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Re: [Discuss-gnuradio] grc amplitude demodulation questions
From: |
Marcus D. Leech |
Subject: |
Re: [Discuss-gnuradio] grc amplitude demodulation questions |
Date: |
Fri, 14 Jan 2011 15:03:17 -0500 |
User-agent: |
Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.1.15) Gecko/20101027 Fedora/3.0.10-1.fc12 Thunderbird/3.0.10 |
On 01/14/2011 02:56 PM, address@hidden wrote:
> 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.
>
Decimation filters will have slightly different output magnitude,
depending on parameters.
You'll always have to re-null after changing decimation. Fact of life.
>
>
> 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,
>
>
>
>
My first thought is evaluation ordering in the
100e6/usrp2_decim/sw_decim. What happens if you
re-express it as: (100e6/usrp2_decim)/sw_decim?
--
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org