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Thread: GPS reception blocked by RFI

  1. #1
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    GPS reception blocked by RFI

    Hi there,

    I am using a Teensy 3.6 audio project using the audioboard getting samples on a high rate and a displaying waterfall or spectrum on an Ili9341 3.2" display. (Teensy bat detector thread).

    Since I wanted to store location information in the waf file metadata I added a GPS receiver.
    I chose a tiny board using an ATGM336H and mini antenna.

    The GPS testcode works fine (TinyGPS) but when the bat detector code is running the GPS reception is lost. Looking at my spectrumanalyzer I can see the whole RF spectrum lifting up when I change from the testcode to the batdetector code. (Can't measure above 1024mHz)

    I can't get a GPS fix indoor and outdoor it takes a very long time to get a fix if I mount the antenna and GPS receiver outside the box. (when running the testcode I can get a GPS fix indoor) Of course I'd like to have all in the small plastic enclosure.

    Do any of you have similar experiences, do you have suggestions for alternative GPS receivers or ideas to improve reception? I am a HAM Radio amateur and already tried some ferrite beads around wires and coaxial cable of the antenna but that does not seem to help. (or at least does not help enough)

    Kind regards,

    Edwin

  2. #2
    Senior Member+ defragster's Avatar
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    Any chance it is running short of power? No doubt you are seeing the RF Noise - just wondering ... Also - What is the satellite transmission freq?

    Years back running GPS and IMU I could power the GPS from Teensy but IIRC @mjs513 with similar setup - maybe then on T_3.5 had to power GPS another way. I never saw his setup - but was never clear why it was different.

    In use here was an adafruit ultimate with antenna on the same PCB and I could get indoor ( in a window ) locks - and I just powered the GPS up the other week and got locks on two diff GPS units - but one took a long time after being off a year or two with a dead battery - then adjusting window position.

  3. #3
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    I tried several power sources, and planned it to be running from the "powerbank" that also powers the Teensy via a small RC filter. It already proved to be hard to keep all the data signals and pulsed power users like SD-card writing out of the recordings.
    Internally it uses 3.3v I guess but it is claimed to work between 3.3 and 5V giving 3.3v serial ttl output

    The frequency used is about 1575mHz and usually there is an amplifier in the antenna which makes it hard to filter out any rf signals that are picked up by the antenna.

    The adafruit GPS is quite large, and for building it into a detector a unit with a separate antenna would probably be better.

    The when I run the testcode in the detector is can see a quick satellite fix via de serial monitor. As soon as the Teensy is busy running the detector code the signal gets lost.

  4. #4
    Senior Member PaulStoffregen's Avatar
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    My guess is the SPI signals. Maybe add resistors between Teensy and the wires going to that display.

  5. #5
    Senior Member+ manitou's Avatar
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    I had problems loosing GPS lock when powering up T4.1+ethernet and using an Adafruit ultimate GPS. I finally bought an external antenna and placed the antenna on windowsill. see thread
    https://forum.pjrc.com/threads/61581...4-1-NTP-server

    I haven't tried it, but sparkfun has an antenna eval board https://www.sparkfun.com/products/15247

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    Thanks Paul, I have a resistor in every dataline (150 ohms). The TeensyBat detector already needed them to keep the the noise out of the bat sound recordings.

    I'm afraid an external antenne in the windowstill is not an option for a small portable device.... The sparkfun board is a nice one to test out some external antennae. A lot of boards use external antennas with built-in amplifiers (LNA) maybe it is beter to use a passive antenna, a good bandfilter and amplify afterwards.

    I guess the Adafruit Ultimate uses a passive external antenna....

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    EMI absorbing sheets might be another approach, such as Kemet FX5. Place behind the GPD antenna...

  8. #8
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    Note that a GPS requires far less noise during acquisition. So in some cases, you can power up the GPS, acquire a lock and then power up the noisy circuitry.

    Distance (even say from 1" to 3") makes a difference. So does a better antenna and a small ground plane. Using a phone + bluetooth is another option.

  9. #9
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    i have a drone w gps - very small antenna. outdoors lock 3-4+ sat usually takes 1-2 minits.

    indoors (tile roof) often takes 5-6 minits

    and as jonr says, once it has aquired, i can carry it inside no problem

  10. #10
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    Thank you for your replies, materials like fx5 are new to me I will look into that.

    My detector is portable only about 3x5x1.5 inches the Teensy is just about in the middle and the display is covering most of the front. It is hard to find a place away from noisy circuits.

    In the meantime I have tried two other GPS units to see if they are performing better tan the ATGM336H. I tried a Beitian BN-180 and a similar style and size Sunsun gps module Both have the antenna mounted directly to the gps unit.
    It seems that the Beitian has much better immunity to the noise the detector creates. The Sunsun has the same problems as the ATGM336H. I liked the ATGM336H better because it seemed to find a solid fix faster and current drawn was less.

    I guess I will keep the Beitian for now and might try some other gps units because there seems to be quite a bit of difference between the ones I tried now.

    It might also be interesting to try and add a tin shield around the hart of the unit.

    Edwin

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