Measuring SRF of a ferrite bead

[fuuton1]

Hi all,

I am in the process of selecting a ferrite bead for a low power application, signal filtering.

I have come across a few articles which I will link below detailing the selection procedure. However, I cant figure out where the capacitive region of the ferrite bead starts, using the impedance, reactance, resistance vs frequency curves. The capacitive region starts after the self resonant frequency of the ferrite but I cant seem to find a way to calculate the self resonant frequency using the info available in the ferrite datasheets.

How can one determine the frequency range over which the ferrite behaves as a capacitor using the info provided in the datasheets?

Any help in this regard will be highly appreciated.

Links of literature:

Ferrite Beads Demystified | Analog Devices

In this article the author details several critical design techniques involved with implementing ferrite beads that must be considered to avoid resonance that can induce noise and ripple in the supply.

www.analog.com

EMC Basics: Chip Bead Ferrites & Reading Datasheets | Würth Elektronik: Electronic & Electromechanical Components > News > Blog: World of Electronics

In this second installment of our five-part blog series on EMC basics, we'll discuss chip bead ferrites and how to read a datasheet.

www.we-online.com

Regards.
fuuton

 

[BigBoss]

Because you don't know the Equivalent Circuit of a Ferrite Bead and this values are generally not given in the datasheets.But you can measure by a VNA.When resonance occurs, you will see approx. \[ \infty \]
Ohm then you will know which part inductive and which is capacitive..

 

[dick_freebird]

The point of a ferrite bead is to be lossy at high
frequency so it may mask the capacitive region
(if any - what's the through-capacitance of 1/4"
of wire in a nonconductive hole?)

 

[Easy peasy]

If the Q of bead is low - which they normally are ( lossy right ) then there is essentially no SRF as the Q is not high enough to allow a resonant ring -they become rather more resistive with freq, in nature rather than inductive ( which they still are by virtue of length ) and less capacitive - unless there is a gnd plane right under the bead ....

 

[FvM]

I don't understand the exact problem. Most ferrite have datasheets with impedance curves. You can even fit an equivalent circuit if it's not given by the manufacturer or the provided model isn't good enough.

Even low Q ferrite beads have an impedance maximum, you might consider the respective frequency as SRF and the region right of it as capacitive.