Re: [Discuss-gnuradio] grc amplitude demodulation questions

[ematlis]

Martin, thanks for responding.  See my follow ups below:
--- On Thu, 1/13/11, Marcus D. Leech <address@hidden> wrote:

> From: Marcus D. Leech <address@hidden>
> Subject: Re: [Discuss-gnuradio] grc amplitude demodulation questions
> To: address@hidden
> 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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