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I am having trouble with the reception of my GPS module - a Quectel L76L. I am connecting the Antenna via a U.Fl connector. I know that the issue is somehow due to my layout because it works on a breakout board that I purchased (which used a chip antenna), but not the PCB I made. I’ve tried a whole bunch of things but none seem to work well. It takes well over 10 minutes to get fixed, and another 10 - infinity minutes to get readings. something is definitely not right.

Things I’ve kept in mind:

  • Transmission line - keeping the trace length below .25in
  • Vias - both many vids all around the signal for the coplanar waveguide and more sparse.
  • Trace width - I’ve used the calculated 50 ohm impedance of 42.2 mil and the 35 mil width that the GPS spec sheet recommends - Since the trace width is larger than the pad, which width should I use? I saw a video recommend to use a trace width as large as possible to match the chip’s pad (this would be 31 mil).
  • Using active antenna, so I am using a 10Ohm resistor and a 47nF inductor

Please see the layout below that did not work.. is there anything wrong that is glaringly obvious? Is it okay that I put the inductor pad on the micro strip?

no polygon pour non working layout

Also, please see this design which I plan to send in for FAB. On this one I brought the antenna way closer to the GPS unit, I reduced the trace width to match the pad width of the GPS unit (31 mil), and added a bunch of bias that are a bit bigger. I also added a passthrough capacitor.

New Design enter image description here

Does this design look like it will work? After about 6 iterations, I am at a loss of what could be the issue... Any advice will be much appreciated.

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  • \$\begingroup\$ I think you are loosing much power due to lack of antenna matching. I am also curious how you chose the patch antenna.. \$\endgroup\$
    – stowoda
    Commented Jun 17, 2020 at 7:20
  • \$\begingroup\$ The [document][1] also states, that an external LNA is recommended for a better receiver sensitivity and TTFF. [1]: quectel.com/UploadImage/Downlad/… \$\endgroup\$
    – stowoda
    Commented Jun 17, 2020 at 7:26
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    \$\begingroup\$ I did not read through the modules spec, but what strikes me: There is no DC blocking cap. Are you sure it's ok to put antenna supply voltage on pin 11? \$\endgroup\$
    – Andreas
    Commented Jun 17, 2020 at 7:32
  • \$\begingroup\$ Hi @stowoda - The manual also states that "There is no need to use external LNA ... because an embedded LNA is already used inside the module". One thing that confuses me is whether or not my antenna is in fact active or passive if it is a patch antenna. In practice I have found that it doesn't work at all if I don't feed it power through an inductor as I have shown... though I thought my antenna was passive. \$\endgroup\$
    – Troy Cados
    Commented Jun 17, 2020 at 21:35
  • \$\begingroup\$ Hi @Andreas - The schematics show it to be this way. Please see the schematics on page 29. I saw in someones video for a Ublox GPS (similar) they added a passthrough capacitor, so I did as well on the newest layout I showed above. quectel.com/UploadImage/Downlad/… \$\endgroup\$
    – Troy Cados
    Commented Jun 17, 2020 at 21:37

2 Answers 2

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I got the board and what I think ended up making the biggest difference was actually just a bigger patch antenna. I think all my models actually worked in the end, just not very well with those tiny 10mm x 10mm antennas (suggests my design could still be better).

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Your design is missing the Pi matching network. For active antennas, you want a 56pF DC block capacitor, just to be on the safe side, in case your active antenna injects DC into your RF feed. For the next rev, try having a solid ground plane and pours, yours have holes in it.

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