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As a rule of thumb the SRF(self resonant frequency) of an inductor should be higher than the operating frequency. I need to know why this inductor has lower SRF and why the operating frequency is much higher than SRF.

As I know the inductor and a chock are the same for using them with the same inductance and SRF. How is it possible the WA8514-AE(SRF<250Mhz) inductor is used as a RF chock for this application which frequency can be up to 2.7Ghz.

John Jin
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The data in the link says it can be operated to 3 GHz yet it has a self-resonant frequency (SRF) of 250 MHz. So, the answer lies in what applications might benefit from this part at frequencies higher than the SRF. Here's its impedance graph vs frequency: -

enter image description here

And this pretty much tells me that it would make a good ferrite bead at frequencies between about 30 MHz and 2 GHz. The peak shown in the graph will be mainly dissipative resistance and that is precisely what ferrite beads do and what applications they are targeted at: -

enter image description here

Image from here. And here's another from this site: -

enter image description here

How is it possible the WA8514-AE(SRF<250Mhz) inductor is used as a RF chock for this application which frequency can be up to 2.7Ghz

Look at the impedance graph and ask yourself at what point does this drop close to anything like 50 ohm across the frequency range you need. Between 10 MHz and 3 GHz it has an impedance greater than 500 ohm. That makes it very usable as a bias setter for the PHA-202+.

Andy aka
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  • I am really appreciative of your response. But based on the diagrams, this inductor would not be an inductor after SRF (it becomes a capacitor). In the PhA-202+ in the "Recommended application circuit" table it uses this inductor. – John Jin Nov 25 '20 at 18:15
  • Absolutely correct and that is why it becomes useful as a ferrite bead - they come in all shapes and sizes of course. FBs are precisely like this - above a certain frequency (usually much less than the SRF) the device becomes very resistive and can remove interference from cables and wires. I'm certainly not saying it's a common application but, this is what it could be used as. You always, with data sheets have to differentiate the "marketing hype" from the hard facts and, if you can't use it as an FB then it has little use above 30 MHz in your "other" lower frequency applications. – Andy aka Nov 25 '20 at 18:21
  • If you need it to be an effective inductor above 30 MHz, you will be disappointed (sadly). L1 will work perfectly fine in this application as that ferrite - look at the impedance graph and ask yourself at what point does this drop close to anything like 50 ohm across the frequency range you need. Between 10 MHz and 3 GHz it has an impedance greater than 500 oh. That makes it very usable as a bias setter for the Pha 202. – Andy aka Nov 25 '20 at 18:23
  • Thank you very much for your explanation. – John Jin Nov 28 '20 at 05:59