Are you sure HackRF really can send 20 MHz of
bandwidth over USB 2.0
continously? Where did you find that info? (Seems I should try to
push SDR manufacturers who use USB 2.0 to supply modes with higher
sampling rates...)
I can't find any specific examples now, but most pages discussing the
HackRF (including those from people who have a prototype) say there is
no problem receiving 20Msps from the device. I find this quite
believable, as one of the aims for the project was to be able to
monitor a 20MHz wi-fi channel at 5.8GHz, so that the 802.11 protocol
could be implemented entirely in software.
The only reason the RTL devices are limited to smaller bandwidths seems
to be a limitation in the performance of the RTL2832 chip itself -
something not entirely surprising to anyone familiar with the
performance of other Realtek products ;-)
Cheers,
Adam.