How to use an oscilloscope_Oscilloscope selection_How to measure with an oscilloscope

I often see that many small companies use oscilloscopes that are too low-end, with low bandwidth and low sampling rate. They think that it is enough to capture the waveform, and they think that it is not necessary to buy such a good oscilloscope. They also think that the oscilloscope is simple to operate and does not have so many specifications. When they operate the oscilloscope, they do not prepare before the test and just use it. In fact, it is incorrect to do so. It may often be this incorrect operation that causes distortion of the test results and affects the analysis. Even some very experienced engineers may not notice some details. Many engineers lack understanding of oscilloscopes, and how to use oscilloscopes better still needs to be improved. The following is to correct the common problems that many engineers I have seen and share some of the knowledge I have mastered.

Many engineers just pick up the probe and test it without checking whether the probe needs compensation or the oscilloscope needs calibration. Only some large companies or trained engineers will do the preparation work before use.

The oscilloscope needs to be self-calibrated and the probe compensation adjusted before use. This adjustment is performed to match the probe to the input channel.

When operating the instrument for the first time and when displaying data from multiple input channels simultaneously, it may be necessary to calibrate the data vertically and horizontally to synchronize the time base, amplitude, and position. This may be necessary, for example, when significant temperature changes (> 5°) occur.

Disconnect any probes or cables from the channel input connectors. Make sure the instrument is running and has warmed up for a while. R&S Oscilloscope From the File menu, select Selfalignment.

On the Control tab, click Start Alignment.

The R&S oscilloscope takes several minutes to complete this process. Some oscilloscopes may take more than an hour. The overall pass/fail result is displayed in the Overall alignment state field. The results of the individual calibration steps for each input channel are displayed in the Results tab.

The steps for adjusting the probe compensation are as follows:

1. Connect the oscilloscope probe to the channel and press the PRESET button on the front panel (in the settings area on the left panel).

Connect the probe signal end and reference ground to the reference output on the oscilloscope panel, and then press Autoset. If you use the probe hook-type front end accessory, firmly connect the signal pin front end to the probe to ensure a correct connection. As shown in Figure 1:

Picture 1-1

Figure 1-2

Figure 1 Probe compensation adjustment

2. Check the shape of the displayed waveform. The possible situation is shown in Figure 2.

Figure 2: Over-compensation, under-compensation and correct compensation waveforms in the second half

Both excess and deficiency require adjustment of the probe to better test the accurate value.

3. If the waveform is incorrect, please adjust the probe, as shown in Figure 3 below, until the waveform is the correct compensation waveform above.

The above two points seem simple, but they are often overlooked by engineers. In order to make the measurement more accurate, please pay attention to the inspection. These two calibration functions should be available in any oscilloscope.

Test voltage ripple

Many power supply engineers do not pay much attention to the ripple measurement and just test it as they please. The different ways of using the oscilloscope lead to very different test results. As shown in Figure 4 and Figure 5 below, for the same product and the same test point, the test results are very different due to the different test methods. Ripple is an important parameter for power supply, but it is not worth it to fail the test due to your own operation problems and waste a lot of manpower and cost to rectify it.

Sometimes your customers may not pay enough attention to the use of the instrument, resulting in wrong test data. However, there is no problem with your product, and no matter how you explain it, it doesn't make sense, so that the customer thinks you are deceiving them. Therefore, the test method is very important. Paying attention to these details can save a lot of time and improve your ability.

The values ​​tested by the oscilloscope have errors (I will not explain this here for the time being). Now many companies require the values ​​of the test waveform as the basis for judgment. In fact, the oscilloscope only tests the process of voltage change over time, mainly to capture waveforms 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 of the oscilloscope is also calibrated with a multimeter as a reference. However, more and more companies and engineers use the values ​​of the oscilloscope as the real values, so we can only try our best to minimize the test error.

Below is a diagram and analysis of the test ripple:

Pic 4-1

Figure 4-2 Group 4

Figure 5-1

Figure 5-2 Group 5

The test ripple result value of Group 4, 3.9921V, is much larger than 0.126V in Figure 5. However, the test value of Group 4 is not real.

Problem analysis: Actually, there is no problem with the product. It's just that the testing method is problematic. Now let's point out the problem:

The first mistake is using a long ground wire.

The second mistake is to place the probe loop and ground lead close to the power transformer and switching components.

The third mistake is excess inductance between the oscilloscope 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 displayed with high-frequency noise mixed in it.

The fourth mistake is that the range is too large.

To accurately test ripple, you need to:

Use bandwidth limiting to measure ripple to prevent picking up high-frequency noise that is not really there. Set the oscilloscope bandwidth to 20M.

Remove the probe "cap" and ground clip to prevent the antenna effect formed by the long ground wire. Use a short ground wire to wrap between the probe and the ground. Rohde & Schwarz provides a special matching short ground wire. You can consider connecting a 0.1uf and a 10uf capacitor in parallel between the signal and the ground for decoupling. The length of the capacitor pin also affects the test value.

Many engineers do not understand oscilloscopes, which leads to misoperation and damage to oscilloscopes or power supplies. They still do not understand why many junior engineers use multiple probes to measure power supplies. As soon as they turn on the power supply, the power supply product "explodes" or even damages the oscilloscope. They will ask me, doesn't the oscilloscope directly connect the probes between the components to be tested? I don't think I connected them wrong, why is this happening? That is because they do not understand the connection method of the channels and ground of the oscilloscope. The multiple probes of the oscilloscope are grounded inside the oscilloscope. So when measuring the primary and secondary sides of the power supply at the same time, if one probe is connected to the ground of the primary side and the other probe is connected to the ground of the secondary side, since the ground of the internal channels of the oscilloscope is connected together, it is equivalent to short-circuiting the primary and secondary grounds of the power supply. However, there is a voltage difference between the primary and secondary grounds, so the large current after the short circuit is easy to burn the product and the probe, and may even damage the oscilloscope. When testing the voltage of the primary and secondary sides, a differential probe should be used on one side and a normal probe on the other side. Even if the same side of the line is tested, the ground wire of the probe should be a common reference point. The ground of the oscilloscope is connected through the power ground. Many companies basically add an isolation transformer in front of the oscilloscope, which is a good method. Some companies directly cut off the PIN pin of the three-phase ground of the power supply. In this way, there is no grounding. If you touch the oscilloscope case with your hand, the leakage current will increase. It is recommended not to use it this way.

In fact, there are more problems, such as dynamic applications, applications of calculations between probes, and matters needing attention in testing voltage values. We all know that oscilloscopes are very powerful, and there are almost no electronic engineers who do not use oscilloscopes. Therefore, when using oscilloscopes, we must think more, do more experiments, and learn more about the functions of oscilloscopes, the differences between internal option keys, and the impact of different oscilloscope parameters on measurements, which can help us better. Don't just do it casually to complete the task. Do it seriously and observe carefully, so that we can make great progress. Experience is accumulated step by step.

Choose a single-trace, dual-trace, or multi-trace oscilloscope, depending on the number of signals to be displayed;

Select slow scan, general purpose, high speed or sampling oscilloscope according to the frequency characteristics of the signal being measured;

Depending on how the measured signal is reproduced, choose an analog or digital storage oscilloscope;

Select the test focus according to the signal under test

Voltage measurement

DC voltage measurement

Measuring principle

The principle of measuring DC voltage by an oscilloscope is to use the measured voltage to present a straight line on the screen. The height of the straight line deviating from the time baseline (zero level line) is proportional to the magnitude of the measured voltage. The measured DC voltage value VDC is