The forced oscillation technique (FOT) provides a simple and accurate approach for pulmonary function testing. However, most current devices are large and high cost, hence the test remains sparingly used. To address this problem, we verified the feasibility of a smart and portable forced oscillation device based on a small subwoofer and ultrasonic sensor that is targeted at point-of-care pulmonary function testing. In this paper, we first develop and optimize the signal processing algorithm for impedance estimation. Then we characterize the signal quality of programmable oscillatory waveforms with varying frequency, amplitude and duration. Finally, we evaluate the performance of the device against both mechanical models and human subjects. The results show that the coherence function is above 0.9 for all frequencies and the measurement error is less than 10%. The device yields good repeatability and satisfies the clinical diagnostic requirements.