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For the single-pass and double-pass wavelength converters based on cascaded sum and difference frequency generation in quasi-phase matched lithium niobate waveguide, we present the criteria for a choice of the waveguide length and the assignment of pump powers to achieve the desired efficiency, ripple and bandwidth with a large pump wavelength difference of 75 nm. It is found that using the double-pass structure with a 95% reflectivity for the sum frequency wave, the mean efficiency is increased at least 5 dB compared to that of the single-pass one with the same length and input pump powers and the bandwidth is large enough to cover the entire conventional telecommunications C-band and long-wavelength L-band. To flatten the excessive response fluctuation of the double-pass scheme, we propose tuning one of the pumps to longer wavelengths incurring a small penalty of a slight reduction in the mean efficiency which can be compensated by increasing the input pump powers. We also show that to achieve the same amount of flatness, double-pass converters have a smaller efficiency penalty compared to the single-pass converters.