An update: As we’re shipping some more backlogged LS1028A devices I’ve looked into this some more. First, we replaced the Molex antenna with one that has 20cm cables and used a new acrylic (from the RCM4 project) that mounts over the SD card slot, so the antenna is at the other end of the device. But I still got interference into eDP on 2.4GHz WiFi (AFAIK not on 5GHz, so one workaround is to disable 2.4GHz).
My working theory is that the pinout of the eDP FPC connector on the LS module combined with a simple flat cable is bad for signal integrity, as there is no ground plane and no managed impedance between the N/P lines of each lane:
As you can see, N/P pairs have a GND pin in the middle, and TX0_DN is closer to TX1_DP than to TX0_DP. It should be the other way around.
So I tried to design a custom flex PCB (cable) that fixes these things:
- Diff pairs are routed together with the right impedance
- Diff pairs, not their lines, are separated by GND
- There’s a GND plane on the bottom layer
- EMI shielding foil will cover the cable and will be connected to GND lines
I’ve ordered 20x at JLC and hope that they will arrive quickly after CNY and will report back with testing results.
We will continue shipping LS1028A Reforms as-is to not let everyone wait more, and if the flex cable works out in tests, mail these to anyone who needs for free.