[Discuss-gnuradio] Getting started with the USRP/RFX2400 and underflows

[Jason Uher]

I am currently trying to get some basic code working to test that my
shiny new USRP is working correctly.  I have been going over the
code/examples/list archives for a few weeks on and off, but most of
the code examples are for the basic RX/TX boards, and I only have two
RFX2400's.

My testing plan was to use the A side to send out some unmodulated
data (which should be the same as OOK? right?) and receive it at the B
side using the WA5VJB antennas, but I can't seem to get it to work.  I
have put together the following code based on the examples and what
help I could find searching the list archives.  I'm thinking it should
play a sine wave out my speakers, but all I hear is static.
Additionally, I get a lot of 'iU' in the output, I was able to find
that U meant underflow, but there was no 'i' listed in the key, just
'a' and 'u'.

So I have two questions that I hope someone has the answer to:
1) Why don't I hear a pretty sine wave?
2) What type of underflow is indicated by the letter 'i'

Thanks
Jason

Output
===================

Using TX board A: Flex 2400 Tx MIMO B
Using RX board B: Flex 2400 Rx MIMO B
>>> gr_fir_fff: using SSE
Press Enter to quit: uUiUiUiU(Lots more iU)

===================
Code

from gnuradio import gr
from gnuradio import audio
from gnuradio import usrp

def build_graph ():


  #Parametrs
  tx_inter_rate = 512
  trans_freq = 24e8

  rx_decim_rate = 128;


  #Create the flow graph
  fg = gr.top_block ()


  #Create the USRP sink (transmitter)

  ut = usrp.sink_c (0, tx_inter_rate)

  #First choose the transmitting device
  #tx_sd = usrp.pick_tx_subdevice(ut)
  tx_sd = (0,0) #manually choose the correct device

  m = usrp.determine_tx_mux_value(ut, tx_sd)
  ut.set_mux(m)
  t_subdev = usrp.selected_subdev(ut, tx_sd)
  print "Using TX board %s" % (t_subdev.side_and_name(),)

  #Transmit at the maximum possible gain
  t_subdev.set_gain(t_subdev.gain_range()[1])

  #Set the transmit frequency and check that it worked
  r = ut.tune(t_subdev._which, t_subdev, trans_freq)

  if not r:
    sys.stderr.write('Failed to set RF transmit frequency\n')
    raise SystemExit

  #Turn on the transmitter
  t_subdev.set_enable(True)


  #Connect the sink to a SIN source
  src0 = gr.sig_source_c (48000, gr.GR_SIN_WAVE, 350, 0.5)
  fg.connect (src0, ut)



  #Create the USRP Source (receiver)
  ur = usrp.source_c(0,rx_decim_rate)

  #Choose the recieving device
  rx_sd = (1,0)
  m = usrp.determine_rx_mux_value(ur, rx_sd)
  ur.set_mux(m)
  r_subdev = usrp.selected_subdev(ur, rx_sd)
  print "Using RX board %s" % (r_subdev.side_and_name(),)

  #Set the recieve frequency
  r = ur.tune(r_subdev._which, r_subdev, trans_freq)

  if not r:
    sys.stderr.write('Failed to set RF recieve frequency\n')
    raise SystemExit

  #Make sure we are using the RX
  r_subdev.select_rx_antenna( 'RX2')

  #Convert complex data to sound samples
  conv2 = gr.complex_to_float ()

  #Decimate the received signal to soundcard rates
  dec_filt = gr.fir_filter_fff(5,(1.0, 1.0, 1.0));


  #Write received data to the soundcard
  dst = audio.sink (48000)

  #Connect the Source to the audio sink
  fg.connect (ur,conv2)
  fg.connect (conv2,dec_filt)
  fg.connect (dec_filt,dst)


  return fg

if __name__ == '__main__':
    fg = build_graph ()
    fg.start ()
    raw_input ('Press Enter to quit: ')
    fg.stop ()