Hi there
To better understand what's going on when you digitize/generate a signal using quadrature sampling, feel free to have a look here
Complex, but not complicated
Quadrature sampling allows you to have Fs=Bandwidth (BW), compared to direct sampling where you need to meet the Nyquist criterion (Fs>2*BW).
There are pros and cons that need to be considered when building a signal generator/reciever to choose between sampling schemes.
On the USRP 2954R (and all USRPs) we use quadrature sampling, so you'll get 160MHz per channel, above 500MHz as said in a post above.
Hope that bring you usefull info.
Regards
Victor F. | Systems Engineer
Certified LabVIEW Developer | Certified TestStand Architect
National Instruments Budapest