分数N频率综合器的杂散分析

Spur analysis of fractional-N frequency synthesizers

杂散是影响分数N频率综合器性能的重要指标。针对传统无补偿频率综合器和采用ΣΔ调制技术的分数N频率综合器的杂散输出进行理论分析。在传统无补偿频率综合器的分析中引入附加相移满足锁定条件,同时采用新的分析方法使得各杂散分量更加明显。在对于ΣΔ调制技术的频率综合器,通过线性分析指明带宽选取与残留相差及高频端噪声抑制能力间的关系,并用时域模型给出环路非线性如鉴相鉴频器的死区和电荷泵充放电电流的失配使得频率综合器带内输出噪声频谱恶化30dB。

Spur is the key specification that affects the performance of fractional-N frequency synthesizers. The spur for traditional compensationless and ΣΔ modulated fractional-N frequency synthesizers was analyzed theoretically to make the spur components more explicit. An extra phase shift was introduced into traditional compensation-less fractional-N frequency synthesizers to satisfy the locked condition. For ΣΔ modulated fractional-N frequency synthe- sizers, a relationship was developed for the loop bandwidth as a function of the residual phase error and the high frequency noise rejection using linear analysis. Analysis using the time field model shows that loop nonlinearities, such as the dead-zone in phase-frequency detectors and the mismatch between the charge and discharge currents in the charge pump, can reduce the output noise by 30 dB.