Why does this diagram reject self-resonance behavior for RF chokes?

Question

Based on this tutorial, after self-resonance the impedance decreases and the inductor behavior becomes a capacitor. But for a real inductor (Murata RF choke):

1. What is the minimum self-resonance frequency? (In the Specifications table)
2. Why does the impedance increase after self-resonance frequency? (Characteristic Data)

New Question:

1. How can I these inductors for high frequency such as 10GHz while their SRF is less than 1GHz? (for example in this application the device works up to 1GHz but the choke SFR is less than 100MHz page 4)

Answer 1

what is the minimum self-resonance frequency?

The table says this: Self resonance frequency (min.) 26MHz

Notice that this is a minimum figure. The graph shows this: -

And the graph is a typical graph (although it doesn't state so). It's pretty normal for graphs in product data to represent typical values.

why does the impedance increases after self-resonance frequency?

Because the table indicates the minimum SRF.

If you dig out the data sheet you'll see that the 10 uH and 15 uH (yours) have the same stated SRF: -

Hence Murata are likely being a tad lazy with their specifications.

How use these inductors for high frequency such as 10ghz while their SRF is less than 1Ghz?

You can't unless you want them to behave like capacitors.

for example in this application the device works up to 1GHz but the choke SFR is less than 100MHz page 4

I think Mini-circuits might be mistaken here but, contact them to find out how they can justify this. Alternatively, the losses may increase so much that the device becomes very resistive at higher frequencies maybe like a ferrite bead: -

Image from here.

Although the SRF is about 60 MHz, the impedance continues rising (due to losses in the ferrite material) up to a usable point beyond 1 GHz. It's not the way I like to design things but you can't always rely on someone else's design matching what you might do. In other words, if an inductor isn't characterized at some arbitrarily high frequency then don't use it. Don't even use it if you have tested a dozen and find them all to be good at the high frequency because how would you ever find a replacement if you needed one?

It's like finding some old paint in your garage and painting your kitchen. After the third wall you run out of paint and, of course, the colour matching process to finish the last wall will be a pig of a job. You'll probably end-up starting all over again and curse yourself and your dog and anyone else who happens to be in the vicinity but, who really was to blame?

Answer 2

The minimum self-resonance frequency is 26 MHz, as it says on the datasheet. Self-resonance frequency can't be accurately controlled during manufacture (or at least it isn't worth the effort to), so manufacturers simply guarantee that the self-resonance frequency is higher than a specified minimum.

This should answer your second question too; that graph doesn't actually get to the self-resonance frequency because the self-resonance frequency of the device those graphs were made from is higher than the minimum, high enough to be off the end of the graph. You can tell by the way it curves at the end that it's not too much beyond the end of the graph, though.

Answer 3

SRF vs SRF& PRF

All inductors have interwinding capacitance and therefore have a parallel self resonant frequency SRF . They ought be called PRF but aren’t (!) only by some in the RF world because caps can have series L and then lumped stray Parallel LC so there is a SRF before the PRF then they behave as inductors above this f.

This is similar but opposite to all capacitors which due to 0.5 to 1nH per mm of wire or electrode length have a series resonant frequency. SRF.

Air coils are “cool” but still has C and are used in TV tuners for in bandshaping filters or VCO’s with precision and cavity shields.

The air core inductors used for RF are high Q and high SRF. ferrite core inductors are smaller due to high mu and higher mu electrical steel reactors have even lower SRF.

Core saturation

”All BUT air coils saturate” and only used well below this as the Z_L(f)=DCR+2Pi L and when the core saturates the BH curve flattens and L which is related to the slope drops towards zero, harmonic currents rise sharply, Pd rises with I^2 as well and then thermal runaway occurs (poof). Yet they are mandatory for Woofer..Tweeter crossover band splitting filters to avoid the harsh effects of ACDC, U2 and other good zombie loud music shorting the Crossovers and “woofing your tweeters”.

Power chokes or Reactors

Power transformers are oil cooled can withstand the heat if interrupted current at line f , when the resume phase is out of phase from off state, Remanence charge still exists and they start up in saturation causing megawatt units to hum loudly on start until flux balanced.

Back to Mainstream inductors

I suspect you were only asking why are some Inductors are others called chokes and other acronyms like short and curlies (j/k)*

Thus is because choke implies choking higher Frequencies than the DC and AC harmonics that it can handle in the lower f current spectrum. The accuracy is not as important, just choke whatever it can into a shunt cap to make a tagteam series L and added shunt C choke ‘n block. no I’m not talking about Asian Martial Arts or the Kickboxing..

Some ferrite chokes are called ferrite beads that surround the wire and add inductance with lossy conductive particles in the slurry so they also absorb energy at the high impedance currents thus higher mV levels to behave somewhat like a 100pF Rf shunt cap except reduce the currents rather than increase them, yet both reduce the Rf ripple voltage but beads reduce current and that helps reduce mutual coupling current to nearby high impedance circuits. Whew. Take a breath. let it sink and say ummmm. Was that bad english? Yes i’m an ungineer.

Hey if you are still with me bravo. I’m going to repeat what might already has been answered.

Chokes are to pass DC and/or signal and block higher noise spectrum.

Inductors are never ideal and the sL/DCR=Q the complex impedance quality ratio with respect to the DC resistance. This is just as important to SMPS as low ESR (effective series resistance) caps where Q =sC/ESR)

If you search my answers I wont repeat the handy “RLC nomograph” that allows you see Impedance of RLC vs f on not 3 but 4 yes 4 axes and plot the “Bode” frequency response . Very handy for SRF, PRF, RC vs f Vs RL and Z(f) or your short and curlies with added resistance.

opinions are my own experience. Since back in the hippie days.