Flattop wavelength conversion over a $70\mathrm{nm}$ bandwidth in a $45\text{-mm}$ -long periodically poled lithium niobate (PPLN) waveguide based on nonlinear cascaded sum frequency generation (SFG) and difference frequency generation processes using two separated wavelengths, is realized. A $2\mathrm{dB}$ peak-to-peak ripple over the conversion bandwidth in the C band is observed when the two pumps are fixed for perfectly phase-matched SFG conversion; this is significantly reduced to $0.5\mathrm{dB}$ by slight detuning of one of the pumps to a longer wavelength. We also demonstrate multichannel broadcasting using two pumps to convert one signal wavelength to three idlers. Using the aperiodically poled lithium niobate waveguide increases the idler separation.