PLL的杂散和相噪怎么理解

tang66521

如图1所示，参考杂散为参考频率处的功率和载波处的功率差，单位为dBc；图2中，相位噪声曲线上的一些脉冲凸起为杂散，单位为dBc/Hz; 单从单位来看，这两个数值的差别为一个是功率差，一个表示单位单宽内的功率差值，那么这两者是怎样转换的呢，因为根据杂散的计算公式得到的单位为dBc；例如，计算得到800KHz 处的杂散要小于-73dBc，那么800KHz 表示的应该是频偏800KHz，即第二张图得到的杂散值吧，但是图2 的曲线不能通过仿真得到，根据计算的结果，怎么转换为图1中的值呢。另外，根据相噪的定义可以从图1中计算得到相位噪声吗？？
求解答，谢谢！

图2

杂散计算公式

tang66521

上面是图2  这楼是图1

bright_pan

图2如果你把span做到1HZ一个点，应该和相噪的图就是一个东西了，我的个人理解。不知道对不对，但我一直这么用的的。

tang66521

回复 3# bright_pan


   你说的是图1吧，那不是要去很多的点，你这样做和用pnoise仿的结果是一致的吗

hoking841112

兄弟，话不多说，杂散抑制是相对值

电子新手是也

同问，已经用Cadence仿真得到PLL的输出频谱的杂散，那根据该图，应该如何计算频偏1MHz处的相位噪声呢？求教了！

gongjiang

Spur tone is a deterministic tone. The unit of the spur is dBc, which is the ratio of the spur tone power to carrier (signal) power.  Phase noise is noise to signal ratio. The noise power is calculated in 1Hz bandwidth, consequently, you have the phase noise unit dBc/Hz. You can also calculate spur based on phase noise. This is related to the resolution bandwidth you are using. If the resolution bandwidth is 1Hz, then tone level in the phase noise plot is the spur.

电子新手是也

回复 7# gongjiang


   Thank you for your replying. Is the resolution bandwidth FFT used?If the resolution bandwidth is not 1Hz, then how to calculate the phase noise using the spur spectrum?

gongjiang

回复 8# 电子新手是也
   Yes. The phase noise plot comes from the psd- fft of the auto-correlation function of the oscillation waveform (traditional definition, or the fft of the oscillation phase(IEEE definition) ). When you do the fft in MATLAB, you have the resolution frequency. Your phase noise plot is spread from 0 to fout/2 with the finite number of points (nr_points). The resolution frequency (fres) is fout/2/_nr_points. If fres is larger than 1, which means the spur value on phase noise plot is underestimated since the spur energy is the same but spread by fres. Consequently,  you can calculate the spur from PN plot by adding 10*log10(fres). But this is typically not the real case (I observed when using R&S) when you calculated from a phase noise analyzer since a phase noise analyzer has different fres at different frequency offset.
Regarding calculating phase noise from the spectrum, you can also do something similar. The spectrum analyzer is used to calculate the spectrum of the oscillation waveform instead of psd. You can calculate the noise to signal ratio from a spectrum at a certain offset frequency on the spectrum analyzer, then subtracting 10*log10(fres).  Basically, you calculated NSR at 1Hz bandwidth...

电子新手是也

回复 9# gongjiang


   Thank you very much~