An important detail in using an oscilloscope

Use a signal generator to input a signal to the board. The oscilloscope can generally only be used to test the waveform between a certain node and the ground on the circuit. If the waveform between two nodes is measured, the ground wire on the probe may short-circuit the rest of the circuit behind the ground wire. Therefore, if you want to measure the waveform between two nodes, you need to reasonably change the circuit form or make some additional circuits for testing. Of course, you can measure different points twice and then compare them, or use the Li Yusha method to measure the phase difference between the two signals .

This is because in order to ensure electrical safety, most electronic instruments are connected to the safety ground through the power cord. The ground wires of oscilloscopes, signal generators, voltage regulators, etc. are also connected to the safety ground, so the two grounds are connected. If the oscilloscope ground is connected to another position in the circuit instead of where the signal source ground is connected, part of the circuit will be short-circuited.

The dual-trace oscilloscope has two probes and can measure two signals at the same time, but the ground wires of the two probes are connected to the safety ground of the oscilloscope, so the ground wires of the two probes cannot be connected to two different points of a circuit at the same time, otherwise the two points will be electrically short-circuited through the oscilloscope. For this reason, in the experiment, the ground wire of one of the probes can be removed and only one of the ground wires can be used. When it is necessary to observe two signals at the same time, the common point of the two measured signals must be found on the circuit, and the ground wire of the probe is connected. The two probes are connected to the signal respectively, so that the two signals can be observed on the oscilloscope at the same time without any accidents.

Of course, this means that there are other devices in your system, and the ground wires of these devices are connected to the safe ground like the oscilloscope. For a homemade system, the power supply is also homemade instead of a dedicated power supply instrument, there are two situations:

1: The transformer used is an isolation transformer, which is the kind with two windings. In that case, if your system or board is not connected to a safe ground, the above situation will not occur, and the signal between two points can be measured, but when two probes are used together, only one ground wire clip can be connected. (This inference still needs to be tested experimentally)

2: If the transformer uses self-coupling, then it cannot be connected randomly. Pay attention when measuring, especially when measuring the power supply, otherwise there may be a risk of short circuit between the power live wire - oscilloscope ground wire - safety ground - neutral wire.

You can solve the second problem in two ways:

1: The safety ground of the oscilloscope is not connected, that is, the three-pin plug only uses the lead and live wires, so the oscilloscope may be charged;

2: Use an isolation transformer as the system power supply, or use a 1:1 isolation transformer at the power supply of the oscilloscope to provide power to the oscilloscope.

You can think about the specific current path yourself. If you have the conditions, such as making a circuit with a power supply, you can test it yourself with a few resistors. This issue is still worthy of attention because there are many safety factors to consider.

The final article with measurement results will be further improved after I conduct experiments.

Everyone is welcome to discuss this issue, put forward your own views, and point out the shortcomings and errors in this article.