangent calculation and a vernier unit 30 which calculates phase angle u03c6 and or transit time t of ultrasonic signal 15 based on the individual phase angles u03b1 1 u03b1 n fig5 shows the demodulator unit 27 of fig4 in detail demodulator unit 27 includes an a d converter 14 with which ultrasonic signal 15 that was received is digitized and a plurality of parallel connected quadrature demodulators 23 a 23 c which process the digitized signal the function of the quadrature demodulator is explained below in greater detail with reference.