RAK RPi Converter Board apparently has bugs/lacks in wiring of GPS module and antenna
Below information is based on U-Blox MAX-7 Data Sheet and U-Blox MAX-7 / NEO-7 Hardware Integration Guide.
As delivered RAK’s RPi Converter Board (the GPS part) seems incorrectly wired, neither suitable/optimal for passive GPS antenna nor for active GPS antenna.
Below the issues:
-
GPS module VCC_RF (pin 14) is hard-wired to 3.3V but it should not.
Hard-wired via a PCB trace below the GPS module from pin 14 to pins 7+8.
For compliance with the U-Blox documentation this trace should be cut as described by @JTAG above (be careful for the via next to pin 13’s soldering pad).
Another trace goes from VCC_RF to the 3.3V pad marked ‘3.3V’ on the outer side on the board. This trace should also be cut to prevent possible future issues because it is not a 3.3V pad but the GPS module pin 14. -
Antenna / GPS module RF_IN (pin 11) is wired directly to 3.3V via 0 ohm resistor R19 but it should not.
For compliance with the U-Blox documentation R19 should be removed. -
According to U-Blox documentation, without required additional components (coil, capacitor and resistor) the U-Blox MAX-7Q will not (properly) support an active GPS antenna.
These components are missing on the converter board and there are no free soldering pads for adding them manually later.
For using the board with an active GPS antenna there are two options:
- Use a U-Blox MAX-7Q or Quectel L70 module, apply the above two fixes and add the required additional components as described by JTAG (note: L70 lacks U-Blox’ binary UBX protocol), or
- Use a U-Blox MAX-7W module (already has components required for active antenna), apply the above two fixes and connect a 10 ohm (Rbias) resistor between V_ANT (pin 15) and VCC_RF (pin 14). (Also includes antenna shortcut protection.)
Optional:
If not implementing JTAG’s modifications (adding the three components, which uses this trace) cutting the following PCB trace will prevent it from having possible negative impact on the antenna signal:
Cut the PCB trace from R19 to the unlabeled 0 ohm resistor that is connected to RF_IN. Cleanly and directly below the unlabeled 0 ohm resistor. The trace is then not connected to RF_IN anymore.