Nrf9160 GPS long time to fix with healthy RF electronics

[bjornshnrf]

Helo Nordic support team,

I am supporting a client that has designed a product using the NRF9160 with GPS. The product has a small patch antenna mounted on the pcb board, a saw filter and a LNA upstream the GPS port of the NRF9160.

The product has been tested extensively in the open and they experience long time to fix - in some cases 10 to 15 minutes from cold start (and some time no fix at all). It was suspected that the antenna gain and match could explain long fix time due to lov SNR for the received GPS signal. The antenna match was investigated using a network analyser and found to be around -10dB RL for the GPS band.

We have compared the product respons with a Thingy:91 using a larger patch antenna with integrated LNA. This gave similar results with long time to fix (not tested as thorough as the product).

Next test was done in a lab version of an anechoic chamber called GTEM cell. It is not a perfect environment for antenna performance tests but gives a good indication on performance. Using this GTEM device with a signal source we were able to apply a -90dbm CW signal to the product (now running only with test software and battery power). The test software logged the SNR and RSSI using the AT AT%XRFTEST=2,1,-90 at command and after the test we extracted the data from the product to a log file using a J-link SEGGER with RTT Viewer. We did a similar test with the Thingy:91 with the external antenna and got similar results.

Our preliminary conclusion is that the product (and the Thingy:91) has good RF performance (around 14dB gain including LNA, SAW and antenna gain).

I tend to conclude that the long time to fix can be explained by software. 

There are no other use of the radio between GPS read, the LTE is not used.

Can the Nordic team comment on why do we experience long time to fix from cold condition even with apparently healthy RF electronics ?

Thanks,

Regards

Bjørn

[bjornshnrf]

We have made some additional tests using both the Thingy:91 with external antenna and the product we have designed based on your NRF9160.

 

The Thingy has been tested using an external antenna. The Coax connector for external antenna has no longer an internal switch so we selected to remove the dc block capacitance downstream of the connector towards the internal GPS antenna. This to try to keep the 50ohm transmission line from the NRF to the external antenna. The external antenna has a LNA and the test software is configured to switch this power on (confirmed by measuring that the bias voltage is present on the external coax line).

 

We have three configurations (four if you add the GPS cold start hit the pedal drive test .. : )  ). The first test was done putting the Thingy:91 w/ext ant. on the roof of my car. The car was parked at Sunland in Drammen with no cars nearby and farily open space around the site. The second test was run at the exact same spot but now with the product as the GPS receiver. The third test was done on the dashboard for both the Thingy and the product. The last test was executed by giving Thingy (and also the product) a cold start and then immidiatly driving away in order to test or get a reference measurement for a moving receiver.  

 

I still have a few more tests to run before I will share the results. I will upload these test when Ithe extended test has concluded.

 

In addition to these tests I have added a more dedicated test on SNR and RSSI. I put the receiver into a suitable GTEM cell and configured the lab. Signal source to give -90dBm field power at the antenna of the test device. The GTEM is EM sealed and very little exernal radiowaves can kreep into the test chamber. Could you please evaluate the data in this log file and comment on the RSSI and SNR?

 

All data in the attached log files have been extracted using the JLINK and the JLinkRTTViewer.exe from Segger.

  

There are two files attached:

• gps_driver_share.zip : Code for test in the field
• gps_rf_test_share.zip : Code to test the gps in the RF chamber
• Chamber_test_Product_boresight_90dbm_vertical.log : log file from chamber test

 These datafiles have been modified to remove client source code.

gps_driver_share.zip

gps_rf_test_share.zip

Fullscreen Chamber_test_Product_boresight_90dbm_vertical.log Download

# SEGGER J-Link RTT Viewer V6.88a Terminal Log File
# Compiled: 15:12:58 on Nov 18 2020
# Logging started @ 14 Feb 2021 13:37:26
00> *** Booting Zephyr OS build v2.4.0-ncs1-24-gdac3d0802faa  ***
00> 
00> AT response: %XRFTEST: 289,-19411
00> 
00> AT response: %XRFTEST: 286,-19391
00> 
00> AT response: %XRFTEST: 284,-19411
00> 
00> AT response: %XRFTEST: 283,-19391
00> 
00> AT response: %XRFTEST: 285,-19411
00> 
00> AT response: %XRFTEST: 284,-19411
00> 
00> AT response: %XRFTEST: 288,-19411
00> 
00> AT response: %XRFTEST: 287,-19411
00> 
00> AT response: %XRFTEST: 286,-19411
00> 
00> AT response: %XRFTEST: 288,-19411
00> 
00> AT response: %XRFTEST: 284,-19391
00> 
00> AT response: %XRFTEST: 288,-19391
00> 
00> AT response: %XRFTEST: 287,-19411
00> 
00> AT response: %XRFTEST: 285,-19371
00> 
00> AT response: %XRFTEST: 289,-19391
00> 
00> AT response: %XRFTEST: 289,-19371
00> 
00> AT response: %XRFTEST: 284,-19411
00> 
00> AT response: %XRFTEST: 285,-19411
00> 
00> AT response: %XRFTEST: 286,-19371
00> 
00> AT response: %XRFTEST: 286,-19391
00> 
00> AT response: %XRFTEST: 285,-19411
00> 
00> AT response: %XRFTEST: 291,-19391
00> 
00> AT response: %XRFTEST: 292,-19411
00> 
00> AT response: %XRFTEST: 287,-19391
00> 
00> AT response: %XRFTEST: 289,-19371
00> 
00> AT response: %XRFTEST: 288,-19391
00> 
00> AT response: %XR1
00> 
00> 1
00> 
00> Measurement: 0, snr: 289, antenna_power: -19411
00> 
00> Measurement: 1, snr: 286, antenna_power: -19391
00> 
00> Measurement: 2, snr: 284, antenna_power: -19411
00> 
00> Measurement: 3, snr: 283, antenna_power: -19391
00> 
00> Measurement: 4, snr: 285, antenna_power: -19411
00> 
00> Measurement: 5, snr: 284, antenna_power: -19411
00> 
00> Measurement: 6, snr: 288, antenna_power: -19411
00> 
00> Measurement: 7, snr: 287, antenna_power: -19411
00> 
00> Measurement: 8, snr: 286, antenna_power: -19411
00> 
00> Measurement: 9, snr: 288, antenna_power: -19411
00> 
00> Measurement: 10, snr: 284, antenna_power: -19391
00> 
00> Measurement: 11, snr: 288, antenna_power: -19391
00> 
00> Measurement: 12, snr: 287, antenna_power: -19411
00> 
00> Measurement: 13, snr: 285, antenna_power: -19371
00> 
00> Measurement: 14, snr: 289, antenna_power: -19391
00> 
00> Measurement: 15, snr: 289, antenna_power: -19371
00> 
00> Measurement: 16, snr: 284, antenna_power: -19411
00> 
00> Measurement: 17, snr: 285, antenna_power: -19411
00> 
00> Measurement: 18, snr: 286, antenna_power: -19371
00> 
00> Measurement: 19, snr: 286, antenna_power: -19391
00> 
00> Measurement: 20, snr: 285, antenna_power: -19411
00> 
00> Measurement: 21, snr: 291, antenna_power: -19391
00> 
00> Measurement: 22, snr: 292, antenna_power: -19411
00> 
00> Measurement: 23, snr: 287, antenna_power: -19391
00> 
00> Measurement: 24, snr: 289, antenna_power: -19371
00> 
00> Measurement: 25, snr: 288, antenna_power: -19391
00> 
00> Measurement: 26, snr: 286, antenna_power: -19411
00> 
00> Measurement: 27, snr: 292, antenna_power: -19391
00> 
00> Measurement: 28, snr: 284, antenna_power: -19371
00> 
00> Measurement: 29, snr: 285, antenna_power: -19371
00> 
00> Measurement: 30, snr: 284, antenna_power: -19391
00> 
00> Measurement: 31, snr: 289, antenna_power: -19411
00> 
00> Measurement: 32, snr: 288, antenna_power: -19371
00> 
00> Measurement: 33, snr: 286, antenna_power: -19411
00> 
00> Measurement: 34, snr: 290, antenna_power: -19391
00> 
00> Measurement: 35, snr: 289, antenna_power: -19391
00> 
00> Measurement: 36, snr: 286, antenna_power: -19411
00> 
00> Measurement: 37, snr: 286, antenna_power: -19391
00> 
00> Measurement: 38, snr: 288, antenna_power: -19371
00> 
00> Measurement: 39, snr: 286, antenna_power: -19371
00> 
00> Measurement: 40, snr: 290, antenna_power: -19411
00> 
00> Measurement: 41, snr: 293, antenna_power: -19411
00> 
00> Measurement: 42, snr: 287, antenna_power: -19351
00> 
00> Measurement: 43, snr: 289, antenna_power: -19391
00> 
00> Measurement: 44, snr: 291, antenna_power: -19391
00> 
00> Measurement: 45, snr: 283, antenna_power: -19411
00> 
00> Measurement: 46, snr: 287, antenna_power: -19391
00> 
00> Measurement: 47, snr: 286, antenna_power: -19411
00> 
00> Measurement: 48, snr: 290, antenna_power: -19411
00> 
00> Measurement: 49, snr: 289, antenna_power: -19391
00> 
00> Measurement: 50, snr: 291, antenna_power: -19441
00> 
00> Measurement: 51, snr: 290, antenna_power: -19391
00> 
00> Measurement: 52, snr: 288, antenna_power: -19391
00> 
00> Measurement: 53, snr: 289, antenna_power: -19391
00> 
00> Measurement: 54, snr: 291, antenna_power: -19411
00> 
00> Measurement: 55, snr: 292, antenna_power: -19391
00> 
00> Measurement: 56, snr: 290, antenna_power: -19391
00> 
00> Measurement: 57, snr: 290, antenna_power: -19391
00> 
00> Measurement: 58, snr: 292, antenna_power: -19411
00> 
00> Measurement: 59, snr: 293, antenna_power: -19391
00> 
00> Measurement: 60, snr: 286, antenna_power: -19441
00> 
00> Measurement: 61, snr: 289, antenna_power: -19391
00> 
00> Measurement: 62, snr: 290, antenna_power: -19371
00> 
00> Measurement: 63, snr: 289, antenna_power: -19371
00> 
00> Measurement: 64, snr: 286, antenna_power: -19391
00> 
00> Measurement: 65, snr: 290, antenna_power: -19411
00> 
00> Measurement: 66, snr: 286, antenna_power: -19411
00> 
00> Measurement: 67, snr: 290, antenna_power: -19411
00> 
00> Measurement: 68, snr: 295, antenna_power: -19391
00> 
00> Measurement: 69, snr: 297, antenna_power: -19411
00> 
00> Measurement: 70, snr: 290, antenna_power: -19391
00> 
00> Measurement: 71, snr: 288, antenna_power: -19371
00> 
00> Measurement: 72, snr: 292, antenna_power: -19411
00> 
00> Measurement: 73, snr: 288, antenna_power: -19411
00> 
00> Measurement: 74, snr: 291, antenna_power: -19391
00> 
00> Measurement: 75, snr: 287, antenna_power: -19411
00> 
00> Measurement: 76, snr: 290, antenna_power: -19411
00> 
00> Measurement: 77, snr: 293, antenna_power: -19371
00> 
00> Measurement: 78, snr: 290, antenna_power: -19391
00> 
00> Measurement: 79, snr: 292, antenna_power: -19391
00> 
00> Measurement: 80, snr: 294, antenna_power: -19371
00> 
00> Measurement: 81, snr: 290, antenna_power: -19411
00> 
00> Measurement: 82, snr: 289, antenna_power: -19391
00> 
00> Measurement: 83, snr: 291, antenna_power: -19411
00> 
00> Measurement: 84, snr: 290, antenna_power: -19391
00> 
00> Measurement: 85, snr: 295, antenna_power: -19411
00> 
00> Measurement: 86, snr: 294, antenna_power: -19411
00> 
00> Measurement: 87, snr: 290, antenna_power: -19411
00> 
00> Measurement: 88, snr: 295, antenna_power: -19391
00> 
00> Measurement: 89, snr: 293, antenna_power: -19371
00> 
00> Measurement: 90, snr: 294, antenna_power: -19391
00> 
00> Measurement: 91, snr: 291, antenna_power: -19411
00> 
00> Measurement: 92, snr: 292, antenna_power: -19411
00> 
00> Measurement: 93, snr: 294, antenna_power: -19391
00> 
00> Measurement: 94, snr: 286, antenna_power: -19411
00> 
00> Measurement: 95, snr: 293, antenna_power: -19411
00> 
00> Measurement: 96, snr: 285, antenna_power: -19391
00> 
00> Measurement: 97, snr: 287, antenna_power: -19391
00> 
00> Measurement: 98, snr: 293, antenna_power: -19391
00> 
00> Measurement: 99, snr: 290, antenna_power: -19391
00> 

# Logging stopped @ 14 Feb 2021 13:40:42

[bjornshnrf]

Hi,

We also added some tests from a location in Drammen.

The test is on a Thingy:91 with external antenna and LNA. The Thingy is placed on the rooftop of the car and 15 cold start measurements were recorded (a cold start is a power on cycle when the unit is completely disconnected from any power supplies).

The next test is with the Thingly located on the dashboard of the car. This time we only took 5 measurements all from a cold start.

The last test was with the product located on the dashboard. 15 tests were recorded.

  thingy (roof) :  Product (dashboard)
1    33        52
2    37        46
3    21        32
4    21        52
5    33        43
6    37        41
7    38        44
8    39        48
9    37        42
10  36        42
11  29        48
12  43        46
13  38        52
14  36        42
15  38        35

      thingy (dashboard)
1    33
2    37
3    30
4    31
5    36

The log files from these measurement where created by connecting the JLink to the Thingy/Product. No USB ocnnection was used.

Thanks,

Regards

Bjørn

Thingy_27_2_Sundland.zip

[bjornshnrf]

Hi Jonathan,

Did you get a chance to comment on tha data logger file:

• Chamber_test_Product_boresight_90dbm_vertical.log : log file from chamber test

thanks,

Regards Bjørn

[JONATHAN LL]

Hi Bjørn, 

Sorry for the slow reply, but what is the gain of the LNA used? 

The camber test summarized:
And the values range (100 runs): 
Antenna power:-19411 to -19371 so that would be -75.8dBm to -75.6dBm
SNR: range from 283 to 297, that would be 17.6dB to 18.5dB


Some feedback on the chamber test. 

Our preliminary conclusion is that the product (and the Thingy:91) has good RF performance (around 14dB gain including LNA, SAW and antenna gain).

So you have to account for the gain when doing the test.


If you give -90 dBm power level in the command, have -90 dBm signal generator, and get -75dBm back as response to the AT command, it means that there is 15dB of gain between the signal generator and the nRF9160. The difference of 15dB in command's power and response indicates that the receiver is possibly saturated and the SNR result is possibly not accurate.

Thus, in order to take the LNA gain into account they should use command

     AT%XRFTEST=2,1,-75,0 

(set <param1> = 0) 

Regarding the signal level setting there is also a note in the AT Guide:

Note: It is recommended to set the signal level defined in the GPS SNR ON parameters to the
expected signal level at the GPS antenna port of the nRF9160 SiP.

Then the SNR should also get higher. Expected level is closer to 30dB than 20dB. The RSSI in response should be close to the value given in input.

 

Regards,
Jonathan

[bjornshnrf]

Hi Jonathan,

Thanks for your answer.

I am not quite sure I understand the explanation on saturated LNA. We do have a LNA, SAW filter and an antenna upstream the gps port of the nrf9160. If we apply -90dBm to the antenna port (using the TEM wave produced by the GTEM cell) and the xRFTEST gives RSSI of -76dBm. then the gain is 14dB and roughly as expected.

My understanding of the AT%XRFTEST=2,1,-75,0 is that it should give 0dB gain? 

Could you explain?

Thanks,

Regards

Bjørn

[JONATHAN LL]

Using this config :  AT%XRFTEST=2,1,-75,0
the result should be close to 75dBm for the antenna power.


since you had a offset the SNR values you got from the measurement from the chamber test there is likely a offset in the SNR value not giving the true result due to the ~15dB offset from not accounting for the LNA.

If that maks sense ?

Regards,
Jonathan

[bjornshnrf]

Hi Jonathan,

I made a few other measurements and the primary objective was to investigate if the receiver goes into saturation.

I used AT commands through the JLINK interface to communicate with the product. The prodcut was positioned inside my GTEM cell. The signal source could be ste to have different levels of RF power.

Test one: Using RF power levet at the GPS antenna port of -90dBm and command "at at%xrftest=2,1,-75". This gave the result 271,-18825 (SNR=17dB and RSSI=-73.5dBm).

Then the RF signal power was increased from -90dBm up to -72dBm (using command "at at%xrftest=2,1,-75"):

 

                              Antennapower(dBm)      SNR       RSSI       SNR(dB)        RSSI(dBm)

1) RF Power = -90dBm   -90                        274        -18805      17,1             -73,5

2) RF Power = -87dBm   -87                        318        -18104     19,9              -70,7

3) RF Power = -82dBm   -82                        393        -16873      24,6             -65,9

4) RF Power = -80dBm   -80                        412        -16392      25,8             -64,0

5) RF Power = -78dBm   -78(edit from -7    434        -15881       27,1             -62,0

6) RF Power = -76dBm   -76                        455        -15351      28,4             -60,0

7) RF Power = -74dBm   -74                        325        -15020      20,3             -58,7

8) RF Power = -72dBm   -72                        384        -14850      24,0             -58,0

(Table 1)

I also tested with "at at%xrftest=2,1,-57" with antenna power of -72dBm followed by "at at%xrftest=2,1,-53" with antenna power of -67dBm. This gave SNR 495(31dB)/RSSI -14352(-56dBm) and SNR 538(34dB)/RSSI -13093(-51dBm).

I have added two graphs from "table 1" above.

It appears that SNR goes into saturation but only at significant higher power than -90dBm. I cannot see evidence that the Product LNA goes into saturation at -90dBm to the antenna (in the GTEM cell).

(Edit 18.3.2021: The LNA in these tests have been upgraded and has 3dB more gain. )

Thanks,

Regards

Bjørn