How to correctly use the channel coupling method of the oscilloscope to select AC?
Ripple: In an ideal state, the DC voltage output by the power supply should be a fixed value, but in many cases it is obtained by rectifying and filtering the AC voltage. Because the filtering is not clean, there will be more or less residual AC components. This clutter signal containing periodic and random components is called ripple.
Even battery power will cause ripples due to load fluctuations. Larger ripples will interfere with high-speed signal quality and affect the normal operation of the CPU and GPU, so the smaller the value, the better. Therefore, in order to ensure the voltage output quality of the power supply, it is necessary to measure the output ripple of the AC/DC or DC/DC module that supplies power to the circuit board. The measurement method of ripple will have a great impact on the determination of this indicator. Today Agitek will simply demonstrate to you some precautions for measuring power supply ripple with an oscilloscope.
When testing power supply ripple through an oscilloscope, only by adopting the correct measurement method can accurate measurement values be obtained. How to correctly use an oscilloscope to test power supply ripple? The following points are what you need to pay attention to when using an oscilloscope to test ripple:
1. The oscilloscope must select a bandwidth limit of 20MHz. Generally, the output ripple of switching power supply is in the range of DC~20MHz. The noise caused by high-frequency synchronous switching noise and signal reflection is in the DC~1GHz range. Therefore, this setting can filter out high-frequency noise and avoid the impact of high-frequency noise on ripple measurement.
Ripple and noise. A: ripple + noise; B: ripple; C: noise.
2. Keep the oscilloscope probe ground wire as short as possible. It is usually recommended to remove the probe cap and use the grounding spring that comes with the probe to ground it. This can avoid the antenna-like loop formed by the probe and the ground wire from coupling into the circuit noise.
3. Try to choose an oscilloscope probe with 1X. Ripple errors caused by the noise of the oscilloscope itself can be avoided. Because after the signal is attenuated by the probe end, in order to still read the actual signal voltage value on the oscilloscope, the oscilloscope will compare the signal through the set probe. If a 10X attenuation probe is used, the actual signal entering the oscilloscope is attenuated by 1/10. In order to display the true voltage value on the oscilloscope, the probe ratio on the oscilloscope needs to be set to 10X. The oscilloscope will multiply the resulting signal by 10 before displaying it. The noise of the probe itself will not be attenuated by the attenuation of the probe, so the noise obtained after multiplying by 10 will become larger. It will have an impact when the test ripple is small. In addition, the bandwidth of many probes at 1X is less than 10MHz, which will attenuate ripples higher than 10MHz and cause the actual test ripple to be too small. So it is best to choose a probe with 1X that is no less than 20MHz for testing. For example, the RIGOL PVP2000 probe has a bandwidth of 35MHz at 1X, which can meet the ripple test bandwidth requirements.
4. Select AC as the channel coupling method of the oscilloscope, which can isolate the DC voltage and facilitate signal observation. Because the ripple is superimposed on the DC signal, its value is smaller compared to the DC voltage. Therefore, you need to reduce the vertical scale and adjust the vertical offset to see the ripple signal. In addition, since the adjustable vertical offset range of the oscilloscope is limited, when the DC signal is too large, the ripple may not be visible. Therefore, choosing AC coupling can only display the AC ripple signal, making it easier to observe the waveform.
