A signal generating circuit for generating a fan driving signal includes a phase adjusting circuit, a direct digital frequency synthesizer, a first operating circuit, a driving signal generator and a second operating circuit. The phase adjusting circuit receives a hall signal and adjusts a phase of the hall signal to generate a synchronization signal. The direct digital frequency synthesizer generates a modulating signal according to the synchronization signal. The first operating circuit receives a load current and generates a modulated signal according to the load current. The driving signal generator generates an original driving signal according to the synchronization signal. The second operating circuit generates a control signal according to the modulating signal and the modulated signal. The original driving signal is selectively outputted as the fan driving signal in response to the control signal.