The USRP-2901 ships with a transformer for a 6V external power supply. The device draws a maximum of 4.5W @ 5V per the device specifications which is within the capability of the bus USB port. Is there any risk to powering the device off of USB without the external supply? It seems to work just fine.
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That's an interesting idea as I've never tried that before, but the logic of the power requirements seems to check out.
That being said I probably would stick to using a power supply since that what the designers intended so there may be more protection surrounding that the power supply port.
Also it might be good to keep our power and data lines separate since if something does go wrong with the USB that would mean cutting power and data at the same time. That could leave the device in an unrecoverable state since It's probably something that we didn't test for.
Hope this helped clarified possible risk!
Would be good to know what the current limit is over USB. Preferably the absolute max rating on whatever IC handles the power.
Outside of what's on the specifications sheet we don't have published specifications of what you're asking for.
As far as the stated recommendations they can be found on the sheet below. Specifically it calls out that USB 3.0 will work better so I'd use the USB 3.0 specification to get an idea of max ratings.
Specifications Talking About USB Power - http://www.ni.com/pdf/manuals/376357b.pdf#page=5
If you'd like to try getting more specifics you can open up a formal case with our department and discuss you specific application. You can do that by calling 866 275-6964, emailing firstname.lastname@example.org, or going to ni.com/ask.