Based On The Ad9858 Wideband Linear Sweep Source

The discovery of The Direct Digital Frequency Synthesizer is a revolution in the development of frequency synthesis technique, which consists of the phase accumulator, the sine look-up table, the digital to analog converter and the low band filter. DDS is of fast frequency switching speed, high frequency resolution, the function to switch frequencies while maintaining constant phase. It is very convenient to achieve many kinds of modulations and to be controlled by micro-processor. The DDS technique has gained its rapid development in recent years, which will be applied more and more extensively in such fields as radar, communications, electronic warfare and electronic measurement instruments.This paper firstly analyzes the essential configuration and the working mechanism of DDS from the two aspects of operational flow and mathematics, and probes into the spectrum characteristic of DDS output in ideal cases. Since the inherent spur performance of DDS is not in perfect condition, so, in order to make DDS work effectually, it is very necessary to understand the spur performance of DDS fully. Based on the thoughtfulness of frequency layout, this paper analyzes the spectrum characteristic of DDS output in practical cases and introduces the location and amplitude of spurs caused by various spur sources.According to the analysis of spur performance, this design makes AD9858 achieve wide-band frequency sweeping of 300MHZ. And DPLL(ADF4113) is used to accomplish the function 1GHZ reference clock source. In order to restrain the 3rd harmonic, this paper work out the method of using filter group which is synchronized-controlled by frequency sweeping. The results of simulation and the measurement by spectrum analyzer are presented here. Also this paper analyzes these results in detail. From this paper, we can find this method can also be applied to the restraint or application of other predictable spurs.