USRP and gnuradio as a data acquisition system

GNU Radio
1

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
http://www.stanford.edu/~karthikv