TLDR: If your FL2K dongle only works on USB 2.0 ports, it may have been wired incorrectly. Rewiring it should make it work.
Two weeks ago I learned about osmo-fl2k from Hackaday and immediately ordered one from Aliexpress. Two days ago, I received my order and tested it: it works with USB 2.0 ports on all of my computers (Windows and Linux, Desktop and Laptop), but when I plugged it in on a USB 3.0 port, it is not detected at all.
I verified that the device ID is as expected by osmo-fl2k and that it works (with fl2k-test) on USB 2.0 port although it is very slow (14 MS/s).
The device is never detected on USB 3.0 ports, with or without hubs. No message at all when typing “dmesg”, and Windows also doesn’t show anything on device manager (not even a blink on device manager when the device is plugged in). I verified that all of my USB 3.0 port works by plugging other USB 2.0 and USB 3.0 devices.
Next step is to open this thing to verify that I really get the correct chip. Fortunately, the chip is correct, although the PCB is a bit different from the one shown in the osmo-fl2k website. The cable soldering seems to be ok, the connection looks good, and they even hot-glue it to make sure it stays that way.
I began to notice that the cabling is a bit different from the one shown in osmo-fl2k website. On the website all 10 cables is on the front side of the PCB, and on mine there are only 8 of them on one side and two on the other side.
I began to suspect something is wrong with the SSTX+/SSTX-/SSRX+/SSRX- cabling. But there is no specification/datasheet for the Chip, so how can I know the correct cabling?
First I tried using my multimeter’s continuity tester to check the color of the cables and the connection. It matches the standard coloring as listed in the USB 3.0 page on Wikipedia. For example, testing D+ pin on the USB port shows that it connects to the green cable and D- connects to the white cable.
I made an assumption: the quality of dongle used on the FL2K website looks good (mine doesn’t even have the SPI flash chip), so I assume they also use standard colors for the cables. Tracing from the pins to the cable shows that I need to rearrange the cables to match. For example, the first two pins on the top right of the chip should go to purple and orange cables (SuperSpeed receiver differential pair).
After rewiring it, it works fine, the dongle is detected on USB 3.0 ports and I can transmit up to 108MS/s. I have an old motherboard (>5 years), and as you can see from the osmo-fl2k website that the transmission speed depends on the USB 3.0 controller that you have. I have successfully transmitted FM radio signal but I haven’t experimented with other types of signal yet.
So the explanation of why it works on USB 2.0 ports is because it only uses VCC, GND, D+, and D- which was wired correctly. While on USB 3.0 ports, it tried to use the SSTX/SSRX pair and it doesn’t work.
I hope this helps others that have the same problem. Just a word of caution: the colors of your cables might be different from the one that I have, the PCB layout that you receive may also be different, so be careful and double check everything.