A circuit for detecting a phase error between a clock signal and a beam position includes a beam generator, sensor, and phase detector. The beam generator directs a beam toward a beam sweeper in response to a clock signal. The sensor, which is disposed at a mid line of a region that the beam sweeper scans, detects the beam from the beam sweeper, and the phase detector detects an error in the clock phase from the detected beam. Such a circuit can automatically detect the phase error in the pixel clock and correct this error, thus eliminating the need for a manual phase-error corrector.