[Discuss-gnuradio] USRP and gnuradio as a data acquisition system

[Karthik Vijayraghavan]

Hi,

I am trying to use the USRP and gnuradio as a data acquisition system for feedback control of my physics experiments on atomic force microscopy (AFM). I am using the LFRX and LFTX daughterboards for my system. The bandwidth that I am interested is DC ~ 1-2 MHz.  I'll be getting in 4 analog inputs which correspond to different physical parameters in the experiment and based on the measurements I send 4 analog output signals for feedback control. I have a few questions and it would be great if somebody can answer them.

1. After reading the archives I did these calculations on what I can expect in terms of the maximum signal frequencies that I can record from the system. The bottleneck is the USB, hence Max data rate of USB  = 32 MB/s = 16 MS/s = 2MS/s per I and Q channel = 2 MS/s per input channel assuming that I set all Qs to zero and route ADC0 to I0, ADC1 to I1 etc. This means that the maximum signal frequency that I can get is 1 MHz. Is this calculation correct? By setting all Qs to zero I am wasting half the bandwidth. Is there a way by which I can increase the max signal bandwidth? For example instead of setting Q to zero if I could alternate samples between I and Q channels, I can make use of the full USB bandwidth. 

2. What is the number of samples that is stored on the on board FIFO? After reading the archives it seems that the number is 32k but I am not sure of this. I would be great is somebody can clarify this. 

3. Does USRP use DMA for transferring data or is the data written to disk through the USB every time? 

Thanks!
Karthik