Optical signal processing devices based on quasi-phase-matched three-wave mixing and cascaded three-wave mixing in guided-wave geometries have been demonstrated to operate efficiently at practical pump-power levels. We describe operation of such devices in a balanced mode that allows mixing without wavelength offset and separation of mixed output from pump and signal input without wavelength-selective filters. We present a design for an optical-frequency balanced mixer using quasi-phase-matched, cascaded second-order nonlinear processes. Using this design, we fabricated a balanced mixer in periodically poled lithium niobate waveguides that has the expected linear and nonlinear optical performance.