Analysis of Detection Methods of RF Vibrometer for Complex Motion Measurement

In this paper, the detection technique of the previously reported wavelength-division-sensing RF vibrometer is thoroughly analyzed by investigating the Bessel function coefficient of harmonics in the baseband spectrum. The analysis provides guidelines on the selection of harmonic pairs to help improve detection accuracy and reliability. A new detection method that utilizes multiple harmonic pairs under a fixed carrier frequency proves to be a more reliable approach for RF vibrometers. The sensitivity of harmonic amplitude ratio to the excess phase angle is explored as well. The results show that by using insensitive harmonic pairs, information about the characteristics of the vibration system and the external driving force are still attainable even when the phase angles are unknown. Real-time monitoring of various harmonic vibrations has been realized through a LabVIEW program with a user interface that provides visualization of the vibration patterns. The function of through-wall detection of the RF vibrometer has also been verified by conducting all measurements behind a wall with a detection distance of 1.5 m.