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Re: [Discuss-gnuradio] iir filter in spectrum analyzer

From: Marcus D. Leech
Subject: Re: [Discuss-gnuradio] iir filter in spectrum analyzer
Date: Sat, 05 Jun 2010 18:02:31 -0400
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv: Gecko/20100430 Fedora/3.0.4-2.fc12 Thunderbird/3.0.4

> Dear all,
> I have a question regarding the usage of single_pole_iir_ff filter in the
> fftsink_nongl.py which is used to construct the spectrum analyzer in
> gnuradio-3.1.3. what is the usage of this iir filter since the alfa value is
> equal to 1 which give the value of input the same as the value of the output
> (refering to the single pole iir filter explanation in
> https://radioware.nd.edu/documentation/a-dictionary-of-the-gnu-radio-blocks
> -- please fixme if i am wrong). in the previous post, firas told that this
> filter is used for the averaging purposes
> (http://old.nabble.com/question-regarding-usrp_fft.py-td24440532.html#a24466469)
> but what is the value that it average? 
> -----
> Mohd Adib Sarijari
> Universiti Teknologi Malaysia
> www.fke.utm.my
> www.utm.my
It averages the values in each of the bins.

If your FFT is 1024 wide, then you essentially have 1024 single-pole IIR
filters, each computing the
  low-pass filtered version of the individual bins.  This is called (at
by me) spectral averaging.
  There will always be a certain amount of "noise" in spectral
estimates, and for weak signals,
  averaging can bring out spectral features that would otherwise be
hidden by the randomness
  of the noise floor.

Principal Investigator
Shirleys Bay Radio Astronomy Consortium

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