I’m working with USB 1.1 and, for several reasons, I need a connection of about 20cm long between the microcontroller and the connector, so I want to ensure that the impedance is right. Although I’ll use Low-Speed mode (1,5Mbps), I want my design to be safe for Full-Speed (12Mbps).
I’m using 1,6mm PCB with two layers, which means that, in order to achieve an impedance of 90 ohm I need traces of 0,85mm with a separation of 0,2mm. Ok to this; it even fits perfectly with the coupling resistors in the microcontroller.
The problem comes when I want to connect it to the USB connector, because it has 0,4 width tracks.
Is it enough to just use two very short tracks to connect to the differential pair? Or do I need to do a smooth transition, like this test that I did using bezier curves with InkScape and manually tracing them using a plane and straight line approximation?
If the best solution is the later, which is the best way of tracing it?