A signal transmitting device includes: a signal processing circuit arranged to process an input signal to generate a processed input signal according to a compensating signal; a signal converting circuit arranged to convert the processed input signal to generate an output signal according to an oscillating signal; and an arithmetic circuit arranged to generate the compensating signal according to the power of a predetermined component in the output signal, wherein the signal processing circuit uses the compensating signal to update the input signal, and the signal converting circuit converts the updated input signal to reduce the power of the predetermined component in the output signal.