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Old Feb 15, 2016, 12:49 PM
JackJones is offline
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Help!
DIY Flight Controller and u-Blox M8Q GPS problems

I am working on a custom flight controller, with integrated GPS module. It's u-blox MAX-M8Q module and external antenna is connected via U.FL antenna connector. Antena connection is designed as shown in u-blox datasheet for active antenna. 10 ohm resistor in series with 27nH inductor connected from VCC_RF power pin to RF_IN, and 47pF capacitor in parallel.
First, I noticed that in same occasions, when I use active Taoglas AP25 antenna, GPS module hangs on startup, like in some reset state. No communication in or out, TIMEP pin in weak pullup.

With some other active antennas (AP17, AGGP25) this does not happen. However, I also seem to get pretty weak signal, and it has difficulties to get a lock. CNO is usually in some 20-30 dB range, hardly ever goes over 30. It is a bit better when using large active patch antenna (black thingy on image ), I can get a lock on satellites, cno go over 40.
Should there be a DC blocking capacitor before RF_IN pin? I am using 3.3V for power supply.

I also have Lora 433Mhz module near ublox module, along with several switching power supplies. Could that be the cause of interference to module or gps antenna?
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Old Feb 16, 2016, 01:14 AM
KiwiDavid is offline
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Hi,
I don't think you need an input capacitor as this module senses remote antenna current (for opens or shorts) by using the drop across the 10R resistor.
It will go in to a protection mode if the current goes too high.

Keep the antenna away from other digital circuitry and also any transmitting antennas. Just remember that these devices are working with signals between about -135dBm and -165dBm. In comparison a microvolt (uV) is -120dBm. System noise will be your biggest problem.
Any dielectric patch antenna is designed for a certain ground plane size which affects the gain and the center frequency. The magnetic mount antenna (auto type) should be designed to go on the car body or rear parcel tray - i.e. large ground plane. In free air it may be somewhat off tune. With a good view of the sky and away from noise sources you should be seeing 40+dB CN ratios.
Good luck.

Cheers,
David
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Old Feb 16, 2016, 02:46 AM
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It maybe worth having at look at their UBLOX reference designs, having worked on GPS/GSM trackers a while back. They do have some PCB layouts which may help on any issues that may be from noise / crosstalk

https://www2.u-blox.com/images/distr...1-cs-10020.pdf
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Old Feb 16, 2016, 02:37 PM
JackJones is offline
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Quote:
Originally Posted by KiwiDavid View Post
Just remember that these devices are working with signals between about -135dBm and -165dBm. In comparison a microvolt (uV) is -120dBm. System noise will be your biggest problem.
That makes sense. I'll try to shield antenna from electronics and see if it helps. Thanks.

Quote:
Originally Posted by bluegoose View Post
It maybe worth having at look at their UBLOX reference designs, having worked on GPS/GSM trackers a while back. They do have some PCB layouts which may help on any issues that may be from noise / crosstalk
Interesting, I'll have a look.
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