On 5/18/18 6:09 AM, Clint Jay wrote:
Interested to know how much noise would be from USB signalling and how much is " machine noise" from the PC as my understanding of USB signalling is that it's differential so such should be low noise?
Every device I've worked with that had a USB interface has radiated significant power around 12 or 24 MHz. (I've not worked much with devices that support 480Mbps USB3..)
Indeed, the data wires are twisted pair (although the "twist rate" in most cables is pretty sketchy), but the signaling is voltage mode, not switched balanced current like LVDS. So I think you get significant current spikes on the power wires as you charge and discharge the capacitance/inductance of the (unterminated) data lines.
The shielding is, in a lot of consumer devices, common to the power supply negative lead. So the shield carries part of the power supply current, and then radiates.
In theory, USB devices sold in the United States should meet FCC Part 15 (and the similar requirements in the EU and elsewhere) - but I'll bet there's a LOT of stuff out there that isn't tested, nor would it pass.
High Speed USB might be better for EMI/EMC - you're just not going to push 480Mbps through an unterminated system with crummy noise properties.
TI has an application note https://www.ti.com/sc/docs/apps/msp/intrface/usb/emitest.pdf _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
