Low Phase Noise Jump Frequency Synthesizer

Many systems, especially the radar system compatible with Doppler Pulse Technology and Code Hopping Technology, require that the frequency synthesizer has high performance in frequency agility and low phase noise characteristics. At present the application of the agility frequency synthesizer that based on the DDS (Direct Digital Synthesis) technology is limited because of the poor spurious suppression performance. This paper provides frequency synthesizer based on the research and analysis of the low spurious suppression and low phase noise. This kind of the frequency synthesizer adopts the new-type GaAs DDS IC and analog phase-locked technology with c distinct advantages as frequency agility, low phase noise and spurious suppression index which is better than-65dBc in the whole band.The difficulty of this project is to achieve low spurious characteristics, low phase noise, wideband and small steps of frequency agility frequency synthesizer. These are two ways to achieve frequency agility frequency synthesizer, one is Direct Analog Frequency Synthesis, and the other is Direct Digital Synthesis (DDS). Because the Direct Analog Frequency Synthesis has the disadvantages of big volume, high cost, complex circuits, this project achieves frequency agility frequency synthesizer with Direct Digital Synthesis through frequency conversion. Currently the essential limitation factor of the DDS is the poor performance of the spurious suppression especially in the condition that bandwidth of the output signal is wide. This paper provides a frequency agility frequency synthesizer based on a new DDS IC adopted the clock frequency up to3.2GHz with analog phase locked technology. This technology improves the spurious indicators which caused by the non-linear of DDS and also achieves a low phase noise.Based on theoretical analysis, a frequency agility frequency synthesizer with low spurious characteristics, low phase noise, wideband and short steps combined with DDS and analog phase locked technology has been researched. The frequency of the output signal is1830MHz～1990MHz; frequency step is1MHz; spurious suppression≤-65dBc; phase noise≤-112dBc/Hz@1kHz.The system is divided into two parts. One part generates baseband signal by DDS, the other part generates low phase noise local oscillator signal by analog phase locked. These two signals then mixed, filtered, amplified and finally get the1830MHz-1990MHz signal.