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?

[AndreasF]

Hi.

The most important thing is that the LNA is turned of or deactivates the signal-chain once GPS_enable is low.  Usually you would turn the LNA off when you transmit, and therefore its important that the signal which is transmitted do not re-enter the radio. The compression characteristics is more important in situations which have a lot of noise. The sensitivity given in the datasheet is probably excluding the LNA.

Best regards,

Andreas

[ABM]

AndreasF said:

The most important thing is that the LNA is turned of or deactivates the signal-chain once GPS_enable is low.  Usually you would turn the LNA off when you transmit, and therefore its important that the signal which is transmitted do not re-enter the radio.

Thanks for spelling that out.  A couple of questions (though if they're already covered in the documentation and I've missed it please do point me at it):

1. How low do we need to reduce the level of the cellular signal present at the GPS input to avoid improper operation?
2. Can you explain what the purpose of U34 and C73 is in nRF9160 DK schematics v0.8.5 is? What it does is shunt the antenna with 33pF when GPS_EN is low. Why is that done as I assume the LNA will be disabled at that time anyway?

[AndreasF]

Hi.

1. If you look at the datasheet for QM14501 LNA, you can read that Input 1dB Compression Point is -2 dBm, so that is the level you will get distortion in the signal.

2. The purpose of U34 and C73 is to reduce the impact of interference from the LTE antenna when transmitting.

Best regards,

Andreas