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We report simultaneous frequency conversion and amplitude modulation in an optical second-harmonic generator by electrooptically controlling the relative phase between the 1064-nm fundamental and the 532-nm second-harmonic fields in a dispersion crystal section between two periodically poled lithium niobate (PPLN) sections. Theoretical derivation and experimental demonstration were carried out for two novel crystal configurations, including a linear cascaded configuration in which a 1-cm dispersion section is sandwiched between two 2-cm PPLN sections, and a folding-crystal high-efficiency configuration in which the mixing waves traverse twice in a 2-cm PPLN section through total internal reflections in a 1.5-cm dispersion section. Due to the coherence enhancement in the constructive phase between the two second-harmonic generation (SHG) fields in the two PPLN sections, we measured a 30% increase in conversion efficiency compared to a 4-cm continuous-grating PPLN under the same condition. The measured half-wave voltage for the amplitude modulation is 1.1 V × d (μm)/ld (cm), where d is the separation of the electrodes and ld is the length of the electrodes.