| Due to the characteristics associated with Millimeter-wave, it is applied in mm-wave radars,guidance,electronic countermeasure,mm-wave communication and remote sensing. As for mm-wave radars, they generally require frequency sources of full-coherence,high spectrum pureness and frequency-agile ability。This task was carried out just aiming at high–performance mm-wave sources.Direct analog frequency synthesis is an important way to yield high-performance frequency source. However it usually leads to high volume and fairly high spur level. Since commercial PLL IC came out, Phase-Locked-Loop frequency synthesis has become widely accepted. But when narrow frequency-step is required, the loop bandwidth has to decrease while cannot meet the demand of frequency-hopping speed. DDS (Direct Digital Frequency Synthesis) can just solve this problem, yet the output has an upper bound and high wideband spurs.According the demands with frequency source by full-coherent mm-wave radars, this paper combined the three styles of frequency synthesis and developed a type of low-phase noise and spur,narrow-step,frequency-agile,coherent mm-wave frequency synthesizer. It comprises of two sections: Microwave section and MM-wave section.Microwave section includes DDS,PLL and X band frequency multiplication chain. The MM-wave section contains quadruple multiplication stage and IQ modulation part. Measurement has testified that triple-tuning of DDS frequency,reference divider ratio,and loop divider ratio avoided frequencies with high spur level while ensuring the realization of total frequencies. The spur level is below -55dBc and frequency resolution reached 100kHz; DDS output frequency was up-converted to drive the M/N PLL, reducing phase noise, which is better than -87dBc/Hz@1kHz, -92dBc/Hz@10kHz, decreasing frequency-hopping time which is less than 14us. Twice-mixing method was adopted to simplify circuits. The synthesizer has been used in××project . |
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