[Discuss-gnuradio] no signal from LFTX when data sent from gr.file_source

[Piotr Swiatkiewicz]

Hi all,
 
I have been testing USRP and Gnu Radio for a while and it's been a very 
interesting, educational experience.
One of the important things for me is to get capture to / playback from file 
capability to work.
I simplified usrp_rx_cfile.py a bit and essentially what my script does is:
 
usrp.source_c --> gr.head --> gr.file_sink
 
It produces huge files (with complex samples, I understand) which I intend to 
"play" back through LFTX
daughterboard. So, another script does something like:
 
gr.file_source (gr.sizeof_complex, fname,true)
\-----> usrp.sink_c (0, interp)
\-----> fftsink2.fft_sink_c
 
Gain on the LFTX is set to -1.02 dB - as read by self.u.pga(0). The problem is 
that I don’t get any signal
from LFTX although I can see the expected spectrum in the FFT window (which is 
connected in parallel
to the same stream). The signal is digital radio transmission with estimated 
S/N around 40 dB.
Since the spectrum matches what I had seen when tuning the radio with some 
receiver script which also
provided FFT window, I assume data stream is OK. Just to check my hardware I 
tried usrp_siggen.py
and checked the outputs of LFTX with an oscilloscope. I could see the expected 
sine wave nicely
phase shifted by pi/2. Hardware is OK, but somehow my “playback” script doesn’t 
want to push data out.
Do you have any suggestions?
 
Thanks,
Peter