The plug-in module of DDS (Direct Digital Synthesis) based function generator is provided as an option of the GDS-2000A series. The function generator is able to generate Sine, Triangle and Square waveforms, with variable duty cycle of the square waveform. Two function generators can be used at the same time to provide dual output signals. With the stimulus source, the verification of electrical characteristics and functionality of the DUT (Device Under Test) can be done in one DSO.