what is the utmost important for ferrite EMI filter?

[yakex]

hi,
there are too many ferrite for emi filter from Murata, but what is the utmost important for the ferrite? the equivalent series resistance? the reactance? or the total impedance?
for example, if the effective radio frequency, then what I should is series mount a ferrite in the supply circuit to BLOCK the inference induced by the supply circuit?
thanks.
yakex

 

[FvM]

For most applications, a standard series "A" type would be suitable, for power supply a "P" type. For circuits dealing with UHF frequencies and it's harmonics, "GHz Noise Suppression" H series and "High GHz" G series may be interesting.

All ferrite beads have an almost resistive impedanz above a certain frequency, in this region resistance and total impedance are effectively equivalent.

 

[yakex]

FvM: Thanks for your reply.

what you said is the general rules for product selection, and it is helpful to me.

But what I want to learn is whether I should select a ferrite of large resistance at the frequency range of interest, in order to block the external interence?

Thank you again.