Signal processing of ultrasound modulated by gas flow

Measurement of gas flow velocity with ultrasound by cross correlation functions were causing intensive investigations of complex modulated signals. The ultrasound signal is modulated in amplitude and in phase as well by various physical effects explained. The demodulation of the signal by undersampling and the separation of amplitude and phase modulated parts by Hilbert-transform are described. Cross correlation functions applied to these separated signals result in well defined measurements of flow velocity.