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nrf51822: Inductor selection

JohnC

I'm interested in working off the reference design for the nrf51822 to create my own board. I have a few questions about the 3 inductors specified on the schematic/BOM:

  1. 0402 SMD size parts are specified. Are larger SMD parts OK? (Say, 0603 or 1005)

  2. What exactly is meant by "high frequency chip inductor"? Does this relate to the resonant frequency of the inductor and, if so, what resonant frequencies would I want to consider?

  3. Are there any recommended (or min/max) current rating or current saturation values?

  4. I assume that a multilayer ceramic inductor is what I want. If not, what should I use?

  5. I assume that the same selection criteria would also govern the selection of inductors for the antenna. If not, what are the differences?

  • 0

    Hi,

    Just to let you know - there are also specialized chips which include all the balun into one component. I use those and I am extremely satisfied with them. For example Johanson technology makes baluns specifically for nRF51 family. I will post a photo here in an hour or so to show my latest prototype design with these.

    1. Yes, but larger parts also induce more parasitic inductance and other not wanted effects. The space occupied by matching circuitry is also larger 0603 are OK in my opinion.

    2. The self-resonant frequency should be at least 2 times the carrier frequency - 2.45 Ghz, so anything above 5 Ghz is fine.

    3. Personally I think this is not an issue when transmitting at 0 dBm (1 mW) power. But I am not sure so wait for answer of anyone else. I always used something around 300 mA minimum.

    4. If I am right there are reference components listed. If possible, use the same components to assure optimum performance.

    5. The parameters of components used to make matching circuitry near the antenna can be the same as the balun circuitry.

    After you make your final design and you are planning to make it commercial, you should make a proper measurement using spectrum analyser to ensure that everything is working fine. This can also save a lot of money during cerfitication process.

  • 0 in reply to mnhs

    Here is the photo - sorry for bad soldering, it is a first working prototype soldered by hand including all the 0201 components which was literally a hell to do only with tweezers and hot air :-).

    www.mnhs.cz/nrf_balun.jpg

  • 0

    Great answers by @mnhs in my opinion. If I may add some comments to a couple of those answers.

    Q1: Yes, you can use larger components, but keep in mind that the component values probably would have to be changed. I would not recommend anything bigger than 0603 as we haven't tried that.

    Q3: Agree and just want to add that this is not a parameter we consider a lot since the currents and power is really small.

    Q4: Multilayer ceramics are the preferred. We use a Murate LQP15 series in our lab. It is also recommended, that as @mnhs states, that you verify the performance on your final prototype and that you use the same inductors in prototyping as you will in your final application.

  • 0 in reply to mnhs

    Thanks to both mnhs & Asbjørn for their excellent answers. I checked out the info on Johanson's site and it looks like an integrated balun and chip antenna is probably the way to go. Oh, and don't apologize for the soldering... I didn't even think it was possible to solder something that small (goodness knows I don't think my hands are steady enough to do it ;)

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