Connect different width tracks

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?


I think it is enough. 12MHz is not that demanding frequency.
But not assume me being an expert in it.

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That connection is so short, I see no reason for that kind of thing. The “fat” track end is rounded anyway (that’s how Gerbers are), so you won’t really have any discountinuances causing reflections.

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So, then, is this just enough?

In my opinion, yes. You can also make a two- or three-stage stepdown if you like.

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In my opinion, also yes.
Think how then pads are connected to socket contacts, and if solder paste while soldering will follow impedance rules.

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USB 1.1 Fullspeed has rise and fall times of under 20ns, so a bandwidth of about 30 MHz. This has a wavelength of 10m, so even a 10mm Impedance irregularity is 0.1% of a wavelength. In transmission line theory this is absolutely insignificant.
USB2.0 at 480 Mbps and USB3.0 are a different story. Especially USB3.0 requires track tapering and attention to how the track crosses the FR4 substrate weave


Thank you! I’m really a programmer, and although I did some RF courses, it is still black magic for me :sweat_smile: