We present a compact, low-noise, and inexpensive optical phase-lock loop (OPLL) system to synchronize the frequency and the phase between two external cavity diode lasers. Based on a direct digital synthesizer technique, a programmable radio-frequency generator is implemented as the reference signal source. The OPLL has a narrow beat note linewidth below 1 Hz and a residual mean-square phase error of $0.06{\mathrm{rad}}^2$ in a 10 MHz integration bandwidth. The experimental test results prove the competent performance of the system, which is promising as a low-budget choice in atomic physics applications.