We’ve already tried near field probing to uncover the source of the desense. However, the sensitivity of our GPS receiver is quite low (-167dBm for NEO-M8N, https://www.u-blox.com/sites/default/files/NEO-M8-FW3_DataSheet_(UBX-15031086).pdf) when compared to the noise floor of our near field probe + spectrum analyzer. So the aggressor is below the noise floor of what we can measure. We can very clearly see the spike at 1.25GHz for the PCIe Gen1 signalling, but there is nothing at 1.575GHz above the noise floor.
I agree that moving the GPS antenna further from the aggressor would be a good solution, but while we may end up there eventually, it’s not acceptable for the overall product requirements today.
Regarding tuning the PCIe PHY values, I’d like to note that we aren’t suggesting this would be a production solution for the GPS issue, especially since the signal integrity impact on cards we may wish to use in the future is unknown. Rather, tuning would help us see if the PCIe_TX or PCIe_RX lines are the source (as is our current hypothesis).
Here are some other knowns based on recent experiments:
-There is no interference whatsoever when using GLONASS instead of GPS (around 1.598 to 1.606GHz). [Signal generator set to broadcast GLONASS only, receiver set to track GLONASS only]
-On the NVIDIA Jetson carrier board, we can install our card in the M2 slot (using a mPCIE to M2 adapter) and can reproduce the desense. Adding copper tape to completely enclose the card and adapter, all the way back to and including the M2 connector, eliminates the desense.
-On the NVIDIA carrier board, changing from TX1 to TX2 has no change in desense (this experiment used the x4 PCIe connector with an adapter to mPCIe since at the time we didn’t have the commands to change the TX2’s MUX for PEX1 vs USB_SS0 to get this to work in the M2 slot)
-Desense reproduces across 3 tested systems, suggesting this isn’t just a problem with one unit.
-Disabling unused PCIe slots or clocks has no impact on desense
-Independently enabling PCIe clocks but without a card in the slot has zero desense (suggesting that there must be traffic on TX/RX lanes for there to be desense). We also tried the same experiment with a card present but prevented enumeration of the card. Again, there is zero desense.
All of this is leading us to believe the desense is either because of the traffic or other noise on PCIE_TX/RX lines, or because of the state of the TX1 or card after enumeration occurs.
Finally, we’re expecting the PCIe Gen2 cards we’ve ordered to arrive Monday so will have more updates soon. We’ve also ordered a half-size minicard to better test the hypothesis about the size of the ground plane in relation to the wavelength of GPS.
Thanks for your continued support and ideas. Please let us know if you have more ideas for us to try. We are continuing to work through our own list of experiments too.