Working at 20 MS/s


I was interested in working with detectors for OFDM 802.11b in my USRP2.
I’ve read on the internet about this and it seems that there are people
able to use sampling rates of 20 MS/s.

However, when I try to send an OFDM signal with 20 MHz of bandwidth, at
the USRP2 receiver board the FFT plot shows the signal blinking.

This effect only disappears for sampling rates as low as ~2 MHz.

This will affect my detectors performance

What is the main cause for this? Can it be solved?