| This paper is a part of hydrogen maser researched by Time and Frequency Research Center of Shanghai Astronomical Observatory Chinese Academy of Sciences, the primary purpose is to generate a L band low cost frequency synthesizer with low phase noise and high frequency stability. Frequency synthesizer is one of the key component in the high performance electronic system , and it is an important research field in communication technology.In this paper, he PLL frequency synthesizer is studied. First, we review the fundamental concepts and basic parts of the PLL, then review the design methods of a PLL synthesizer and the one constituted by charge pump PFD(phase and frequency detector) and passive loop filter is focused .Because the VCO determines the out-band phase noise, it is an important step to design a high performance VCO in this paper, to reduce the phase noise, we use a shorted coaxial resonator below λ/4 as the inductor in the resonator ,then we simulate it with ADS ,the simulated data and the real test results are provided. At last, the control method and the real test results of the PLL synthesizer constituted by ADF4212 and passive lead-lag loop filter are provided, ADF4212 simplifies the design and is propitious to hydrogen maser miniaturization.The real test results satisfy the following specifications, phase noise is-92dbc/Hz@ 10kHz,Spurs is 75dBc, short-term frequency stability is 1.9 × 10-13 / s 。.
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