Standard as well as multiwavelength pulse oximetry as established methods for measuring blood oxygen saturation or fractions of dyshemoglobins suffer from different kinds of interference and noise. Employing lock-in technique as a read-out approach for multiwavelength pulse oximetry is proposed here and strongly decreases such signal disturbance. An analog lock-in amplifier was designed to modulate multiple LEDs simultaneously and to separate the signals detected by a single photodiode. In vivo measurements show an improved signal-to-noise ratio of photoplethysmographic signals and a suppression of interference by means of the lock-in approach. This allows the detection of higher order overtones and, therefore, more detailed data for pulse wave analysis, especially for implantable sensors directly applied at arteries.