A modulation method includes sampling the first input signal by using the first local oscillation signal and the second local oscillation signal to generate the first sampled signal, sampling the second input signal by using the third local oscillation signal and the fourth local oscillation signal to generate the second sampled signal, sampling the second input signal by using the first local oscillation signal and the second local oscillation signal to generate the third sampled signal, sampling the first input signal by using the third local oscillation signal and the fourth local oscillation signal to generate the fourth sampled signal, adding the first sampled signal and the second sampled signal to produce the first modulation signal, adding the third sampled signal and the fourth sampled signal to generate the second modulation signal, and adding the first modulation signal and the second modulation signal to generate an output signal.