We’re using the Jetson TX1 module and have designed our carrier board to also accept PCIe full mini cards. We’re using the 2.5GT/s rate. While we are able to achieve a successful PCIe link between the TX1 and the add in card, we also observe significant EMI interference which is desensing a nearby GPS receiver (~1.575GHz) when the PCIe link data lanes are signalling. We’ve narrowed down the issue sufficiently to suspect a relationship with the PCIe data lanes themselves, but not necessarily a root cause.
After reviewing the TX1 design guide, I noticed that the recommended differential impedance for the PCIe link was not followed by our layout engineer. It was routed at 95 ohm differential, as opposed to the recommended 85 ohm. Intrapair matching was exact and all other design rules were followed (except voiding of connector pads and ideal GND via placement. GND return vias are on the same grid as the Jetson connector pins so aren’t ideal.)
What is the 85 ohm recommendation based on? Is the Jetson TX1 module itself routed at 85 ohm? Is the source impedance of the transmitter in the TX1 chip 85ohm? If not, what are the differential impedances of these elements?
Thanks very much,