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I am trying to understand what ferrite bead to use for the Ublox M10S GPS unit. I am using as a reference the design from SparkFun GNSS Receiver Breakout - MAX-M10S (Qwiic) (https://www.sparkfun.com/products/18037) - under Documents. The design calls for a 2.66 Ohm Ferrite Bead on the line being used to power the active GPS antenna (VCC_RF). I have looked at the provided Eagle files in KiCad and it does not show the exact ferrite bead component used. It just says 2.66 Ohm (package 0402).

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My understanding on the FB usage is that it is there to reduce the noise on the RF line. I know that the GPS antenna operates at around 1500Mhz. Does this mean that I am looking for a FB with high impedance at that frequency? (Then why are they using 2.66 Ohm?). Most Ferrite beads that size don't have a high impedance at that frequency (go down after 1000Mhz). Others only show the impedance at the resonant frequency and no graph (e.g. 2 Ohms @ 100Mhz).

What should I be using?

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  • \$\begingroup\$ Did you check the MAX-M10S documentation; Integration Manual in particular? content.u-blox.com/sites/default/files/… \$\endgroup\$ Oct 30, 2023 at 20:36
  • \$\begingroup\$ yes. They show a slightly different circuits. Simplest, uses a passive antenna without a ferrite. The other type uses an antenna supervisory design which does use active antenna and a ferrite bead, called F1. They provide values for all components except for F1. The sparkfun circuit is much simpler than the supervisory antenna since it is just providing power \$\endgroup\$
    – Enk73a
    Oct 31, 2023 at 21:16

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C1-FB2 looks to be nothing more than a bias tee. It's not clear from the block diagram, but I assume the RF amplifier needs bias or power, and this would be a typical way to feed it in.

Beads are generally rated in impedance at 100MHz, but 2.66Ω would be a preposterous value, both in smallness and undeserved precision. The next most likely candidate parameter would be DCR, but most chip beads have much less resistance than this.

Parts are suggested in MAX-M10S Integration Manual section 4.3.4 (p.70). They also indicate a resistor for current limiting, which it appears your example has adopted as well.

The type and value of L3 is selected to have a resonance peak at GNSS frequencies. This provides a high series impedance above 500 Ω at GNSS L1 frequencies, creating an impedance mismatch with respect to the 50 Ω RF signal line. This minimizes the effect of the feed point on the RF signal line, and isolates the antenna supply from the RF signal line at GNSS frequencies.

They recommend a 27nH inductor, from several families. None of which seem to have a DCR matching your example, so it's probably not one of these either.

Ferrite beads are available with comparable impedances at these frequencies, if you like. Offhand, Würth 74269244182 seems about as good as anything.

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