How to Select and Use Ferrite Cores
How to Select a Ferrite Core 2
Turns on a core
‘Turning’ in regards to ferrites refers to how many times a cable has passed through a core. A cable that is not wound around the ferrite is referred to as a single turn, because it has only passed through the core once.
Impedance is increased by square of turns.
The effectiveness of turning a ferrite is equivalent to having N^2 ferrites on the cable, where x is equal to the number of turns. For example, if a ferrite has one turn, the effectiveness of the ferrite is equal to 1^2=1. However, if a cable has 3 turns, the efficiency of the core is equal to having 3^2=9, or 9 ferrites on the cable.
As the number of turns within a core increase, so too does the impedance performance. This is efficient in regards to increasing impedance without attaching more ferrites.
If the problem frequency is under 200MHz, the number of turns of the ferrite should be altered or a lower frequency ferrite core should be considered. If the target frequency is over 200MHz, adding more ferrite cores or using a higher target frequency ferrite core is recommended.