Displacement measurement by dual wavelength and single grating integrates the single grating diffraction theory and the
heterodyne interference theory. By taking advantage of the two theories, it solves the contradiction between large range
and high precision in grating displacement measurement quite well. In order to obtain nanometer resolution and
nanometer precision, high-power subdivision of interference fringe must be realized accurately. According to phase
demodulation theory for heterodyne interference signal, a digital phase measuring method is proposed by combining
frequency-mixing technique and pulse-filling method. The whole signal processing part, which is based on FPGA and
PLL, has been designed to realize the integer period measurement and high-powered subdivision of the decimal phase.
Through experiments, it is validated that the phase range is [-180°, 180°], and the phase measurement resolution and the
phase precision are above 0.03° and 0.1°, respectively. Moreover, the displacement measurement resolution and the
displacement precision, corresponding to the phase indexes, are 0.167nm and 0.556nm, respectively.