I can't send signal with frequency beyond sampling rate in GNuRadio

I’m a student working on a FRID project about ASK modulation and demodulation. I use USRP to send and receive but when I used 915MHz sine wave as carrier and set the sampling rate as 64KHz, I found the frequency of the signal I received is no more than 16KHz. And if I use a filter to process the signal, the GnuRadio will give a warning that I should follow the nyquist theorem. However the signal in my system is narraw band. Will someone help me? Thanks!