A signal adjustment device includes a frequency adjustment circuit, a filter circuit, and a power estimation circuit. The frequency adjustment circuit is configured to receive a two-tone signal from a signal generator and to generate a first signal according to the two-tone signal, wherein the signal generator generates the two-tone signal according to a first coefficient and a second coefficient. The filter circuit is configured to filter the first signal, in order to generate a second signal. The power estimation circuit is configured to detect a power of an intermodulation distortion from the third order signal component, which is associated with the two-tone signal, in the second signal, and to adjust at least one of the first coefficient and the second coefficient according to the power, in order to reduce the power.