Re: [Discuss-gnuradio] grc amplitude demodulation questions

[Marcus D. Leech]

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