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LMX2594: 全辅助模式下跳频时间不稳定

Part Number: LMX2594

上图所示:是LMX2594输出后接固定四分频器到相噪仪的结果。SPI时钟频率为25MHz,每200us切换一次频率,在图中可以看到正常情况下在全辅助模式可以得到极短的跳频时间,但是偶尔会出现图中一些不正常的现在,跳频时会出现一个大过冲,然后需要大概100us才能稳定到目标频点。这种现象在同一个VCO段内出现的概率小,跨段的时候出现的概率大一些。下面是初始化的代码。初始化完成以后置位VCO_SEL_FORCE, VCO_CAPCTRL_FORCE, and VCO_DACISET_FORCE进入全辅助状态。 

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跳频时寄存器的写入顺序如下,其中R20,R19和R16是通过回读以后记录下来的值,然后用FPGA采用读表的方式写入频率对应的寄存器值。

1. change R36(0x24)   (note:PLL_N)

2. change R43(0x2b)   (note:PLL_NUM)

3. change R20(0x14)   (note:VCO_SEL)

4. change R19(0x13)   (note:VCO_CAPCTRL)

5. change R16(0x10)   (note:VCO_DACISET)