External GPS antenna on Thingy:91/PCA20035 1.4.0

[DurandA]

I received a bunch of Thingy:91/PCA20035 version 1.4.0 which upgraded the SWF connector for GPS antenna to a uFL/IPX connector which is great. I also have a few PCA20035 version 1.0.0 and the GPS fix time was horrible with the internal antenna. The documentation still mentions a SWF adapter so I guess it wasn't updated.

I have a few questions reguarding the use of an external antenna:

• It the Errata v1.0 still valid? I guess so because the Fractus internal antenna is the same reference.
• Do you have a suggestion for an external GPS antenna (that may fit inside the case)? I tried an active antenna but the performance is terrible.

[Heidi]

Hi!

The update to the documentation is coming. 

The Tingy:91 v1.0.0 Errata is no longer valid and has been fixed. I'm still waiting on the person responsible for that to comment on some more details, so I'll let you know when I have that for you. 

And we do not have any suggestions for an external GPS antenna that fits inside the case, unfortunately. We don't have any specific recommendations for external GPS antenna's but please refer to this white paper (nRF9160 Antenna and RF Interface Guidelines nWP-033) when choosing.

Best regards,

Heidi

[DurandA]

Can you explain in greater details how the Thingy:91 v1.0.0 Errata was solved?

The FR01-S4-210 seems to have a very poor reception at GPS L1 (1575.42 Mhz) frequency.

[Heidi]

Here is the comment from the developer:

1. Basically what we did was to focus on "pre LNA" loss:
   1. Replaced inductors with higher Q variants.
   2. Removed SWF connector that was before LNA
   3. Shortened the transmission lines before LNA
   4. Replaced LNA with higher gain and lower noise figure.
   5. Retuned the antenna after these changes was made.
   6. In total we had an efficiency increase from ~20% to ~40%
2. Not sure we have tried any external antenna, the connection is equal to the nRF9160-DK, so the antenna following the DK should work, but it would not fit inside the casing. (Remember to keep on-board LNA disabled when testing external antenna)

The plot is from the FR01-S4-210 user manual. It is not relevant at all for the Thingy:91 performance

1. The measurements are done on a big evaluation board (142 x 60 mm) and with a fixed matching network
2. Thingy:91 is much smaller ~50 x 50 mm and we are doing active tuning.

Please note: We are not supporting L2, so no need to consider that band.

[Heidi]

2. Here is the antenna recommended for the DK: 

no.mouser.com/.../206640-0001

[DurandA]

Thank you for the detailed explanation.

Is it correct than we can either use a passive antenna and keep the LNA enabled or an active antenna and disable the LNA?

Also, is it sufficient to set NRF9160_GPS_ANTENNA_EXTERNAL to disable the LNA?

According to the Thingy:91 schematic, the LNA is enabled with COEX0.

It seems that the configuration disable COEX0 when NRF9160_GPS_ANTENNA_EXTERNAL=y:

config NRF9160_GPS_COEX0_STRING
	string "COEX0 string"
	default "AT\%XCOEX0=1,1,1565,1586" if BOARD_NRF9160DK_NRF9160NS && NRF9160_GPS_ANTENNA_ONBOARD
	default "AT\%XCOEX0" if BOARD_NRF9160DK_NRF9160NS && NRF9160_GPS_ANTENNA_EXTERNAL

However, the default is enabled only for NRF9160DK_NRF9160NS and not for THINGY91_NRF9160NS.

[Heidi]

DurandA said:

Is it correct than we can either use a passive antenna and keep the LNA enabled or an active antenna and disable the LNA?

 Yes.

Yes, the configuration you referred to does not have support for Thingy:91 yet. However, the PR 3077 (which I see you've already found) will fix this. 

As for your questions on the PR:

• Does the LNA act as a passthru when COEX0 is high?

First could you clarify what you mean by "passthru".

The answer depends on how you configure COEX0. 

COEX0 is input to the LTE modem from the external device. When active high, indicates that the external device transceiver is turned on.

When the internal GPS is used, COEX0 can be used as active-high control for the external LNA component.

• How is MAGPIO used?

The MAGPIO pins can be used for example to control an external antenna tuner or any GPIO controller device whose state depends on the modem's RF frequency. They are configured using the AT command: %XMAGPIO

• For Thingy:91, is it ok to use an external antenna with this configuration:
  • passive antenna with LNA enabled
  • active antenna with LNA disabled

Yes.

You can read more about the LTE modem coexistence interface and the LTE modem RF control external interface in the links attached. 

[DurandA]

Many thanks for the extensive answer. I misread the schematic and thought that the external antenna connector was connected to ANT_GPS while it is J2.

In that case, the signal from the external antenna does not go through the LNA so I assume only an active GPS antenna will work.