nRF9160 external LNA requirements?

[tj]

The Qorvo QM14501 LNA in nRF9160 reference schematic seems to be consuming several mA of current, which is quite a lot for a GPS LNA. There are other LNA chips in the market, which use less current, but their IIP3 / 1dB compression point is lower. Are there any specific demands to compression characteristics of the GPS LNA, as it sits next to the cellular TX antenna etc.? Has the higher compression point LNA (QM14501) selected to the ref. design, because lower-compression point (and lower current consumption) GPS LNAs would saturate or produce cellular TX harmonics due to the high RF field generated by the cellular TX antenna of nRF9160?

Are these nRF9160 datasheet GPS sensitivity figures including or excluding the use of QM14501 LNA ?

- Acquisition sensitivity: -144 dBm for cold start, -147 dBm for hot start
- Acquisition time: 30 seconds for cold start, 5 seconds for hot start
- Tracking sensitivity: -149 dBm (open sky received power ≥ -130 dBm)

If the LNA is not used at all, how much less sensitivity there is for the GPS? What are those figures then?

[Andreas]

Update Oct 3 2019: Please see the White paper on nRF9160 Antenna and RF Interface Guidelines for latest recommendations and specifications for GPS front-end. Summarized requirements:

1. GPS LNA recommended minimum gain is 15 dB (maximum gain not limited, but anything beyond 15 dB won’t improve the system performance noticeably).
2. GPS LNA recommended maximum NF is 1 dB (with filter 1.5 dB).
3. GPS LNA input and output nominal impedance 50 Ohm.
4. Interference at any frequency coupling into SIP GPS input should be < -7 dBm when GPS is OFF.
5. Interference at any frequency coupling into SIP GPS input should be < -30 dBm when GPS is ON.
6. Interference at any frequency coupling into GPS LNA input should be verified from selected GPS LNA datasheet, but same figures can be used as rule of thumb as for SIP GPS input case (4 and 5).
   • Recommended minimum attenuation for own LTE TX signal coupling into GPS LNA input is 30 dB. Attenuation can be implemented by combination of LTE and GPS antenna isolation and using filter in GPS LNA input.