Re: [Discuss-gnuradio] grc amplitude demodulation questions

[Marcus D. Leech]

> 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.

> 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.


--
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org