The paper addresses the development and performance-analysis of a lung function device for non-invasive forced oscillatory technique (FOT). The applied identification methodology allows estimating the level of noise and the level of non-linearities generated by the device and correct their influence on the patient's excitation signal. The repeatability of the system is assessed under different experiments using respiratory extensions and a calibration structure. The device allows assessing the mechanical properties of the respiratory impedance in the frequency band 0.05–5 Hz. From clinical point of view, this low-frequency band has particular interest when lung's viscoelastic properties need to be assessed, especially when airway remodeling and structural changes occur.