[N210, Basic TX/RX] Generating a chirp waveform with very short duration

[Sahlin]

Hello,

I am attempting to generate a linear chirp (frequency modulated waveform) using LabView and a USRP N210. However, I've run into some issues primarily concerned with how the USRP is processing samples.

My goal is to generate a chirp spanning a major portion of available bandwidth (say 25MHz) and doing so in a very short time, say 1ms. The chirp samples are generated through a common algorithm (https://en.wikipedia.org/wiki/Chirp).

To my understanding, the IQ Rate setting in the Configure Signal.vi (in LabView) sets the number of samples the USRP outputs over 1 second. So, a setting of 200k (200 kHz sample rate) would indicate that one sample is being procured every 5 microseconds.

Now, my problem is that this seems to indicate that it is not possible to have the USRP generate a very high-frequent waveform produced from discrete samples. For example, setting the IQ Rate to 50M will have the USRP output 50 000 samples in 1 ms. So, any waveform to be sent during 1 ms will be limited to a resolution of 50 000 samples. For a chirp range of 1-50Khz, this leaves one sample per Hz in resolution which is obviously inadequate. Let alone a chirp ranging from 0-25Mhz.

However, looking at say http://www.jpier.org/PIERB/pierb53/20.13052904.pdf figure 12 on page 15 (and text on the following page) seem to indicate that they managed to sweep over a bandwidth of 25Mhz in just 655 microseconds (with an older USRP model).

I'm very confused here and would very much appreciate input.

Hardware/software details:

USRP N210 Rev4

LabView 2013

NI-USRP Feb 2014