Systems and methods are provided for patient monitors which apply phase detection operations to analog signals to identify differential pulse transit time (DPTT). Photoplethysmograph (PPG) signals measured at two sensor sites may be processed by a phase detection system to identify phase information that allows the calculation of a DPTT. The phase detection system may process analog PPG signals in the analog domain to determine phase information. In some embodiments, the phase detection system may process optical oximetry sensor signals to determine phase information using, for example, interferometric methods.
