Oscilloscope frequency domain measurement power supply noise measurement problem
In the process of analyzing power supply noise, the more classic method is to use an oscilloscope to observe the power supply noise waveform and measure its amplitude, so as to determine the source of the power supply noise. However, as the voltage of digital devices gradually decreases and the current gradually increases, power supply design becomes more difficult, and more effective testing methods need to be used to evaluate power supply noise. This article is a case of using the frequency domain method to analyze power supply noise. When the fault cannot be located by observing the time domain waveform, time-frequency conversion is performed through the FFT (Fast Fourier Transform) method, and the time domain power supply noise waveform is converted to the frequency domain for analysis. When debugging the circuit, viewing the signal characteristics from the time domain and frequency domain perspectives can effectively speed up the debugging process.
During the single board debugging process, it was found that the power supply noise of a network reached 80mv, which exceeded the device requirements. In order to ensure that the device can work stably, the power supply noise must be reduced.
Before debugging this fault, review the principles of power supply noise suppression. Different frequency bands in the power distribution network use different components to suppress noise. Decoupling components include power regulation modules (VRM), decoupling capacitors, PCB power ground plane pairs, device packages and chips. VRM includes a power chip and peripheral output capacitance, which operates approximately from DC to low frequency (around 100K). Its equivalent model is a two-component model consisting of a resistor and an inductor. It is best to use decoupling capacitors with capacitors of multiple orders of magnitude to fully cover the mid-frequency band (around 10K to 100M). Due to the existence of wiring inductance and package inductance, even if a large number of decoupling capacitors are stacked, it will be difficult to function at higher frequencies. The PCB power supply ground plane forms a plate capacitor, which also has a decoupling effect, approximately tens of megabytes. Chip packaging and chips are responsible for high-frequency bands (above 100M). Current high-end devices generally add decoupling capacitors to the package. At this time, the decoupling range on the PCB can be reduced to tens of megabytes or even several megabytes. Therefore, when the current load remains unchanged, we only need to determine which frequency band the voltage noise appears in, and then optimize the decoupling components corresponding to this frequency band. The two decoupling elements will cooperate in adjacent frequency bands, so the decoupling elements in adjacent frequency bands must also be taken into consideration when analyzing the critical points of the decoupling elements.
Based on traditional power supply debugging experience, some decoupling capacitors were first added to the network to increase the impedance margin of the power supply network to ensure that the impedance of the power supply network in the mid-frequency band could meet the needs of the application scenario. The result is only a few mV reduction in ripple, a minimal improvement. There are several possibilities for this result: 1. The noise is at low frequency and is not within the range of these decoupling capacitors; 2. Adding capacitance affects the loop characteristics of the power regulator VRM, and the impedance reduction caused by the capacitance is related to VRM. The deterioration is offset. With this question in mind, we considered using the frequency domain analysis function of the oscilloscope to view the spectral characteristics of the power supply noise and locate the source of the problem.
The frequency domain analysis function of the oscilloscope is realized through Fourier transform. The essence of Fourier transform is that any time domain sequence can be expressed as an infinite superposition of sine wave signals of different frequencies. We analyze the frequency, amplitude and phase information of these sine waves, which is an analysis method that switches the time domain signal to the frequency domain. The sequence sampled by a digital oscilloscope is a discrete sequence, so the Fast Fourier Transform (FFT) is most commonly used in our analysis. The FFT algorithm is optimized from the Discrete Fourier Transform (DFT) algorithm. The amount of calculations is reduced by several orders of magnitude, and the more points that need to be calculated, the greater the savings in calculations.
