The present invention provides a broadband and small-step frequency source circuit. A small-step output signal with the frequency thereof to be hundreds of megahertz is outputted through the DDS, so that the small-step output signal of a voltage controlled oscillator is realized. A broadband frequency source circuit is realized through selecting the voltage controlled oscillator having the broadband characteristics. According to the technical scheme of the invention, an IQ mixer is adopted as an RF mixer. The IQ mixer is used for extracting the different signal of a local oscillator signal and an intermediate-frequency (IF) signal. Meanwhile, the sum signal of the local oscillator signal and the intermediate-frequency (IF) signal is extracted, and the suppression of the different signal on the local oscillator signal is over 20 dBc. Therefore, RF signals outputted by the IQ mixer can enter a digital phase detector without passing through an extremely narrow-band filter. Compared with a conventional DDS circuit, the circuit is low in debugging difficulty and extremely high in frequency accuracy is, and can be miniaturized.