First of all, with the improvement of test standards, the frequency range required for EMI experiments becomes wider, so the frequency stepping needs to be increased to cover a wider frequency range.
Second, equipment performance also has an impact on frequency stepping. For example, the performance of the filter may limit the size of the frequency step. If the device can't keep up with rapidly changing frequencies, this can lead to larger steps.
In addition, the stability of the signal source is also a factor in the frequency step of the electromagnetic immunity experiment.