GPS (pin 42) is a 50 Ω single-end interface for the Global Navigation Satellite System (GNSS) antenna.
To ensure best possible GNSS reception performance, antenna impedance and the characteristic impedance of the transmission line connecting the antenna to the GPS pin should be 50 Ω. Impedance mismatch leads to reduced performance.
Loss deteriorates the received power and GNSS reception of nRF9151. To minimize loss, the length of the transmission line from the antenna to the GPS pin should be kept as short as possible. A maximum of 0.5 dB transmission line loss is acceptable. If an external GNSS Low-Noise Amplifier (LNA) is used, and it is located close to the GNSS antenna, the routing impedance and length recommendations can be relaxed.
To achieve good GNSS reception under real-world field conditions, it is recommended to protect GNSS from interference. GNSS operating band and out-of-band rejection requirements are given in nRF91 Series Antenna and RF Interface Guidelines.
When selecting an external GNSS LNA and filter module, it is recommended to select a module topology where the filter is placed before the LNA. This reduces the penetration of external wideband interference into the nRF9151 GNSS input and protects the LNA from potential coupling nRF9151 high-power cellular transmit signals during cellular activity. The nRF9151 GNSS cannot be active simultaneously with the cellular transceiver. However, a high-power cellular transmit signal, if coupled into external LNA, might damage the LNA or generate unwanted distortion or both even when LNA is not active. Distortion generated in the external LNA might cause radiated emissions during cellular transmission.
If IoT Non-Terrestrial Network (NTN) band 255 (1626.5–1660.5 MHz) support is required, the filtering requirements might be too stringent for conventional SAW and BAW RF filters. In this case, use an RF switch instead of a conventional RF filter. The RF switch must have sufficient isolation to achieve a minimum of 30 dB isolation from the LTE ANT input to the external GNSS LNA input. This protects the external GNSS LNA from the nRF9151 high-power cellular transmit signal. If the RF-switch is placed before the LNA, place the GNSS wideband interference filtering after the LNA. Set the filtering requirements according to out-of-band rejection requirements, and take into account the LNA gain at out-of-band frequencies. The requirements for out-of-band rejection are given in nRF91 Series Antenna and RF Interface Guidelines.
PCB layout design
When designing the PCB layout, consider the following recommendations for the GPS pin:
- To ensure good radiated performance, begin the design by locating the GNSS antenna on the PCB.
- Ensure preliminary antenna performance on a chosen PCB location, for example, by simulation.
- When multiple antennas, for example, Long-Term Evolution (LTE), GNSS, and Bluetooth® Low Energy are present, minimize antenna crosstalk.
- GPS 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 GPS pad on the next metal layer.
- When opening ground planes, avoid exposing noisy routings under the GPS pad.
- 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 GPS routing.
- Consider adding a test connector to the GNSS path for production test and diagnostic purposes. Ensure that the test connector or solution does not deteriorate the impedance of the transmission line.