Abstract: A Universal/Upgrading Pulse Oximeter (UPO) comprises a portable unit and a docking station together providing three-instruments-in-one functionality for measuring oxygen saturation and related physiological parameters. The portable unit functions as a handheld pulse oximeter. The combination of the docked portable and the docking station functions as a standalone, high-performance pulse oximeter. The portable-docking station combination is also connectable to, and universally compatible with, pulse oximeters from various manufacturers through use of a waveform generator. The UPO provides a universal sensor to pulse oximeter interface and a pulse oximetry measurement capability that upgrades the performance of conventional instruments by increasing low perfusion performance and motion artifact immunity, for example.