How to select AC using the correct channel coupling method for oscilloscopes?
Ripple: Ideally, the DC voltage output from a power supply should be a fixed value, but very often it is obtained by rectifying and filtering the AC voltage. As the filtering is not clean, there will be more or less residual AC components, this contains periodic and random components of the spurious signal we call ripple.
Even with battery power, ripple can occur due to load fluctuations. Larger ripples can interfere with the quality of high-speed signals and affect the normal operation of the CPU and GPU, so the smaller the value, the better. Therefore, in order to ensure the quality of the voltage output 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 board. And the ripple measurement method for its determination of this indicator will have a very big impact, today Antai test Agitek will briefly give a demonstration of the oscilloscope to measure the power supply ripple of some of the precautions.
When testing power supply ripple through an oscilloscope, only by adopting the correct measurement method can you get accurate measurement values. How to correctly use the oscilloscope to test power ripple? The following points for the use of oscilloscopes to test ripple need to pay attention to a few points:
1, the oscilloscope should be selected 20MHz bandwidth limit. General switching power supply output ripple in the range of DC ~ 20MHz. And high-frequency synchronous switching noise and signal reflection and other noise caused by the DC ~ 1GHz range. So this setting can filter out high-frequency noise and avoid the impact of high-frequency noise on ripple measurement.
2, the oscilloscope probe earth 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, so as to avoid the noise coupled into the circuit by the antenna-like loop formed by the probe and the ground.
3, try to choose an oscilloscope probe with 1X. Can be avoided by the oscilloscope itself noise caused by the ripple error. Because the probe end of the signal attenuation, in order to still read the actual signal voltage value on the oscilloscope, the oscilloscope will be set up by the probe ratio of the signal operation. If you use a 10X attenuation probe, the actual signal attenuation into the oscilloscope is 1/10, in order to display the real voltage value on the oscilloscope, the probe ratio on the oscilloscope needs to be set to 10X, and the oscilloscope will multiply the resulting signal by 10 for display. 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. This will have an effect on the small ripple of the test. In addition, the bandwidth of many probes at 1X is only less than 10MHz, which will cause the ripple higher than 10MHz to attenuate, resulting in the actual test ripple is small. Therefore, it is best to choose not less than 20MHz with 1X probe testing. Such as RIGOL PVP2000 probe, 1X bandwidth of 35MHz, can meet the ripple test bandwidth requirements.
4, the oscilloscope channel coupling mode selection of AC, DC voltage can be isolated, convenient signal observation. Because the ripple is superimposed on the DC signal, compared to the DC voltage, its value is small. So you need to turn down the vertical scale and adjust the vertical offset to see the ripple signal. In addition, since the adjustable range of the vertical offset range of the oscilloscope is limited, the ripple may not be seen when the DC signal is too large. Therefore, selecting AC coupling can display only AC ripple signal, which is convenient to observe the waveform.
5, if the use of isolated power supply to ensure that the oscilloscope power supply ground and the measured power supply ground isolation, to avoid the introduction of common mode interference.
