Signal processing of complex modulated ultrasonic signals

Abstract

Ultrasound is a natural choice for the measurement of turbulent flow. The ultrasonic signal is complex modulated by the structures in the fluid. The modulation frequency is low resulting in narrowsidebands to the high carrier frequency. The modulated signal can be separated by digital undersampling. By means of hardware- or softwareapplied Hilbert-transform the complexmodulated signal can be determined for crosscorrelation or vortex measurements. Phase angles can exceed the limited range of the arctan function caused by offset or strong vortices in the fluid. The densitydistribution function of the phase angle admits the reconstruction of the phase angle. Extended Kalman filters have been proved to be suitable for phase demodulation. Also, analog signal processing will be explained for an electronic Hilbert-transform and amplitude demodulation in vortex measurement. Measurement results are represented for cross-correlation and vortex measurements at low and high flow velocities. © Springer-Verlag Berlin Heidelberg 2005.