TTGO T-Beam GPS Antenna Design Fault

Hi,

I did find a design fault in the TTGO T-beam design.
There is a coil placed in the wrong path, see RED marked square for the correct design.

To get mine working better i changed also the resistor to 10 ohm (was 22ohm on board) and replaced the coil

ttgo-tbeam%20fault

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If i hold the T-beam to close to other gps devices, then i lose fix on those other devices.
So i guess that the t-beam is also jamming its own gps.

Are you sure that diagram and pcb match? With TTGO devices i several times had the finding that the pcb did not match with the diagram.

I started to investigate this because it did not get a GPS fix sometimes, now after modifications i have fix in 15 seconds.

It seems that the PCB has the same fault. I did also noticed that the battery charger is a total different chip

There a are different versions of TTGO T-Beam, and they changed the battery chip and charging logic.

What version is your T-beam? Mine is T22_V07 and has a date of 20180711 and appears to have the incorrect placement of the coil, and has the 22 ohm resistor. I thought i had one of the later T-beam versions.

Thats not an unusual situation, GPS receivers are operating at very low signal levels. You can see a reduced sensitivity next to anything that generates wideband RFI, such as a fast processor. The same issue can affct LoRa recievers when operating at well below noise level.

You can quite easily check the impact of ‘changes’ on the GPS. With a simple program that echoes the serial output from the GPS to the serial monitor port, you can then connect the PC Ublox Ucenter application which will read the echoed GPS data and produce a graph of the satellites received signal strength. A before and after comparison would show how the signals were actually being affected.

15 seconds for a fix, is I presume, using hot fix, its not too good a guide on performance since the even in good signal situations hot fix time can vary from 2 seconds to maybe 30 seconds, depends how long it has been since the last fix.

Well before it was not having a fix for hours …

My aliexpress contact confirmed that they changed the design now.
So i guess you still can buy the wrong versions if there is some stock left and you buy to cheap.

Well that a good guide.

But for anyone else, so afflicted, it should be easy to see whats actually happening to the GPS with Ucenter.

I wonder why you had no fix for hours. I have that same layout bug but can get a fix in the house in 3 to 5 minutes most of the time, or in two minutes if outside.

Also, I’m curious why you wanted to change the 22 ohm resistor to 10 ohms. I saw a 102 mV drop across it, were you worried that the voltage to the GPS antenna was dropping below spec?

To me a fix time of two minutes outside is suggestive of a poor GPS antenna, a decent setup should get a fix (from cold) in around 40 seconds.

the wrong placed coil will block the higher frequency’s, so you will only get a lock on some but not all satellites, when the blue led is blinking it does not say it has a fix, i only tells you that is has a correct time.

I changed the coil and resistor just to make sure that it was correct in my opinion, since i soldered already a couple of hundreds neo6 modules for some other projects.

When comparing fix times you should also consider that receiving conditions may vary in a wide range. Anything (building, trees, etc.) above the antenna will attenuate the signal. Large objects next to the antenna can reflect signals (multipath). Atmospheric conditions will have an effect, too. Plus, the constantly changing satellite constellation will produce different conditions. And, of course, it largely depends on the pre-known data in the device (⇒ hot vs. cold fix), as already mentioned in a post above.

That is true to an extent.

However if the TTGo is being used with one of those small rectangular ceramic patch antennas, I would suggest using a bigger antenna. When I tested those small antennas, on a properly working GPS, I could not get a fix after several minutes. The larger 25mm ceramic patch antenna, in the same location, a few minutes earlier, on the same GPS took 35 seconds to get a fix.

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I experienced the same when testing indoors, nothing beats an external GPS antenna
x1389

still want to buy these… but min Q 20 :cold_sweat: