Grc amplitude demodulation questions

GNU Radio
1

Hello-

I am using GRC 3.3 to create a flow graph using the USRP2 and a LFRX to
do amplitude demodulation. I have a few questions.

  1. What are good values for “Audio Pass” and “Audio Stop” within the “AM
    Demod” block?

  2. I observe a DC offset at the output of the demodulation. Is this
    expected, and what is the best way to remove it? I am currently using a
    high-pass filter. My incoming signal is a 27 kHz sine modulated at 1
    kHz.

  3. I am sending the result to an FFT sink. I instituted a variable
    slider to control the decimation within the AM Demod block, but when I
    change that decimation, the displayed fft trace does not adjust
    correctly to the new sample rate. In fact, the fft trace remains in a
    fixed position; the only thing that changes in the plot is the x-axis,
    ie frequency. I have a second slider for the usrp2 decimation which
    does change the displayed spectrum correctly, so that the peaks move to
    the correct position. Any thoughts?

  4. Related to the above- I thought I’d mention that if I move the usrp2
    decimation slider bar too quickly, the fft plot then fails to adjust the
    fft trace correctly - I have to move the bar slowly.

  5. Occasionally I am seeing a random error which causes the flowgraph to
    freeze:

Executing: “/home/matlis/Downloads/top_block.py”

Error: failed to enable realtime scheduling.

gr_fir_fff: using SSE
Traceback (most recent call last):
File
“/usr/local/lib64/python2.6/site-packages/gnuradio/wxgui/plotter/plotter_base.py”,
line 187, in _on_paint
for fcn in self._draw_fcns: fcn1
File
“/usr/local/lib64/python2.6/site-packages/gnuradio/wxgui/plotter/plotter_base.py”,
line 59, in draw
self._draw()
File
“/usr/local/lib64/python2.6/site-packages/gnuradio/wxgui/plotter/grid_plotter_base.py”,
line 395, in _draw_p
oint_label
label_str = self._populate_point_label(x_val, y_val)
File
“/usr/local/lib64/python2.6/site-packages/gnuradio/wxgui/plotter/channel_plotter.py”,
line 149, in _populate
_point_label
y_value = (samples[x_index_high] - samples[x_index_low])*scale +
samples[x_index_low]
IndexError: index out of bounds

Done

Thanks for any suggestions!

eric

2

Hello-

I am using GRC 3.3 to create a flow graph using the USRP2 and a LFRX to do
amplitude demodulation. I have a few questions.

  1. What are good values for “Audio Pass” and “Audio Stop” within the “AM Demod”
    block?

  2. I observe a DC offset at the output of the demodulation. Is this expected,
    and what is the best way to remove it? I am currently using a high-pass filter.
    My incoming signal is a 27 kHz sine modulated at 1 kHz.
    There will always be a certain amount of DC offset in an AM demodulator,
    since it’s just a power detector, and since the RF chain always
    produces at least some noise, the detected version of that noise
    results in a small residual DC offset. You can null it out with an
    an “ADD” block.

  1. I am sending the result to an FFT sink. I instituted a variable slider to
    control the decimation within the AM Demod block, but when I change that
    decimation, the displayed fft trace does not adjust correctly to the new sample
    rate. In fact, the fft trace remains in a fixed position; the only thing that
    changes in the plot is the x-axis, ie frequency. I have a second slider for the
    usrp2 decimation which does change the displayed spectrum correctly, so that the
    peaks move to the correct position. Any thoughts?
    The FFT sink isn’t psychic. It needs to be told what the current sample
    rate is, so if you change decimation on the fly, you’ll need to
    arrange for the FFT sink to know about the new sample rate.

–
Marcus L.
Principal Investigator
Shirleys Bay Radio Astronomy Consortium