![]() This is due to the fact that at those frequencies the capacitive coupling between the toroids becomes comparable to the magnetic coupling, so the two sum or subtract depending on the sign of the magnetic coupling. Note that at low frequency the sign of the (magnetic) coupling coefficient makes little difference but at high frequency there may be a notch or a flat region in the coupling, depending on the orientation. The coupling between the toroids when driven/terminated by a 50 ohm resistance is shown in the following graph: The same set of measurement was also repeated when swapping the connections of one of the toroids, to change the sign of the coupling coefficient see below why this gives some slightly different results. The distance, as noted in the graphs below, is between the windings, so 0 mm means that the toroids windings were in close contact. The toroids were placed parallel to each other, with the centers on the same line, and the measurements were repeated for several distances between the toroids along this line. With 19 turns each, that I happened to have around to build some CWAZ filters (see also here). Have done some measurement on the coupling between two T37-6 toroids, I didn't notice that the manufacturer already provides some data on the coupling vs distance for the SMD inductors. ![]()
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March 2023
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