GPS interface and antenna

nRF9160 includes a Global Positioning System (GPS) (L1 C/A) receiver with a SE 50 Ω RF interface (GPS pin). The GPS receiver can be active when the LTE modem is inactive or in power saving mode.

The GPS input of nRF9160 includes ESD components, but external active components may require ESD filtering. The instructions in the component’s datasheet should be followed. The GPS input is DC grounded.

Note: It is not recommended to use the GPS receiver without an external Low-Noise Amplifier (LNA) since it causes a sensitivity drawback of a few dBs.

Using matching components in the antenna output is recommended to ensure a good impedance match. High-Q and tight tolerance components should be used. For information on the recommended matching circuit and layout of each antenna, see the relevant GPS antenna datasheets.

There are several different alternatives for GPS front-end implementation. Some examples of them are described in detail in Dedicated GPS antenna with LNA module, Combined GPS and LTE antenna, and Active GPS antenna module. In other implementations than Combined GPS and LTE antenna, having a filter in the external LNA input is highly recommended to filter out high power LTE transmit signals as it may lead to, for example, an increase in radiated spurious emissions or deterioration in the sensitivity of the GPS receiver.

To avoid RF issues, 30 dB isolation at LTE TX frequencies between the nRF9160 ANT pin and the GPS input pin should be targeted. In other words, the maximum allowed coupling from the ANT pin to the GPS input pin at a maximum LTE transmit power of 23 dBm is -7 dBm. Coupling should be minimized on the Printed Circuit Board (PCB). An additional RF switch may be used instead of a filter in the GPS front-end to reach the required attenuation to LTE TX frequencies in the LNA input. However, a switch does not provide rejection against external interference sources, such as Bluetooth® , WLAN, and mobile networks, and therefore a GPS Band-Pass Filter (BPF) is recommended in the LNA output if an RF switch is chosen in the GPS LNA input.

To minimize power consumption, an external LNA should be supplied only when the GPS is active. The COEX0 pin can be configured to be high state during GPS receive. Thus, COEX0 can be used to control external LNA. For more information, see nRF91 AT Commands Reference Guide. In the GPS front-end implementation examples in Dedicated GPS antenna with LNA module, Combined GPS and LTE antenna, and Active GPS antenna module, COEX0 is used as GPS LNA enable and VDD_GPIO for LNA supply.

Note: Noise filtering capacitors may be required on GPS supply and enable. Noise can be coupled to these lines on the device PCB. As the COEX interface conducts VDD_GPIO noise, additional filtering capacitors are recommended.