Research And Design Of Signal Generator Based On DDS Technology

Automatic test system(ATS) was first only used in weapon equipment and instrument system, and now is widely applied to aviation, marine and other fields. As a key module in ATS, signal generator has maken tremendous progress with the development of ATS. AS the third generation of frequency synthesis, Direct Digital Frequency Synthesis should naturally be applied to the signal generator of digital ATS. Signal generator using DDS technology has many advantages, such as high frequency resolution, fast frequency switching speed, and the ability to output arbitrary waveform. But digital DDS technology has two major drawbacks: the rich spurs and narrow output frequency range, which greatly limited the further promotion of DDS signal generator.First, this paper introduces in detail the working principle of DDS, and function of four main parts in DDS which consist of phase accumulator, waveform data ROM table, digital to analog converter, low pass filter. And then using spectrum analysis merthod, the paper discusses why DDS produces rich spurs caused in phase truncation, amplitude quantization error and DAC nonlinearities. For the theoretical results of spectrum analysis, this paper uses three methods to reduce spurs, including improving the phase accumulator structure, injecting the phase jitter, and compressing ROM table. These were shown to have good results by computer simulation.In this paper the above three techniques reducing spurs is achieved in hardware. DSP chip was the main chip including phase accumulator and waveform data ROM table of DDS. Compared with the general methods that ROM is outside the DSP, this method that only depending on program design achieves function of phase accumulator and waveform data ROM table of DDS inside the DSP is more easier.In order to get some waveform in any specific frequency, only several parameters be changed, such as sampling frequency and waveform amplitude data in above method. This design is Software-based hardware design, and saves the cost significantly.