What is an oscilloscope’s trigger mode? How to use an oscilloscope well and effectively?

Figure 3 Compensation probe method The above two points seem simple, but they are often ignored by engineers. In order to make the measurement more accurate, please be sure to pay attention to inspection. These two calibration functions should be available on any oscilloscope.  

2 Test voltage ripple

Many power engineers don't pay that much attention when measuring ripple and take the test for granted. Different methods of using the oscilloscope lead to very different test results. As shown in Figures 4 and 5 below, for the same product and the same test point, due to differences in test methods, the test results vary greatly. Ripple is an important parameter for power supplies, but it is not worth it to fail the test due to operational problems and waste a lot of manpower and cost on rectification.

Sometimes your customers' test data is wrong due to insufficient use and attention of the instrument. However, there is no problem with the product here, but in the end it doesn’t make sense no matter how hard it is, so that customers think they are being deceived, so the testing method is very important. Paying attention to these details can save a lot of time and improve your abilities. The value tested by the oscilloscope itself has errors (I won’t explain it here for now). Many companies now require the value of the test waveform diagram as the basis for judgment. In fact, the oscilloscope only tests the process of voltage changes over time, mainly to capture the waveform during debugging. The specific accuracy of measuring the effective value of DC voltage is not as good as the value of a digital multimeter. The DC accuracy index calibration of the oscilloscope is also based on the multimeter as a reference. However, more and more companies and engineers regard the value of the oscilloscope as the real value, so we can only do our best to minimize the test error. The following is a diagram and analysis of the test ripple:  

Group picture four  

Group Picture 5: The test ripple result value of Group Picture 4, 3.9921V, is much larger than the 0.126V in Picture 5, but the test value of Group Picture 4 is unreal. Problem analysis: In fact, there is no problem with the product. There is just a problem with the testing method. Now let's point out the problem areas: The first mistake is using a long ground wire. The second mistake is to place both the probe loop and the ground wire near the power transformer and switching elements. The third error is excess inductance between the scope probe and the output capacitor. Due to these carelessness, a lot of high-frequency signals, the magnetic field of the transformer, and the electric field of the switch are picked up, so that the waveform captured by the oscilloscope is mixed with high-frequency noise and displayed. The fourth mistake is that the range is too large.  

Testing ripple accurately requires: Using bandwidth limits to measure ripple to prevent picking up high-frequency noise that is not really there. The oscilloscope bandwidth can be set to 20M. Remove the probe "hat" and ground clamp to prevent the antenna effect caused by the long ground wire. Wrap a near-ground wire between the probe and ground. Rohde & Schwarz specializes in providing matching short ground wires. You can consider connecting a 0.1uf and a 10uf capacitor in parallel between the signal and ground for decoupling. The length of the PIN pin of the capacitor also affects the test value.