How can you recieve a 6 GHz signal with only 64MS/s ADC?
Have a look at the updated block diagram of the chip on the B200.

https://github.com/analogdevicesinc/iio-oscilloscope/blob/master/block_diagrams/AD9361.svg

A couple things:


the max ADC rate is not 61.44, it's actually 640MSPS, but that
assumes some decimation along the way.
the max data output is 61.44 MSPS
the max RF bandwidth is 56 MHz, and is set by the analog filters, not the ADC sample rate.


Many more questions can be answered on ADI's web site:
https://ez.analog.com/community/wide-band-rf-transceivers