Column: EMC
between measurements performed in the near and far fields. While it is generally true that the
stronger the field near the source, the stronger it will register in the far field, this correlation is not precise enough to provide reliable predictions.
Step 2: Measuring cable radiation After a near-field assessment, the next step is to use an RF current probe to measure a sampling of cables. In this case, it is best to make several measurements along the cable, as standing waves can cause readings to differ between parts. Harmonics between 30MHz and 500MHz should be noted.
Step 3: Far-field measurement In the final step, the radiated emissions from the product are measured using antennas, either full- or reduced-sized. While reduced-size antennas can be advantageous for far-field measurements
Previous near-
fi eld measurement results can be useful in determining
whether the far-fi eld radiation comes
from the product or from ambient noise
above 200MHz (since they are easily moved around and fit into spaces not suitable for full-sized antennas), because of their inherent lower sensitivity and higher antenna factor they may not be suitable for measuring radiation fields between 30MHz and 200MHz. This will typically result in a higher system noise
floor that can exceed the test limits being used for comparison. Therefore, it is always best to use a full-size antenna for measuring radiated emissions between 30MHz and 200MHz. Previously-recorded near-field
measurement results can be useful in determining whether the far-field radiation comes from the product or from ambient noise. Software that shows multiple results is helpful in comparing and analysing both the near- and far-field measurements; see Figure 2. In the figure, the red trace shows the
near-field measurement results, and the green the far-field. As can be seen, the ambient noise can be distinguished so that we can focus on the noise generated by the product (the blue pointers in the figure). Quasi-peak scans can then be performed on selected points to determine whether the noise exceeds the limit.
Figure 2: Results from near-fi eld measurements
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