AUX (pin 37) is a 50 Ω single-end auxiliary port that can be used to loop back the signal that is fed into the ANT pin. AUX can be used when two radios share an antenna. If AUX is not needed, it can be left floating or unconnected.
AUX can be configured in software so that when the GNSS receiver is on, AUX is connected to ANT inside nRF9151. This enables the routing of the GNSS signal from the shared LTE and GNSS antenna to AUX and through a recommended external Low-Noise Amplifier (LNA) to the GNSS input. To minimize mismatch losses, it is recommended to use external matching components between AUX and LNA.
When the nRF9151 LTE modem or GNSS is not active, ANT can be terminated to 50 Ω or connected to AUX in a static manner. This increases the current consumption by 0.7 mA from a battery voltage of 3.7 V. Configuring AUX in a static manner is supported only in Non-Terrestrial Network (NTN) firmware (mfw_nrf9151-ntn).
For more information on the antenna interface, see nRF91 Series Antenna and RF Interface Guidelines. For more information on configuring the AUX port, see nRF91x1 Cellular AT Commands and nRF91x1 NTN AT Commands.
PCB layout design
When designing the PCB layout, consider the following recommendations for the AUX pin:
- AUX transmission line impedance should be 50 Ω. Take this into account when selecting the PCB stack-up.
- Avoid excessive capacitance in routing by opening ground layers under component pads. For example, if the routing is on the top PCB layer, open the ground plane under the AUX pad on the next metal layer.
- When opening ground planes, avoid exposing noisy routings under the AUX pad.
- Favor routing in shielded PCB layers to minimize radiated noise coupling to the AUX line.
- Avoid long routing because it causes excessive insertion loss which deteriorates RF performance.
- Ensure a continuous reference ground plane above, below, or both of the AUX routing.