Study Of Micorwave DDS Frequency Synthesizer Technology

The performance of the frequency synthesizer plays a decisive part in theperformance of the whole electronic system. With increasingly demand on theperformance of the electronic system, the demands of research on microwavehigh-performance frequency synthesis technology has been urgently raised. The maincontent of this thesis is the research on high-performance synthesizer of wide bandwidth,low spur, high hopping speed and fine frequency step with combination of variousfrequency synthesis technologies[1,2].Firstly, in this thesis the DDS+DS (Direct Frequency Synthesize) design program isdetailed analyzed on theoretical principle and index estimating. To further verify thefeasibility of the scheme, some essential sub-units are designed, processed and tested.Analyzing test results, the DDS+DS design program, in narrow output band, presentshigh-performance in frequency step, phase noise, spur depression, frequency hoppingspeed.Secondly, in order to avoid the high spur of DDS+DS program when widebandoutput is required, the DDS+PLL program is formulated and detailedly analyzed on theprinciple and index expectation. The PLL key circuits are designed and tested, the testresults say that when choose the appropriate loop bandwidth, with500MHz frequencyhopping band, the frequency hopping time is about5us, which can meet the expectationof the research; besides, when the output frequency at9GHz, the test result of phasenoise is:-87dBc/Hz@1kHz，-99dBc/Hz@10kHz.Lastly, aiming to improve the phase noise, a modified DDS+PLL program has beendesigned. A down-conversion circuit is fixed to the feed back circuit to decrease thefrequency dividing ratio, and then improve the phase noise. Overall considering thedemand of index and the circuit complexity, the design proposal is determined finally.According to the improved design proposal, the frequency synthesizer is designed,processed and tested. The test results show that:The frequency synthesizer’s output frequency band is34~36GHz; the phase noise inthe output frequency band is better than-88dBc/Hz@1kHz,-96dBc/Hz@10kHz,-102 dBc/Hz@100kHz, which compared to the original DDS+PLL program, the indeximproves about9dB; In X band, when the frequency hopping band is700MHz and1MHz, the test result of hopping time is7.9us,370ns;the frequency step is300uHz;the output power of the frequency synthesizer is greater than3dBm; in the output bandthe depression of spur is better than45dB.