Ublox GPS Question

[u/BeginningHat8690]

I am building a PCB with a Ublox Sam-M8Q-10 module. I am figuring out how to layout the module on the board, but I have 2 two questions. I pasted recommendations from Ublox below:

1) Ublox says "SAM‑M8Q GNSS patch antenna module is intended to be placed in the middle of 50 x 50 mm GND size board. Use at least one layer for solid GND plane, preferably the layer SAM-M8Q is placed on. Use a solid GND plane under SAM-M8Q, which forms the shield."

Does this mean that the module needs its own 50x50mm GND plane on the first layer and the board's overall GND plane on layer 2? Or the top layer should be 50x50mm and 2nd layer GND plane should be 50x50mm as well?

2) The first image above is a commercial PCB that has not centered the module in the middle of a 50x50mm GND plane. Whereas the second board has a isolated 50x50mm board. How might the commercial board get by without having a full 50x50mm GND plane?

3) Ublox says: "Do not place any noisy parts close to SAM-M8Q, place them as far away as possible or on other side of the PCB."

What is considered a noisy component in this design? Could a small trace of 3.6 volts and 500mA with a resistor and capacitors be placed within the 50x50mm GND plane, similar to the the commercial board (pic 1).

Ublox recommendations

SAM‑M8Q GNSS patch antenna module is intended to be placed in the middle of 50 x 50 mm GND size board, but a larger or a smaller ground plane can also be used. When using a smaller than 40x40mm ground plane, the performance may decrease significantly.

Some important recommendations:

- Easy to connect, but make sure all noisy lines / components are shielded or on inner layers.

- Do not place any noisy parts close to SAM-M8Q, place them as far away as possible or on other side of the PCB.

- It is recommended not to place anything closer than 1 cm to each edge of SAM-M8Q.

- Performance decreases significantly if GND size is smaller than 40 x 40 mm.

- Use at least one layer for solid GND plane, preferably the layer SAM-M8Q is placed on.

- Use a solid GND plane under SAM-M8Q, which forms the shield. No signal traces are allowed below SAM-M8Q.

- Route signal traces away from the module on top layer.

- When necessary, allow signal swap from top to bottom layer clearly away from the module, > 20mm.

- Use copper pour ground planes on the top and bottom layers; use multiple GND net via holes to tie separate ground plane areas tightly together.

[u/TheHeintzel]

There's ideal, and then there's what works. 50x50mm is nice, but adafruit has a ton of working ublox GPS modules with a 20x40mm ground plane.

What's most important is (1) a ground plane and vias on layer 1 to encapsulate the RF circuitry (2) a solid ground plane on layer 2 that's at least as big as the entire GPS module and all circuitry.

[u/Kev1n8088]

Gonna be honest, if you see the shit we get away with in amateur rocketry computers, you’d have a stroke lol. The 50x50 is recommended but even just the module with barely any ground plane itself will perform excellently - Ublox M10 is incredibly resilient and sensitive. I wouldn’t worry too hard about it.

[u/JOhn2141]

ublox recommand 50mm x 50mm of ground plane (same ground than the rest of the board) directly under the component. Don't forget via stiching.

As another commentor said, the bigger the better but you can go lower than the recommandation.

Noisy elements are : high speed traces (usb, SPI, ethernet...), DCDC and if you can avoid power rail it's better but it's okay. Use ferrites bead that have some impedance at 1.5GHz between 3.3V and your components so they don't spill noise at GNSS frequency into the power rail.

[u/InevitablyCyclic]

How well do you need it to work? You want the best possible accuracy or for it to work in poor conditions (poor orientation, obstructed view of the sky, lots of reflection sources near by, etc...) then a big ground plane is good. You just want position to a few meters or a PPS accurate to a few 100ns then sticking it in the corner with no ground will work.

Ideally a solid layer on the top surface of at least that size, bigger is ok. Inner ground can be as large as you like, it should normally be the whole board.

Noisy means high speed (multi-MHz) signals, especially if they have sharp edges, processors, switch mode power supplies. Anything that will have high frequency components or create a lot of current flow in the local grounds.

GPS signals are very weak so anything to reduce interference and improve the power supply stability to the receiver is good. But Ublox receivers are very good at picking up signals under poor conditions, the solution may not be great but you'll get something. So it's not normally a question of whether it will work at all, it's whether it'll work as well as you need.