[Discuss-gnuradio] problem with USRP/FFT sink in GRC

[Anastasopoulos Achilleas]

Hi,

I am running a simple graph in GRC-0.65 with complex USRP as source
(with TVRX at the B side) and a complex FFT as a sink.
What I see is not expected: I see the FFT of a real signal
(symmetric around 0).

When I run the python example usrp_fft.py I get
the expected result.
./usrp_fft.py -RB -f 93.1e6 -d 16

I attach the grc graph for anyone who wants to take a look at it
and give me some hint as to what I might be doing wrong.

Thanks
Achilleas
<?xml version='1.0' encoding='UTF-8'?>
<flow_graph>
  <timestamp>1177702644.69</timestamp>
  <hostname>jefe.eecs.umich.edu</hostname>
  <version>0.65</version>
  <valid>True</valid>
  <window_width>1600</window_width>
  <window_height>1200</window_height>
  <vars>
    <var>
      <key>decimation</key>
      <value>16</value>
      <min/>
      <max/>
      <step/>
    </var>
  </vars>
  <signal_blocks>
    <signal_block>
      <tag>About</tag>
      <id>About0</id>
      <x_coordinate>20</x_coordinate>
      <y_coordinate>20</y_coordinate>
      <rotation>0</rotation>
      <params>
        <param>Default Flow Graph</param>
        <param>Your Name</param>
      </params>
    </signal_block>
    <signal_block>
      <tag>Note</tag>
      <id>Note0</id>
      <x_coordinate>20</x_coordinate>
      <y_coordinate>100</y_coordinate>
      <rotation>0</rotation>
      <params>
        <param>Fill this flow graph</param>
      </params>
    </signal_block>
    <signal_block>
      <tag>USRP Source</tag>
      <id>USRP Source0</id>
      <x_coordinate>51</x_coordinate>
      <y_coordinate>244</y_coordinate>
      <rotation>0</rotation>
      <params>
        <param>0</param>
        <param>0</param>
        <param>2</param>
        <param>93.1e6</param>
        <param>$decimation</param>
        <param>80</param>
        <param/>
      </params>
    </signal_block>
    <signal_block>
      <tag>FFT Sink</tag>
      <id>FFT Sink0</id>
      <x_coordinate>483</x_coordinate>
      <y_coordinate>235</y_coordinate>
      <rotation>0</rotation>
      <params>
        <param>0</param>
        <param>FFT</param>
        <param>64e6/$decimation</param>
        <param>10</param>
        <param>50</param>
        <param>1024</param>
        <param>15</param>
        <param>0</param>
      </params>
    </signal_block>
  </signal_blocks>
  <connections>
    <connection>
      <input_signal_block_id>FFT Sink0</input_signal_block_id>
      <input_socket_index>0</input_socket_index>
      <output_signal_block_id>USRP Source0</output_signal_block_id>
      <output_socket_index>0</output_socket_index>
    </connection>
  </connections>
</flow_graph>

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