Signal processing of ultrasound modulated by gas flow

Ultrasonic measurement of gas flow velocity using cross-correlation functions motivated an intensive investigation of complex modulated signals. The ultrasound signal is modulated in amplitude and also in phase by various physical effects. This paper describes signal demodulation by undersampling and subsequent separation of amplitude and phase modulated parts by the Hilbert-transform. Cross-correlation functions applied to the phase-modulated signal component produce well-defined measurements of flow velocity.