Beauty of Aliasing : DAC

The hardware on the USRP has a DAC capable of 128MS/s (iirc), which means that AT MOST you could get it to put out a 64 MHz signal.

Another trick to get higher frequencies is to use an image that is produced the DAC.   If you program the DAC to 29.3 MHz, you'll also get RF at the SampleRate +/- DAC Freq.  So you'll get a signal at 128 - 29.3 and 128 + 29.3. So in this case 98.7 and 157.3. The signal levels will be lower, but with the proper bandpass filter and amplifier chain, you can generate a complete signal. 

The signal levels will be lower, but with the proper bandpass filter and amplifier chain, you can

generate a complete signal.  The signal levels fall off at a sin(x)/x rate. 

A DAC typically has a low pass filter to cut all the alias images.  But you can also bandpass filter those images to produce the desired signal.  The system is very linear and isn't a mixer.