The effect of crosstalk between oscilloscope channels

At present, the mainstream oscilloscope channels of almost all common brands are not isolated. Therefore, when multi-channel testing is performed, the channel and the channel will interfere with each other to some extent. Therefore, the channel isolation index is very important, and the oscilloscope measurement with higher isolation is performed. The more precise.

The oscilloscope as an engineer's "eye" can help to find many problems. As a tool for finding problems, the accuracy is crucial. When the test environment has no interference to the oscilloscope, the noise isolation will affect the test results. It will have a typical impact on the test results.

First, what is the channel isolation?

As the name suggests, channel isolation is the degree to which signals interact with each other. If we are testing with two channels at the same time, will the signals between channel 1 and channel 2 interfere with each other? How much is the degree of interference? By quantifying these issues, you can understand the channel isolation.

Second, how to test the channel isolation?

According to the general specifications of the digital storage oscilloscope, first set the vertical sensitivity of the oscilloscope interference channel to a larger file, set the vertical sensitivity of the interfered channel to the most susceptible level, and shield the input.

We adjust the vertical gear of channel 1 (interference channel) to 500mV/div, the vertical gear of channel 2 (interfered channel) to 2mV/div, and the input of channel 2 is suspended.

Then set the signal source to output the sinusoidal signal, and feed it into the interference channel (channel 1), adjust its output so that the display amplitude reaches 80% of the height of the waveform display effective area, and the center is stable.

Finally, keep the output voltage of the signal source unchanged, and read out the voltage amplitude values ​​displayed by the interference channel (channel 1) and the interfered channel (channel 2).

The figure below shows the setup interface.

Figure 1 setting interface

Open the parameter measurement statistics and view the amplitude options.

Figure 2 Voltage amplitude of interference channel and interfered channel

Third, how to calculate the channel isolation?

According to the definition of national standard, the channel isolation is: Where K represents channel sensitivity, A represents the amplitude of the channel display, subscript 1 represents the interference channel (ie, channel one in this article), and subscript 2 represents the interfered channel (ie, the channel in this article) two), The calculated channel isolation is 133.58dB.

Fourth, the role of channel isolation

The greater the value of the channel isolation, the smaller the crosstalk between the channels, and the more accurate the test results! It is not difficult to see from the parameter display in Figure 2. In the channel one, the sine wave signal with the amplitude of 3V is connected, and the channel 2 has the amplitude of only 157uV in the gear position of 2 mV/div. The crosstalk between the channels is very small. The accuracy of the test results is guaranteed.

Finally, I secretly told you that the test instrument selected in this article is the ZDS2024Plus, and the isolation between the measured channels is as high as 133dBå“Ÿ.

(The manuscript is provided by ZLG Zhiyuan Electronics )

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