The recent improvement in medical devices that are safe, economical and efficacious has led to significant interests in the near infrared (NIR) optical characteristics of strongly scattering medium such as human tissues, particularly breast tissue, brian and skeletal muscles. Since the frequency domain equipment involves lower peak powers, slower rise times and hence smaller bandwidths than the time domain circuits, they appear to be more economic and portable as medical device. We have developed a 50 MHz time-sharing 3-wavelength (754 nm, 790 nm, 830 nm) single sideband (SSB) phase modulation system, which measures the essential characteristics of light propagation in strongly scattering medium. Some new techniques, such as phase locked loop (PLL), dynode feedback are used to get high accuracy (better than 0.05 degrees in a 1 Hz bandwidth), high sensitivity (6 cm separation on forehead), low noise (less than 0.05 degrees), low drift (less than 0.01 degrees/hr within 11 hours) and low phase-amplitude cross-talk (less than plus or minus 0.038 degrees/dB). A blood model test has been given. Significant results of hemoglobin oxy/deoxygenation measurements re shown in this paper.