LDO噪声来源以及对射频频综输出相噪的影响

[{"attributes":{"color":"#222222"},"insert":"相位噪声是时钟、射频频综最为关注的技术指标之一。"},{"attributes":{"color":"#c60c23","bold":true},"insert":"影响锁相环相噪的因素有很多，比如电源、参考源相噪、VCO 自身的相噪、环路滤波器的设置等。其中，电源引入的低频噪声往往对锁相环的近端相噪有着很大的影响。"},{"attributes":{"color":"#222222"},"insert":"对于高性能的时钟和射频频综产品，为了获得极低的相噪性能，往往采用低噪声的 LDO 供电。然而，采用不同的 LDO 给频综供电，取得的相噪性能往往会有很大差别，同时，LDO 外围电路设计也会影响到频综的相噪性能。"},{"attributes":{"align":"justify"},"insert":"\n"},{"insert":" "},{"insert":{"image":"https://community-1252773949.cos.ap-guangzhou.myqcloud.com/article/2034/c68d1271e1b7e192.png"}},{"attributes":{"align":"center"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":" "},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":"本文首先简要地介绍了 LDO 的噪声来源及环路稳定性对输出噪声的影响；"},{"attributes":{"color":"#222222","bold":true},"insert":"其次，根据调频理论推导出 VCO 的相位噪声与 LDO 的噪声频谱密度的理论计算关系。"},{"attributes":{"color":"#222222"},"insert":"在此基础上，为了验证 LDO 噪声对射频频综输出相噪的影响，"},{"attributes":{"color":"#222222","bold":true},"insert":"分别采用 TPS7A8101 和 TPS74401LDO 评估板给 TRF3765 射频频综评估板供电，对比测试这两种情况下的 TRF3765 相噪曲线；"},{"attributes":{"color":"#222222"},"insert":"同时，为了验证 LDO 环路稳定性对频综相噪的影响，针对 TPS7A8101 评估板的参考电路做出部分修改，并对比测试了电路修改前后的 TRF3765 输出相噪。"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222","bold":true},"insert":" "},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222","bold":true},"insert":"LDO噪声对输出噪声影响"},{"attributes":{"align":"justify"},"insert":"\n"},{"insert":" "},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#c60c23","bold":true},"insert":"1LDO 噪声来源"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":"LDO 的噪声分为 LDO 内部的噪声和 LDO 外部的噪声。"},{"attributes":{"color":"#222222","bold":true},"insert":"LDO 内部的噪声来自于内部电路的带隙基准源，放大器以及晶体管。LDO 外部的噪声来自于输入。在 LDO 的手册中，PSRR 是表征 LDO 抑制外部噪声的能力，但 PSRR 高并不代表 LDO 内部噪声小。"},{"attributes":{"color":"#222222"},"insert":"LDO 的总输出噪声才是表征 LDO 内部噪声抑制的参数，一般在电气特性表里用单位µVRMS 表示，或者在噪声频谱密度图上表示。"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":" "},{"insert":{"image":"https://community-1252773949.cos.ap-guangzhou.myqcloud.com/article/2034/a7122f153ee7d0bd.png"}},{"attributes":{"align":"center"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":"图 2 是 LDO 内部结构框图，VN 代表等效噪声源。"},{"attributes":{"color":"#222222","bold":true},"insert":"噪声源包括带隙基准源产生的噪声 VN(REF)，误差放大器产生的噪声 VN(AMP)，FET 产生的噪声 VN(FET)以及反馈电阻产生的噪声 VN(R1)和 VN(R2)。"},{"attributes":{"color":"#222222"},"insert":"在大多数情况下，由于带隙基准源电路是由很多不同的电阻、晶体管和电容组成，"},{"attributes":{"color":"#222222","bold":true},"insert":"它所产生的噪声会远远大于反馈电阻产生的噪声。"},{"attributes":{"color":"#222222"},"insert":"而且带隙基准源是误差放大器的输入，它所产生的噪声也会经由误差放大器放大来控制 FET，所以误差放大器本身以及 FET 所产生的噪声也会比带隙基准源的噪声要低。可以说，LDO 内部最大的噪声源就是带隙基准源。我们把 LDO 输出噪声 VN(OUT)表示为"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":" "},{"insert":{"image":"https://community-1252773949.cos.ap-guangzhou.myqcloud.com/article/2034/e2978609a12fccb9.png"}},{"attributes":{"align":"center"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":"VN(Other)是 VN(AMP)以及 VN(FET)的和。由公式 1 可以得出，输出噪声最小值出现在 R1 短接到 FB，误差放大器的增益近似为 1 的时候。"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#c60c23","bold":true},"insert":{"image":"https://community-1252773949.cos.ap-guangzhou.myqcloud.com/article/2034/38639c123a5430b6.png"}},{"attributes":{"color":"#c60c23","bold":true},"insert":" "},{"attributes":{"align":"center"},"insert":"\n"},{"attributes":{"color":"#c60c23","bold":true},"insert":"2 LDO 噪声抑制方法"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":"为了抑制带隙基准源产生的噪声，有三种办法。"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":"一是降低误差放大器的带宽，抑制了带隙基准源的高频噪声。但是降低带宽会使 LDO 的动态性能降低。"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":"二是在带隙基准源和误差放大器之间加低通滤波。"},{"attributes":{"color":"#222222","bold":true},"insert":"高性能的 LDO 都会有一个噪声抑制 NR 管脚，CNR 并联在带隙基准源和 GND 之间，起到低通滤波的作用。"},{"attributes":{"color":"#222222"},"insert":"如图 3 所示。"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":{"image":"https://community-1252773949.cos.ap-guangzhou.myqcloud.com/article/2034/e9acfabfe6845c34.png"}},{"attributes":{"color":"#222222"},"insert":" "},{"attributes":{"align":"center"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":"三是在反馈电阻 R1 上增加前馈电容 CFF。在增加了 CFF 和 CNR 后，输出噪声可以表示为"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":{"image":"https://community-1252773949.cos.ap-guangzhou.myqcloud.com/article/2034/d1de301ea06e8eb7.png"}},{"attributes":{"color":"#222222"},"insert":" "},{"attributes":{"align":"center"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":"从式 2 可以得出，CFF 越大，输出噪声就越小。频率越高，输出噪声越小。"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":"图 4 是不同 CFF 下的噪声频谱密度图。"},{"attributes":{"color":"#222222","bold":true},"insert":"可以看出，CFF 越大，噪声从低频开始都能被很好的抑制。"},{"attributes":{"color":"#222222"},"insert":"CFF 太小的时候，抑制噪声的作用就不太明显。当频率很高的时候，不管用多大的 CFF，噪声频谱密度相差不会太大。所以，增加合适的前馈电容 CFF，对改善 LDO 低频噪声有非常好的效果。"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#c60c23","bold":true},"insert":" "},{"insert":{"image":"https://community-1252773949.cos.ap-guangzhou.myqcloud.com/article/2034/8b93a95295c94d69.png"}},{"attributes":{"align":"center"},"insert":"\n"},{"attributes":{"color":"#c60c23","bold":true},"insert":"3 LDO 环路稳定性与输出噪声的关系"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":" "},{"insert":{"image":"https://community-1252773949.cos.ap-guangzhou.myqcloud.com/article/2034/2b8753c567752ebd.png"}},{"attributes":{"align":"center"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":"从 LDO 的小信号分析可以看出，"},{"attributes":{"color":"#222222","bold":true},"insert":"LDO 有两个低频极点，如果没有合适的零点补偿，LDO 的稳定裕度不够，就有可能产生震荡。"},{"attributes":{"color":"#222222"},"insert":"稳定裕度不够的 LDO 产生的内部噪声会更大。上节中提到第三种噪声抑制方法，即增加前馈电容 CFF 是实际上为了改善系统稳定裕度。由 CFF 与 R1 组成一个低频零点，"},{"attributes":{"color":"#222222"},"insert":{"image":"https://community-1252773949.cos.ap-guangzhou.myqcloud.com/article/2034/2ac07370e4b461bb.png"}},{"attributes":{"color":"#222222"},"insert":"。"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":" "},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":"由下图的频率响应可以看出，零点是相位裕度有了很大的提升，增加了系统稳定性，从而减小了系统低频噪声。"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222","bold":true},"insert":" "},{"insert":{"image":"https://community-1252773949.cos.ap-guangzhou.myqcloud.com/article/2034/8eda6d79a2ef8056.png"}},{"attributes":{"align":"center"},"insert":"\n"},{"attributes":{"color":"#222222","bold":true},"insert":"LDO 噪声与 VCO 输出相噪的关系"},{"attributes":{"align":"justify"},"insert":"\n"},{"insert":" "},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":"电源引入噪声对锁相环中各个有源器件都可能造成影响，"},{"attributes":{"color":"#222222","bold":true},"insert":"其中最为敏感的部分是 VCO，本文将着重讨论 LDO 输出噪声对 VCO 相噪的影响。"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":"一个典型的 LDO 供电的频综系统框图如图 7 所示：加载在电源上的噪声信号通过频率调制过程调制到 VCO 的输出，造成 VCO 输出相噪恶化。"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":{"image":"https://community-1252773949.cos.ap-guangzhou.myqcloud.com/article/2034/584dbd9549d2393d.png"}},{"attributes":{"color":"#222222"},"insert":" "},{"attributes":{"align":"center"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":"根据经典调频系统理论，调制指数β由式（3）来表示"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":{"image":"https://community-1252773949.cos.ap-guangzhou.myqcloud.com/article/2034/24682e4bde646610.png"}},{"attributes":{"color":"#222222"},"insert":" "},{"attributes":{"align":"center"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":"对于电源噪声调制，式中的频率背离（FrequencyDeviation）可由下式得到"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222","bold":true},"insert":{"image":"https://community-1252773949.cos.ap-guangzhou.myqcloud.com/article/2034/b0ea815fa0ae7443.png"}},{"attributes":{"color":"#222222","bold":true},"insert":" "},{"attributes":{"align":"center"},"insert":"\n"},{"attributes":{"color":"#222222","bold":true},"insert":"式中，Kpush 是 VCO 的电源推压指数，它表征的是 VCO 对电源噪声波动的灵敏度，单位用 MHz/V 来表示；A 是电源噪声信号幅度。"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":"对于采用 LDO 供电的射频频综来说，通常用 LDO 的指定频率偏移的频谱噪声密度 Sldo（f）（NoiseSpectrumDensity）来表征电源噪声，由于它是一个 RMS 电压值，所以式（4）可以表示为"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":{"image":"https://community-1252773949.cos.ap-guangzhou.myqcloud.com/article/2034/eb36d2b381016d0b.png"}},{"attributes":{"color":"#222222"},"insert":" "},{"attributes":{"align":"center"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":"将式（5）带入式（3），可以得到"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":{"image":"https://community-1252773949.cos.ap-guangzhou.myqcloud.com/article/2034/54e64681bc20bbf0.png"}},{"attributes":{"color":"#222222"},"insert":" "},{"attributes":{"align":"center"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":"式中，f 是相应的频率偏移。"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":"由不同频率成分噪声调制到载波输出引起的单边带噪声，由下式表示"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":{"image":"https://community-1252773949.cos.ap-guangzhou.myqcloud.com/article/2034/86981991544a1746.png"}},{"attributes":{"color":"#222222"},"insert":" "},{"attributes":{"align":"center"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":"将式（6）带入式（7）有"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":{"image":"https://community-1252773949.cos.ap-guangzhou.myqcloud.com/article/2034/1e319636af93ee37.png"}},{"attributes":{"color":"#222222"},"insert":" "},{"attributes":{"align":"center"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":"由式（8）可见，对于给定的 VCO，由于 Kpush 是一个确定的值，因此由 LDO 噪声引起的 VCO 输出相噪是由 LDO 的噪声频谱密度（NoiseSpectrumDensity）决定的。"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":" "},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222","bold":true},"insert":"采用不同 LDO 进行射频频综供电对比测试"},{"attributes":{"align":"justify"},"insert":"\n"},{"insert":" "},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222","bold":true},"insert":"1 TPS7A8101/TPS74401 频综供电对比测试"},{"attributes":{"align":"justify"},"insert":"\n"},{"insert":" "},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":"TPS7A8101 和 TPS74401 是 TI 推出的两款高性能 LDO 芯片。"},{"attributes":{"color":"#222222","bold":true},"insert":"与 TPS74401 相比,由于具有更高的环路增益和带宽，TPS7A8101 具有更高的电源噪声抑制比（PSRR）；"},{"attributes":{"color":"#222222"},"insert":"然而，由于具有更好的系统稳定性，TPS74401 拥有更低的噪声频谱密度（NSD），如下图 8 所示。"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":" "},{"insert":{"image":"https://community-1252773949.cos.ap-guangzhou.myqcloud.com/article/2034/c0966685c071414b.png"}},{"attributes":{"align":"center"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":"下面我们分别采用 TPS7A78101 和 TPS74401 评估板对 TRF3765 评估板进行供电，比较两者的输出相噪。"},{"attributes":{"color":"#222222","bold":true},"insert":"测试设置如下图 9 所示，LDO 的输入 5V 电源由 AgilentE3634 提供，通过 LDO 评估板后转变成 3.3V 给 TRF3765 供电。"},{"attributes":{"color":"#222222"},"insert":"TRF3765 采用评估板上自带的 61.44MHZ 晶振作为参考输入，输出频率为 2.28GHz。TRF3765 的射频输出连到 R&SFSQ8 相噪分析仪上测试相应的相噪曲线。"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":" "},{"insert":{"image":"https://community-1252773949.cos.ap-guangzhou.myqcloud.com/article/2034/825dd8c59bd2cc31.png"}},{"attributes":{"align":"center"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":"两者对比测试结果如下图 10 所示，"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":" "},{"insert":{"image":"https://community-1252773949.cos.ap-guangzhou.myqcloud.com/article/2034/272b3840f0c2bb43.png"}},{"attributes":{"align":"center"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":"由上图看见，采用 TPS7A8101 供电，TRF3765 在整个积分区间内（1KHz~10MHz）的 RMS 抖动为 0.62ps；而 TPS74401 的 RMS 抖动仅为 0.44ps。"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":" "},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#c60c23","bold":true},"insert":"2 TPS7A8101 输出电路优化及其对频综相噪的影响"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":"TPS7A8101 评估板初始原理图如图 11 所示，"},{"attributes":{"color":"#222222","bold":true},"insert":"由上节的测试结果可知，采用该电路给 TRF3765 供电，RMS 抖动为 0.62ps。"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222","bold":true},"insert":" "},{"insert":{"image":"https://community-1252773949.cos.ap-guangzhou.myqcloud.com/article/2034/a73ce7ad35e3bd19.png"}},{"attributes":{"align":"center"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":"第一章中我们已经讨论了 LDO 加一个前馈电容可以有效的提高电源的环路稳定性，从而降低 LDO 的输出噪声频谱密度。基于此，我们在 TPS7A8101 输出加一个 0.47µF 的前馈电容，修改后的原理图如下图 12 所示。"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":" "},{"insert":{"image":"https://community-1252773949.cos.ap-guangzhou.myqcloud.com/article/2034/cb0ae243eebd2664.png"}},{"attributes":{"align":"center"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":"针对修改前后的设计，我们对比测试了相应的 TRF3765 相噪曲线，如图 13 所示，由图可见，增加 0.47µF 输出电容后，1KHz 到 10MHz 的 RMS 抖动由 0.62ps 提高到 0.49ps。"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":" "},{"insert":{"image":"https://community-1252773949.cos.ap-guangzhou.myqcloud.com/article/2034/78cd899d0cddde86.png"}},{"attributes":{"align":"center"},"insert":"\n"},{"attributes":{"color":"#c60c23","bold":true},"insert":"结论"},{"attributes":{"align":"justify"},"insert":"\n"},{"insert":" "},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":"综合以上两组测试的测试结果，可以得到下表"},{"attributes":{"align":"justify"},"insert":"\n"},{"insert":" "},{"insert":{"image":"https://community-1252773949.cos.ap-guangzhou.myqcloud.com/article/2034/1ec6b61323f77c8c.png"}},{"attributes":{"align":"center"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":"由表 1 可以看到，"},{"attributes":{"color":"#222222","bold":true},"insert":"由于 TPS74401 的噪声频谱密度最小，在给频综供电的时候可以取得最好的相噪性能；TPS7A8101 噪声频谱密度相对较大，在给频综供电的时候取得的相噪性能相对较差；"},{"attributes":{"color":"#222222"},"insert":"但是通过优化 TPS7A8101 的输出电路设计，频综的相位噪声得到了明显的改善。"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":" "},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"#222222"},"insert":"实测结果很好的验证了前文的理论分析，"},{"attributes":{"color":"#222222","bold":true},"insert":"即：LDO 的噪声频谱密度参数（NSD）决定了由电源噪声引起的 VCO 相噪恶化；"},{"attributes":{"color":"#222222"},"insert":"通过提高 LDO 的环路稳定性可以达到降低噪声频谱密度的目的，从而改善频综的输出相噪。"},{"attributes":{"align":"justify"},"insert":"\n"},{"insert":"——转自君鉴科技"},{"attributes":{"align":"right"},"insert":"\n"},{"insert":"\n"}]