A kind of keying broadband signal source based on the direct digital frequency synthesis (DDS) chip is designed. It can output sine wave from 0 to 100 MHZ, the amplitude adjustable range is from 1mV to 50V, harmonic distortion is less than 4%. The paper introduces the architecture of the keying broadband signal source system, the design of low pass filter and impedance matching. Various technical indexes, such as output frequency, output amplitude, harmonic, is achieved by actual test.