We present a phase difference range system with about 1.5 GHz of modulation frequencies. To reduce the phase difference drift, the high-frequency signals are synthesized by digital fractional phase-locked loops, and a differential measurement method is employed by introducing an optical switch. In order to eliminate the electrical crosstalk, a photoelectric modulator working as a mixer is used. The experimental results show that the distance error of the proposed system is within ±0.04mm and the phase difference error is less than 0.15°.