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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 rad 2