Does the interference waveform come from probe ground or channel crosstalk?

When many engineers use an oscilloscope to measure the switching power supply output signal, they will find mutual interference between the two measurement channels. Does this interference come from between channels? Let's test it out.

I. Background Overview <br> When using an oscilloscope to measure the switching power supply output signal, it is found that there is mutual interference (crosstalk) between the two measurement channels. So will the two measurement channels interfere with each other?

Test method: channel 1 measures small signal (100mv/div), channel 2 measures large signal (5V/div), and observes the mutual influence of signals.

Ø First use the standard signal source to verify the influencing factors of channel isolation;

Ø Then use different measurement methods to verify the influencing factors of crosstalk.

Second, the channel isolation test verification <br> Figure 1 oscilloscope two-channel input signal directly from the signal generator output, you can find that the channel 2 signal has no effect on channel 1, channel isolation is good. Of course, the channel isolation index has a more rigorous test method, here is a simple test.

Figure 1 CH1, CH2 measurement signal generator output, good grounding

Third, the real cause of interference - ground!
The above interference is mainly related to the use of the probe, especially related to the connection of the probe ground. The measurement process is as follows, and the crosstalk of the test mode is recorded:

1. Channel 1 probe grounded with alligator clip, channel 2 closed (proving channel 1 signal as it is, see Figure 2)

Figure 2 CH1 uses the clip to measure the power output, CH2 is left floating Note: Although the signal amplitude of channel 1 is very small, it can still locate the interference of 600K frequency by FFT analysis.

2. Channel 1 and 2 probes use only alligator clips to connect one ground (crosstalk is very serious, see Figure 3);

Figure 3 CH1 uses a clip to measure the power output, CH2 uses a clip to measure the inductance, single point ground

3. Channels 1 and 2 are grounded separately using the alligator clips (serious crosstalk, see Figure 4);

Figure 4 CH1 uses a clip to measure the power output, CH2 uses a clip to measure the inductance, each grounded

4. Channel 1 is grounded with spring, channel 2 is grounded with alligator clips (crosstalk is small, Figure 5);

Figure 5 CH1 uses a grounding spring to measure the power output, and CH2 uses a clip to measure the inductance, each grounded

5. Channel 1 and 2 probes are grounded separately using springs (minimum crosstalk, see Figure 6);

Figure 6 CH1 uses a grounding spring to measure the power output, and CH2 uses a grounding spring to measure the inductance, each grounded

Fourth, summary
l When the measuring probe and the ground wire are well connected, the inter-channel interference of the oscilloscope is small;

l Interference from the influence of grounding parasitic parameters at the test probe, such as lead inductance;

l When measuring two signals at the same time, in order to avoid mutual interference of the ground loop, separate grounding;

l Use a grounding spring when measuring sensitive signals and use interface terminals if necessary.