GPS Module Layout (ublox)

[snx]

Hi,

I want to build a project that includes a gps module for data logging. For this project i want to use the Ublox M8. There are quit a few ready to purchase modules on the market, but most of them only have supply+uart. I also need the shutdown and pps pin, so i'm considering to integrate the ublox device on my pcb.

For the antenna i wanna use one of these "micro sma (ufl)" connectors and attach a either a ceramic or helix antenna (commercial manufactured antenna). So basically i just want to put the module on my pcb and thenn connect the connector to the module.

But i have a few questions:

1.) Most of the available Modules have a ground plane on the top side and alot of vias to the bottom gnd plane, my guess is that these are just made this way because they are only 2 layer boards. I'm having a 4 layer board with an internal ground plane, so i guess i don't need a ground plane on top or bottom layer, right?

2.) A few modules uses a capacitor/resistor in paralel with the antenna connector, other modules do not. What are these for / when do i need them?

Thanks for your help

[LapTop006]

I want to build a project that includes a gps module for data logging. For this project i want to use the Ublox M8. There are quit a few ready to purchase modules on the market, but most of them only have supply+uart. I also need the shutdown and pps pin, so i'm considering to integrate the ublox device on my pcb.

I built a somewhat similar board a months back for a project.

For the antenna i wanna use one of these "micro sma (ufl)" connectors and attach a either a ceramic or helix antenna (commercial manufactured antenna). So basically i just want to put the module on my pcb and thenn connect the connector to the module.

You might be interested in the Taoglas AP.10H.01, it's a surprisingly decent antenna, if the form factor works for you.

1.) Most of the available Modules have a ground plane on the top side and alot of vias to the bottom gnd plane, my guess is that these are just made this way because they are only 2 layer boards. I'm having a 4 layer board with an internal ground plane, so i guess i don't need a ground plane on top or bottom layer, right?

I suspect if you're using the LEA or NEO modules you can get away without a ground plane entirely, mine has one on the bottom since that's simply my standard (and this was a two layer board), no attempt was made to bring ground out to the top (other than for electrical connections) for either the module or antenna.

2.) A few modules uses a capacitor/resistor in paralel with the antenna connector, other modules do not. What are these for / when do i need them?

Possibly just a filter, or possibly an external bias network.

[kruszi]

2.) A few modules uses a capacitor/resistor in paralel with the antenna connector, other modules do not. What are these for / when do i need them?

These are used to do what's called antenna matching (google it!). The GPS/GNSS module input impedance needs to be matched to the impedance of the antenna. Most GPS modules have a 50 ohm input but not all antennas have 50 ohms under all conditions. Especially housings or objects in the nearfield of the antenna will affect the match and thus power transfer from the antenna to GPS module. The resonance frequency of the antenna is shifted (mostly to lower frequencies). Since the link budget (google) of GPS is very tight you need a very good match and the resonance frequency of the antenna exactly at the L1 frequency. With this "network" the antenna and GPS module impedance can be matched (e.g. https://de.tek.com/blog/5-easy-steps-antenna-matching-vector-network-analyzer). You usually just place the footprints for a pi-filter and just bridge it with a 0R resistor. If you need it you can do the matching properly.

Filters for these frequencies are usually not implemented with discrete elements (so-called lumped elements) since filter performance of single stage LC-filters is very bad and hardly usable above 1 GHz. You would expect a crystal or SAW filter at a GPS input (which is rarely done).

Regarding the metal housing: I suggest you to build a stetup of your GPS-logger and test it. And the get a feeling for the sensitivity of GPS/GNSS. A Antenna suggestions ist the "Tango"-series of GPS/GNSS antennas (google for "tango gps"). Those could be mounted onto a robust metal housing. There are differenz sizes available. Please note that the performance of nearly any gps antenna relies on a (as large as possible) groundplane.

[deBug]

>2.) A few modules uses a capacitor/resistor in paralel with the antenna connector, other modules do not. What are these for / when do i need them?
I haven't seen that but I have seen a resistor and coil in series with the Antenna input. They provide DC power for an active antenna.

//Harry

[cdev]

In many GPS's there is a "bias tee" for feeding 3-5 v DC power at around 50 ma. to the 1.5 GHz antenna which typically uses its own LNA MMIC to overcome the feed line loss. Some GPSs may not even need an amplified antenna too. So they may leave some of that up to you.

What you need is the Ublox document for product implementation which goes into great detail on the best ways to implement ublox modules, explaining how to successfully get the RF to the module without losing it along the way.

The components in the signal path under some conditions, may be a bias tee, and are likely best explained by that document, which is free.

There are good reasons to consult it because it has a wealth of implementation information for GPS hardware developers.