A Method for Improving LFM Signal Spectrum Quality in Direct Digital Synthesis

In this paper, the distributing rules of spurs in the DDS output spectrum is analyzed. A effective method to design reasonable external circuits and choose appropriate instruction words is presented to improve the spectrum purity of the LFM signal. DDS chip (AD9854) is used to generate a broad band LFM signal. The experiments result shows that the LFM signal can achieve 62 dB SFDR with using this method in 30 MHz bandwidth. It increases the SFDR in about 10 dB than that without using this method.