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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:1.9.1.9) 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
http://www.sbrac.org