How to use an oscilloscope to measure power supply ripple noise and precautions

In college, many electronics enthusiasts like to make some small electronic products. As for the power supply method on the circuit board, 7805 and 7812 are the well-deserved kings of cost performance, fast, good and economical! When the small products we made fail, almost no one will take the impact of the power supply into consideration, because most of the things made in college have simple circuit topology and low signal frequency, so even if there is a fluctuation on the power supply end, it will not have much impact on the subsequent circuits.

Today's electronic circuits (such as electronic measuring instruments and multimedia products) have higher level switching speeds and higher signal complexity than before, while chip packaging and signal amplitudes are getting smaller and smaller, making them more sensitive to power supply fluctuations. Therefore, circuit designers are more concerned about the impact of the power supply than before.

Take our ZDS2024 oscilloscope itself as an example. The internal main power supply is a switching power supply. The power distribution network on the motherboard needs to convert this DC power supply into DC power supplies of various voltages (such as: +-5V, +3.3V, +12V, etc.) to power the CPU and various chips. At the same time, our fans are also changing dynamically at any time. The CPU, IC and fan consume a lot of power, and the power consumption is dynamic. The instantaneous current may be large or small, but the voltage must be stable (that is, the ripple and noise must be small) to keep the CPU and IC working properly. This puts high demands on the stability of the power supply.

All digital oscilloscopes use attenuators and amplifiers to adjust the vertical range. After the attenuation is set, the noise of the oscilloscope itself will be amplified. Therefore, when measuring noise, the most sensitive range of the oscilloscope should be used as much as possible. However, the oscilloscope usually does not have enough offset range at the most sensitive range to pull the measured DC voltage to the center range of the oscilloscope screen for testing, so it is usually necessary to use the oscilloscope's AC coupling function to filter out the DC component and only measure the AC component.

For the same reason, you should also use a 1:1 probe instead of the 10:1 probe that comes standard with the oscilloscope when measuring power supplies. Otherwise, the oscilloscope's noise will also be amplified.

The noise brought by the probe is coupled in front of the attenuator, so no matter how much the attenuation ratio is set, the noise contributed by the probe is certain. However, under some incorrect usage methods, the probe may bring additional noise. A typical example is the use of a long ground wire. In order to facilitate testing, the passive probe of the oscilloscope usually uses a long ground wire in the form of an alligator clip of about 10cm, but this is not applicable to the test of power supply ripple, especially when there is a switching power supply on the board. Since the switching of the switching power supply will generate a large amount of electromagnetic radiation in space, and the long ground wire of the oscilloscope probe is just equivalent to an antenna, it will introduce large electromagnetic interference from space into the measurement circuit. A simple verification method is to connect the ground wire and the front end of the probe together, close to the circuit under test (not directly in contact), you may see relatively large switching noise on the oscilloscope. Therefore, the shortest possible ground wire should be used during the measurement process.

Nowadays, many DUTs require the measurement of ripple with a peak-to-peak value of several millivolts. At this time, it is best to use coaxial cable for measurement. Although the impedance of coaxial cable is only 50 ohms, for the power supply under test at the millivolt level, the load effect is very small and the test accuracy is very high.

Figure coaxial cable

The last point to note is that power supply tests usually specify ripple and noise within a certain frequency range, such as within 20 MHz, and the bandwidth of a general oscilloscope is greater than this requirement. Therefore, the bandwidth limit function of the oscilloscope can be turned on during testing, which will also have a better effect on reducing high-frequency noise.

To summarize, for the test of power supply ripple noise, we usually need to pay attention to the following points:

Try to use homemade power test probes

Try to use the most sensitive range of the oscilloscope;

Use AC coupling function as much as possible;

Try to use a probe with a small attenuation ratio;

The ground wire of the probe should be as short as possible;

Use bandwidth limiting feature as needed.