Modifying SparkFun MAX-M10S schematic for Passive FPC antenna (Smartwatch project)

GPS/GNSS
1

Hi everyone,

I’m currently working on a highly constrained 4-layer smartwatch PCB (ESP32-S3) and integrating the u-blox MAX-M10S GPS module. I will be using a Passive FPC Antenna connected via a U.FL/IPEX port.

I am basing my GPS sub-circuit heavily on the open-source SparkFun MAX-M10S Breakout schematic. However, since the SparkFun board is designed to support active antennas, it routes VCC_RF (Pin 14) into the RF_IN line via a bias-tee network made of FB2, R14, and C1 to inject DC power.

Since my passive FPC antenna requires no power (and might present a DC short to GND), here is my modified approach:

  1. Removing the Bias-Tee: I completely removed FB2, R14, and C1. I left the VCC_RF pin completely floating (NC).

  2. Direct RF Routing: I routed RF_IN straight to the U.FL connector (trace is under 5mm, impedance matched to 50 ohms). The only component on the RF line is a PESD0402-140 ESD protection diode down to GND.

  3. Power Supply: I am using a dedicated RT9080-33GJ5 LDO to supply a clean 3.3V to VCC, V_IO and V_BCKP. Decoupling is done with a 10uF and a 100nF capacitor.

  4. Damping Resistors: I kept the 33 ohm series resistors on the UART TX/RX lines as per SparkFun’s design.

My Questions:

  • Is completely removing FB2, R14, C1 and leaving VCC_RF floating the correct and safest way to modify this reference design for a passive antenna?

  • Is this ultra-minimal RF trace sufficient to get a reliable fix, or should I at least add a Pi-network placeholder (0-ohm series, NC shunt) just in case antenna impedance tuning is needed?

I would really appreciate any insights or pointing out any glaring mistakes before I send this to fab. Thanks!

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2

I think you’ve got it…pages 8, 85 here https://content.u-blox.com/sites/default/files/MAX-M10S_IntegrationManual_UBX-20053088.pdf indicate vcc_rf is supposed to be NC if using passive and the internal LNA gain is sufficient for passive…so:

    • Is completely removing FB2, R14, C1 and leaving VCC_RF floating the correct and safest way to modify this reference design for a passive antenna? -
      Yes
    • Is this ultra-minimal RF trace sufficient to get a reliable fix, or should I at least add a Pi-network placeholder (0-ohm series, NC shunt) just in case antenna impedance tuning is needed? -
      It should be good, but if you have the board space I’d go with adding the 0-ohm placeholder resistor to give yourself the flexibility to make some adjustments later if needed :wink:
3

Thank you so much