[Discuss-gnuradio] Computing Power Spectra

[Scott Johnston]

Hello,

I wrote a simple grc graph to compute power spectra of the output of the 
usrp. My test setup is a signal generator outputting a tone at the same 
frequency that I set as the usrp center frequency, directly connected to 
the usrp. But when I examine the output in Matlab, the first vector is 
exactly what I expect, a peak at the center and then the noise floor. 
The following vectors show the peak at lower and lower power levels 
until, after 10 or so, it disappears into the noise. My code is 
attached. Anybody know what could be causing this?

Any help would be appreciated.

Scott

--
Scott Johnston
MIT Lincoln Laboratory
244 Wood Street, Lexington, MA 02420-9108
(781) 981-8196
address@hidden

#!/usr/bin/env python
##################################################
# Gnuradio Python Flow Graph
# Title: Logpowertest
# Generated: Tue Mar 15 17:36:43 2011
##################################################

from gnuradio import blks2
from gnuradio import eng_notation
from gnuradio import gr
from gnuradio import uhd
from gnuradio import window
from gnuradio.eng_option import eng_option
from gnuradio.gr import firdes
from optparse import OptionParser

class logpowertest(gr.top_block):

        def __init__(self):
                gr.top_block.__init__(self, "Logpowertest")

                ##################################################
                # Variables
                ##################################################
                self.samp_rate = samp_rate = 32000

                ##################################################
                # Blocks
                ##################################################
                self.blks2_stream_to_vector_decimator_0 = 
blks2.stream_to_vector_decimator(
                        item_size=gr.sizeof_gr_complex,
                        sample_rate=samp_rate,
                        vec_rate=30,
                        vec_len=1024,
                )
                self.gr_complex_to_mag_squared_0 = 
gr.complex_to_mag_squared(1024)
                self.gr_fft_vxx_0 = gr.fft_vcc(1024, True, 
(window.blackmanharris(1024)), True)
                self.gr_file_sink_0 = gr.file_sink(gr.sizeof_float*1024, 
"/home/user/Desktop/logpowertest.bin")
                self.gr_file_sink_0.set_unbuffered(False)
                self.gr_nlog10_ff_0 = gr.nlog10_ff(10, 1024, 0)
                self.uhd_single_usrp_source_0 = uhd.single_usrp_source(
                        device_addr="addr=192.168.10.2",
                        io_type=uhd.io_type.COMPLEX_FLOAT32,
                        num_channels=1,
                )
                
self.uhd_single_usrp_source_0.set_clock_config(uhd.clock_config.external());
                self.uhd_single_usrp_source_0.set_samp_rate(.2e6)
                self.uhd_single_usrp_source_0.set_center_freq(800e6, 0)
                self.uhd_single_usrp_source_0.set_gain(0, 0)

                ##################################################
                # Connections
                ##################################################
                self.connect((self.gr_fft_vxx_0, 0), 
(self.gr_complex_to_mag_squared_0, 0))
                self.connect((self.gr_complex_to_mag_squared_0, 0), 
(self.gr_nlog10_ff_0, 0))
                self.connect((self.uhd_single_usrp_source_0, 0), 
(self.blks2_stream_to_vector_decimator_0, 0))
                self.connect((self.blks2_stream_to_vector_decimator_0, 0), 
(self.gr_fft_vxx_0, 0))
                self.connect((self.gr_nlog10_ff_0, 0), (self.gr_file_sink_0, 0))

        def set_samp_rate(self, samp_rate):
                self.samp_rate = samp_rate
                
self.blks2_stream_to_vector_decimator_0.set_sample_rate(self.samp_rate)

if __name__ == '__main__':
        parser = OptionParser(option_class=eng_option, usage="%prog: [options]")
        (options, args) = parser.parse_args()
        if gr.enable_realtime_scheduling() != gr.RT_OK:
                print "Error: failed to enable realtime scheduling."
        tb = logpowertest()
        tb.start()
        raw_input('Press Enter to quit: ')
        tb.stop()

<?xml version='1.0' encoding='ASCII'?>
<flow_graph>
  <timestamp>Tue Mar 15 17:36:42 2011</timestamp>
  <block>
    <key>variable</key>
    <param>
      <key>id</key>
      <value>samp_rate</value>
    </param>
    <param>
      <key>_enabled</key>
      <value>True</value>
    </param>
    <param>
      <key>value</key>
      <value>32000</value>
    </param>
    <param>
      <key>_coordinate</key>
      <value>(10, 170)</value>
    </param>
    <param>
      <key>_rotation</key>
      <value>0</value>
    </param>
  </block>
  <block>
    <key>gr_complex_to_mag_squared</key>
    <param>
      <key>id</key>
      <value>gr_complex_to_mag_squared_0</value>
    </param>
    <param>
      <key>_enabled</key>
      <value>True</value>
    </param>
    <param>
      <key>vlen</key>
      <value>1024</value>
    </param>
    <param>
      <key>_coordinate</key>
      <value>(910, 187)</value>
    </param>
    <param>
      <key>_rotation</key>
      <value>0</value>
    </param>
  </block>
  <block>
    <key>uhd_single_usrp_source</key>
    <param>
      <key>id</key>
      <value>uhd_single_usrp_source_0</value>
    </param>
    <param>
      <key>_enabled</key>
      <value>True</value>
    </param>
    <param>
      <key>type</key>
      <value>complex</value>
    </param>
    <param>
      <key>nchan</key>
      <value>1</value>
    </param>
    <param>
      <key>dev_addr</key>
      <value>addr=192.168.10.2</value>
    </param>
    <param>
      <key>ref_clk</key>
      <value>ext</value>
    </param>
    <param>
      <key>sd_spec</key>
      <value></value>
    </param>
    <param>
      <key>samp_rate</key>
      <value>.2e6</value>
    </param>
    <param>
      <key>center_freq0</key>
      <value>800e6</value>
    </param>
    <param>
      <key>gain0</key>
      <value>0</value>
    </param>
    <param>
      <key>ant0</key>
      <value></value>
    </param>
    <param>
      <key>bw0</key>
      <value>0</value>
    </param>
    <param>
      <key>center_freq1</key>
      <value>0</value>
    </param>
    <param>
      <key>gain1</key>
      <value>0</value>
    </param>
    <param>
      <key>ant1</key>
      <value></value>
    </param>
    <param>
      <key>bw1</key>
      <value>0</value>
    </param>
    <param>
      <key>center_freq2</key>
      <value>0</value>
    </param>
    <param>
      <key>gain2</key>
      <value>0</value>
    </param>
    <param>
      <key>ant2</key>
      <value></value>
    </param>
    <param>
      <key>bw2</key>
      <value>0</value>
    </param>
    <param>
      <key>center_freq3</key>
      <value>0</value>
    </param>
    <param>
      <key>gain3</key>
      <value>0</value>
    </param>
    <param>
      <key>ant3</key>
      <value></value>
    </param>
    <param>
      <key>bw3</key>
      <value>0</value>
    </param>
    <param>
      <key>_coordinate</key>
      <value>(182, 75)</value>
    </param>
    <param>
      <key>_rotation</key>
      <value>0</value>
    </param>
  </block>
  <block>
    <key>gr_fft_vxx</key>
    <param>
      <key>id</key>
      <value>gr_fft_vxx_0</value>
    </param>
    <param>
      <key>_enabled</key>
      <value>True</value>
    </param>
    <param>
      <key>type</key>
      <value>complex</value>
    </param>
    <param>
      <key>fft_size</key>
      <value>1024</value>
    </param>
    <param>
      <key>forward</key>
      <value>True</value>
    </param>
    <param>
      <key>window</key>
      <value>window.blackmanharris(1024)</value>
    </param>
    <param>
      <key>shift</key>
      <value>True</value>
    </param>
    <param>
      <key>_coordinate</key>
      <value>(654, 83)</value>
    </param>
    <param>
      <key>_rotation</key>
      <value>0</value>
    </param>
  </block>
  <block>
    <key>gr_nlog10_ff</key>
    <param>
      <key>id</key>
      <value>gr_nlog10_ff_0</value>
    </param>
    <param>
      <key>_enabled</key>
      <value>True</value>
    </param>
    <param>
      <key>n</key>
      <value>10</value>
    </param>
    <param>
      <key>k</key>
      <value>0</value>
    </param>
    <param>
      <key>vlen</key>
      <value>1024</value>
    </param>
    <param>
      <key>_coordinate</key>
      <value>(914, 266)</value>
    </param>
    <param>
      <key>_rotation</key>
      <value>0</value>
    </param>
  </block>
  <block>
    <key>gr_file_sink</key>
    <param>
      <key>id</key>
      <value>gr_file_sink_0</value>
    </param>
    <param>
      <key>_enabled</key>
      <value>True</value>
    </param>
    <param>
      <key>file</key>
      <value>/home/user/Desktop/logpowertest.bin</value>
    </param>
    <param>
      <key>type</key>
      <value>float</value>
    </param>
    <param>
      <key>vlen</key>
      <value>1024</value>
    </param>
    <param>
      <key>unbuffered</key>
      <value>False</value>
    </param>
    <param>
      <key>_coordinate</key>
      <value>(1108, 286)</value>
    </param>
    <param>
      <key>_rotation</key>
      <value>0</value>
    </param>
  </block>
  <block>
    <key>blks2_stream_to_vector_decimator</key>
    <param>
      <key>id</key>
      <value>blks2_stream_to_vector_decimator_0</value>
    </param>
    <param>
      <key>_enabled</key>
      <value>True</value>
    </param>
    <param>
      <key>type</key>
      <value>complex</value>
    </param>
    <param>
      <key>sample_rate</key>
      <value>samp_rate</value>
    </param>
    <param>
      <key>vec_rate</key>
      <value>30</value>
    </param>
    <param>
      <key>vlen</key>
      <value>1024</value>
    </param>
    <param>
      <key>_coordinate</key>
      <value>(419, 89)</value>
    </param>
    <param>
      <key>_rotation</key>
      <value>0</value>
    </param>
  </block>
  <block>
    <key>options</key>
    <param>
      <key>id</key>
      <value>logpowertest</value>
    </param>
    <param>
      <key>_enabled</key>
      <value>True</value>
    </param>
    <param>
      <key>title</key>
      <value></value>
    </param>
    <param>
      <key>author</key>
      <value></value>
    </param>
    <param>
      <key>description</key>
      <value></value>
    </param>
    <param>
      <key>window_size</key>
      <value>1280, 1024</value>
    </param>
    <param>
      <key>generate_options</key>
      <value>no_gui</value>
    </param>
    <param>
      <key>category</key>
      <value>Custom</value>
    </param>
    <param>
      <key>run_options</key>
      <value>prompt</value>
    </param>
    <param>
      <key>run</key>
      <value>True</value>
    </param>
    <param>
      <key>realtime_scheduling</key>
      <value>1</value>
    </param>
    <param>
      <key>_coordinate</key>
      <value>(10, 10)</value>
    </param>
    <param>
      <key>_rotation</key>
      <value>0</value>
    </param>
  </block>
  <connection>
    <source_block_id>gr_fft_vxx_0</source_block_id>
    <sink_block_id>gr_complex_to_mag_squared_0</sink_block_id>
    <source_key>0</source_key>
    <sink_key>0</sink_key>
  </connection>
  <connection>
    <source_block_id>gr_complex_to_mag_squared_0</source_block_id>
    <sink_block_id>gr_nlog10_ff_0</sink_block_id>
    <source_key>0</source_key>
    <sink_key>0</sink_key>
  </connection>
  <connection>
    <source_block_id>uhd_single_usrp_source_0</source_block_id>
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    <source_key>0</source_key>
    <sink_key>0</sink_key>
  </connection>
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    <source_key>0</source_key>
    <sink_key>0</sink_key>
  </connection>
  <connection>
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    <source_key>0</source_key>
    <sink_key>0</sink_key>
  </connection>
</flow_graph>