Power supply noise analysis

Power supply noise analysis

When it comes to power supply noise , I believe it will resonate with many electronic engineers . What is the power supply noise we usually talk about ? Is it equivalent to power ripple ? in fact , Power supply noise is different from power supply ripple , It is a high-frequency component other than the ripple between the output terminals . Ripple is a kind of frequency between output terminals and input terminals 、 Components of switching frequency synchronization , It is the AC interference signal superimposed on the stable DC signal .

Power noise waveform

During the analysis of power supply noise , The classic method is to use an oscilloscope to observe the power supply noise waveform and measure its amplitude , Based on this, the source of power supply noise is judged . But as the voltage of digital devices gradually decreases 、 The current rises gradually , Power supply design becomes more difficult , When the fault cannot be located by observing the time domain waveform , Can pass FFT( fast fourier transform ) Method to perform time-frequency conversion , Transform the time domain power noise waveform to the frequency domain for analysis . During circuit commissioning , Check the signal characteristics from time domain and frequency domain respectively , It can effectively speed up the debugging process .

The frequency domain analysis function of oscilloscope is realized by Fourier transform , The essence of Fourier transform is that any time-domain sequence can be expressed as the infinite superposition of sine wave signals with different frequencies . We analyze the frequency of these sine waves 、 Amplitude and phase information , It is the analysis method of switching the time domain signal to the frequency domain . The sequence sampled by the digital oscilloscope is a discrete sequence , So we usually use the fast Fourier transform in our analysis (FFT). FFT The algorithm is based on the discrete Fourier transform (DFT) Algorithm optimization , The amount of computation is reduced by several orders of magnitude , And the more points you need to calculate , The greater the computation savings .

The noise waveform captured by the oscilloscope is analyzed FFT Transformation , There are several key points to note ：

1、 According to Nyquist sampling law , Spectrum broadening after transformation (Span) Corresponding to the sampling rate of the original signal 1/2, If the sampling rate of the original signal is 1GS/s, be FFT After that, the spectrum will be broadened at most 500MHz;

2、 Frequency resolution after transformation (RBW Resolution Bandwidth) Corresponds to the reciprocal of the sampling time , If the sampling time is 10mS, Then the corresponding frequency resolution is 100Hz;

3、 Spectrum leakage , That is, the spectral lines in the signal spectrum interfere with each other , Low energy spectral lines are easily submerged by the leakage of adjacent high-energy spectral lines . To avoid spectrum leakage, the acquisition rate can be synchronized with the signal frequency as much as possible , Extend the signal acquisition time and use appropriate window functions .

When measuring the power supply noise, it is required that the collected signal time can be long enough , It can be considered that the time span of the whole effective signal is covered .