Martin, thanks for responding. See my follow ups below:
--- On Thu, 1/13/11, Marcus D. Leech <mle...@ripnet.com> wrote:
> From: Marcus D. Leech <mle...@ripnet.com>
> Subject: Re: [Discuss-gnuradio] grc amplitude demodulation questions
> To: discuss-gnuradio@gnu.org
> Date: Thursday, January 13, 2011, 4:26 PM
> > 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.
I've tried an ADD block, but I noticed that if my decimation is not a power of
2, the offset changes.
>
> > 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?
> 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.
I believe the code is already doing this. Here are code segments for 2 fft
guis. The first shows the fft of the raw output of the USRP2. This gui
responds correctly. Notice the line: "sample_rate=100e6/usrp2_decim", so I
assume that as I dynamically adjust usrp2_decim, this value is being updated.
However, with the second fft, while there is a similar line:
"sample_rate=100e6/usrp2_decim/sw_decim" that includes the software decimation
value, this fft gui does not correct the position of the fft when I dynamically
change sw_decim. The x-axis DOES change, but the peaks in the spectrum just
stay fixed in their relative positions. Any thoughts?
1st FFT: self.wxgui_fftsink2_0 = fftsink2.fft_sink_c(
self.notebook_0.GetPage(1).GetWin(),
baseband_freq=0,
y_per_div=20,
y_divs=10,
ref_level=10,
ref_scale=2.0,
sample_rate=100e6/usrp2_decim,
fft_size=1024,
fft_rate=10,
average=False,
avg_alpha=None,
title="Total Signal FFT",
peak_hold=False,
)
2nd FFT: self.wxgui_fftsink2_1 = fftsink2.fft_sink_f(
self.notebook_0.GetPage(3).GetWin(),
baseband_freq=0,
y_per_div=20,
y_divs=10,
ref_level=10,
ref_scale=2.0,
sample_rate=100e6/usrp2_decim/sw_decim,
fft_size=1024,
fft_rate=10,
average=False,
avg_alpha=None,
title="Demodulated FFT Plot",
peak_hold=False,
>
>
> -- Marcus Leech
> Principal Investigator
> Shirleys Bay Radio Astronomy Consortium
> http://www.sbrac.org
>
>
>
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