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提出一种适用于小数分频技术的delta-sigma调制器(DSM)结构。该调制器采用MASH三级四阶结构,并在第二级引入二阶DSM来提高调制器阶数以此达到抑制低频噪声的目的。同时,在调制器输入端加入随机噪声来打断输出序列的周期性,使得量化噪声功率谱更接近于高斯白噪声,这样,对于锁相频率合成器而言,系统带宽内的相位噪声在加到VCO之前,便会被滤除。给出设计框图,并进行计算机仿真。其结果表明,新结构的噪声基底比MASH1-1-1结构噪声基底降低10dB,并且谱线更加平滑。
A delta-sigma modulator (DSM) structure suitable for fractional frequency division technique is proposed. The modulator adopts MASH three-stage fourth-order structure and introduces second-order DSM at the second stage to improve the order of the modulator to achieve the purpose of suppressing the low-frequency noise. At the same time, random noise is added at the input of the modulator to interrupt the periodicity of the output sequence so that the power spectrum of the quantization noise is closer to Gaussian white noise. Therefore, for the phase-locked frequency synthesizer, the phase noise in the system bandwidth is VCO before it will be filtered out. Given the design block diagram, and computer simulation. The results show that the noise floor of the new structure is 10 dB lower than that of MASH1-1-1 structure noise floor and the spectral line is smoother.