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19th June 2019, 12:18 #1
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Impedance frequency curves for ferrite beads
I was going through the application note AN1368 by Analog devices regarding Ferrite bead selection and design.
Attached picture is the impedance vs frequency graph of Ferrite bead part number: BMB2A1000LN2.
I understand the inductive region lies on the left side of the Z= XL point in the Impedance vs frequency graph.
But how is the capacitive region determined? I mean how to determine the frequency range of capacitive region shown in the attached graph
Thanks in advance

19th June 2019, 15:28 #2
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Re: Impedance frequency curves for ferrite beads
But how is the capacitive region determined? I mean how to determine the frequency range of capacitive region shown in the attached graph

19th June 2019, 20:38 #3
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Re: Impedance frequency curves for ferrite beads
The region where the capacitance starts to dominate is to the right, as the impedance starts to decrease.
Zapper
Curmudgeon Elektroniker

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20th June 2019, 05:58 #4
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Re: Impedance frequency curves for ferrite beads
To be more clear, my query is ..
Inductive region is on the left side of the X=R point.. Likewise capacitive region is on the right side of the frequency where absolute value of the capacitance equals R. From graph it is evident about Inductive region. But i am not clear how this particular area is obtained as the capacitive region.
Thanks

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20th June 2019, 10:10 #6
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Re: Impedance frequency curves for ferrite beads
I suggest this view
The impedance is at no frequency purely inductive or purely capacitive, there's always a resistive (lossy) component.
The diagram marks regions of mostly inductive and mostly capacitive impedance, how far they extend is a matter taste, I think.
   Updated   
In the diagram, "mostly" inductive/capacitive can be roughly translated to phi(Z) > 70°

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20th June 2019, 10:22 #7
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Re: Impedance frequency curves for ferrite beads
they are choosing where XL is greater than the resistance to be the inductive part, and similarly where Cparallel Xc is greater than the resistive part ( at that freq) to be the capacitive part.
Note they are assuming a parallel LC ckt where the impedance is maximum at resonance  as you go very high in freq you get just Rac, and very low in freq you get Rdc
The peak in XL is shown  and is interesting  the peak in Xc (curve) is, disappointingly, not shown  but could be calculated.

20th June 2019, 11:02 #8
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20th June 2019, 23:13 #9
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Re: Impedance frequency curves for ferrite beads
Hello FvM, how does R go down with ever increasing freq .... ?

20th June 2019, 23:43 #10
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Re: Impedance frequency curves for ferrite beads
But i am not clear how this particular area is obtained as the capacitive region
Part of the world that you live in, You are the part that you're giving ( Renaissance )

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21st June 2019, 00:14 #11
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Re: Impedance frequency curves for ferrite beads
the dissipative region is not when the core is saturated  saturation is generally to be avoided if you want best bead performance  a lot of engineers overlook this when they put DC thru a bead ...

21st June 2019, 08:59 #12
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Re: Impedance frequency curves for ferrite beads
Hello FvM, how does R go down with ever increasing freq .... ?
I derived a good matching equivalent circuit for the above shown 600 ohms ferrite bead
   Updated   
the dissipative region is not when the core is saturated  saturation is generally to be avoided if you want best bead performance  a lot of engineers overlook this when they put DC thru a bead ...
FairRite did a good job in documenting ferrite bead saturation behavior. You are often searching in vain in other vendor's data sheets.
Referring to the above equivalent circuit, saturation reduces the inductance but almost keeps the lossy components. The inductance may drop to 50% of the initial value at only 1020% of rated current. Consider that chip bead current rating is only thermal related.

21st June 2019, 12:25 #13
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Re: Impedance frequency curves for ferrite beads
Hello FvM, how does R go down with ever increasing freq .... ? ...
I believe (pardon me) that R is a part of X (LC component) and as the dissipation is frequency dependent, R too becomes frequency dependent.
Also note that X changes sign (LC transition point) is not exactly when the Z peaks. And that is also due to dissipation.
R decreases at still higher frequency because Z deceases; R graph cannot go above the Z graph.
And there are impedance models that cannot be explained in terms of LCR (see e.g., https://en.wikipedia.org/wiki/Warburg_element).
Clearly R is frequency dependent (and in a complex way) because both L and C are (but the graphs remind me of absorption and dispersion processes).
Only Z is measured directly and R and X are the two quadrature components.
I may be wrong but that is how I understand these experimental graphs.
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