Are Common Mode Choke Coils needed on USB?
The USB signal is not entirely differential, so it's not a great idea. (The end-of-packet (EOP) signal is both pins pulled low, which, I believe, is why there's always noise at 1 kHz and harmonics in USB systems, since it's sending common-mode signals every 1 ms.)
- A common mode (CM) choke should be used to terminate the high speed USB bus if they are need to pass EMI testing. Place the CM choke as close as possible to the connector pins. See Section 5.1 for details.
Note: Common mode chokes degrade signal quality, thus they should only be used if EMI is a known problem.
Common mode chokes distort full speed and high-speed signal quality. The eye diagram above shows full speed signal quality distortion of the end of packet, but still within the specification. As the common mode impedance increases, this distortion will increase, so you should test the effects of the common mode choke on full speed and high-speed signal quality.
High Speed USB Platform Design Guidelines
Note: additional filtering may be achieved by winding the 4 wires through the ferrite bead an additional turn. As with the use of ferrite beads in signal paths, care should be taken to insure that the signaling meets rise and fall times, especially the EOP signaling. EOP signaling is single ended and may be strongly affected by a single bead, which acts as a common mode only filter.
Intel EMI Design Guidelines for USB Components
There's a good chance that the choke is included on the Gumstix board because they have to pass FCC Title 47 CFR Part 15 emissions testing for unintentional radiators to sell their device.
You might not care about that, but if it's for a commercial product I think it's cheaper to add the choke now and then remove it if you discover it's not needed.
If your product sits "floating" on the USB cable, the common-mode choke is probably not necessary.
However, if your device is electrically attached through any other paths that could form a loop back to where your USB cable originates, you will have a potential for inductively picking up or generating noise that can affect the performance of your product, or other product attached to it. At that point, you will need the choke.