Abstract: A signal converting device includes: a reference signal-mixing circuit arranged to generate a reference mixing output signal according to an input signal, a reference gain, and a reference local oscillating signal; a plurality of auxiliary signal-mixing circuits, each arranged to generate an auxiliary mixing output signal according to the input signal, an auxiliary gain, and an auxiliary local oscillating signal; and a combining circuit arranged to combine the reference mixing output signal and a plurality of the auxiliary mixing output signals to generate an output signal, and at least one of the auxiliary signal-mixing circuits is configured by the corresponding auxiliary gain to compensate phase imbalances between the reference mixing output signal and each of the auxiliary mixing output signals to reduce a power of a harmonic component in the output signal.