An ultrasonic flow measurement system comprising: a signal generator configured to generate a first excitation signal for a first ultrasonic transducer and a second excitation signal for a second, different ultrasonic transducer; and a phase differentiator configured to generate a differential phase angle signal indicative of a difference in phase angle between first and second receive signals (27, 28) received from the first and second transducers, wherein the phase differentiator comprises: an XOR gate having first and second inputs and an output; and a low-pass filter having an input and an output; wherein the first input of the XOR gate is configured to receive the first receive signal or a signal obtained from the first receive signal and the second input of the XOR gate is configured to receive the second receive signal or a signal obtained from the second receive signal, wherein the output of the XOR gate is coupled to the input of the low pass filter; and wherein the output of the low-pass filter provides the differential phase angle signal; and characterised in that the ultrasonic flow measurement system further comprises a controller comprising a processor configured to control the signal generator, wherein the controller is configured to vary frequency of a clock signal used to generate the first and second excitation signals so as to reduce the differential signal at zero flow. a signal generator configured to generate a first excitation signal for a first ultrasonic transducer and a second excitation signal for a second, different ultrasonic transducer; and a phase differentiator configured to generate a differential phase angle signal indicative of a difference in phase angle between first and second receive signals (27, 28) received from the first and second transducers, wherein the phase differentiator comprises: an XOR gate having first and second inputs and an output; and a low-pass filter having an input and an output; wherein the first input of the XOR gate is configured to receive the first receive signal or a signal obtained from the first receive signal and the second input of the XOR gate is configured to receive the second receive signal or a signal obtained from the second receive signal, wherein the output of the XOR gate is coupled to the input of the low pass filter; and wherein the output of the low-pass filter provides the differential phase angle signal; and characterised in that the ultrasonic flow measurement system further comprises a controller comprising a processor configured to control the signal generator, wherein the controller is configured to vary frequency of a clock signal used to generate the first and second excitation signals so as to reduce the differential signal at zero flow. an XOR gate having first and second inputs and an output; and a low-pass filter having an input and an output; wherein the first input of the XOR gate is configured to receive the first receive signal or a signal obtained from the first receive signal and the second input of the XOR gate is configured to receive the second receive signal or a signal obtained from the second receive signal, wherein the output of the XOR gate is coupled to the input of the low pass filter; and wherein the output of the low-pass filter provides the differential phase angle signal; and characterised in that the ultrasonic flow measurement system further comprises a controller comprising a processor configured to control the signal generator, wherein the controller is configured to vary frequency of a clock signal used to generate the first and second excitation signals so as to reduce the differential signal at zero flow.