Abstract: Phase comparison of the heterodyne signal determines the performance of the heterodyne interferometer to some extent, such as precision and resolution. To resolve the conflict between the resolution and the velocity, methods used presently to design the phase comparator were introduced and analyzed. Key design points to achieve high measureable velocity with fine resolution were discussed. Solutions to the problems were also proposed. The full period sampling method based on field-programmable gate array(FPGA) can improve the precision of autocorrelation and the zero-across detection with frequency mixing can improve the test speed.