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参照LMK04800.PDF里的FIGURE19,如何选择CPOUT2的外接电容(LF2_C1、LF2_C2)、电阻(LF2_R2)呢 ?
内部滤波是针对PLL2的,而外部滤波是针对PLL1的,
The external loop filter for PLL1 should be narrow to provide an ultra clean reference clock from the external VCXO or crystal to the OSCin/OSCin* pins for PLL2.
Integrated internal filter: The LMK048xx features programmable 3rd and 4th order loop filter poles for PLL2.
外部环路滤波器和内部环路滤波器共同组成了一个四阶的滤波器。
外部环路滤波器决定前两个极点,内部环路滤波器决定了后两个极点。
系统对环路滤波器的要求决定了是否需要使用外部的环路滤波器。
James,
PLL1和PLL2都有外部的环路滤波器。
PLL1的外部环路滤波器是用来过滤控制外部VCXO的电荷泵的电压。
PLL2的环路滤波器由内部和外部两部分组成,用来过滤控制内部VCO的电荷泵的电压。