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Re: [Discuss-gnuradio] USRP Input Calibration / CIC Filter Response

From: Eric Blossom
Subject: Re: [Discuss-gnuradio] USRP Input Calibration / CIC Filter Response
Date: Mon, 23 Mar 2009 14:03:14 -0700
User-agent: Mutt/1.5.18 (2008-05-17)

On Mon, Mar 23, 2009 at 10:44:21AM -0700, Eric Blossom wrote:
> On Mon, Mar 23, 2009 at 12:58:17PM -0400, Erich Stuntebeck wrote:
> > The daughterboard gain was set to the mid-point.
> >
> > I don't have the range of the time domain samples as I did the FFT  
> > processing in real-time and discarded the raw data. The data was  
> > collected using a script I wrote to link the USRP block with a custom  
> > processing block that I wrote. This custom block take the FFT of the  
> > incoming data and outputs the amplitude of the appropriate bin. It  
> > determines the appropriate bin based on a given frequency as an input,  
> > and searches a few neighboring bins in case of clock errors to select  
> > the bin nearby with the maximum amplitude.
> >
> > I'm using the firmware without the half-band filter.
> >
> > The signal generator is an Agilent 33220A. I don't have a spectrum  
> > analyzer, but I do have two of the 33220A's and they both perform the  
> > same way so I'm guessing its not a calibration issue with the signal  
> > generator.

A person who desires to remain anonymous passes on this comment:

    eric, i saw your response on the mailing list (below). i think the
    problem has nothing to do with gain.

    in my opinion the mistake is that erich is using FFT without applying
    a window-function first. in other words, he's using a 'rectangle'
    window. a rectangle window is a bad choice for amplitude flatness
    measurements, because the amplitude response depends on whether the
    signal is at bin center, or between 2 bins (the worst case). there is
    a difference of ~3.7dB between these two scenarios.

    if he wants to measure flatness, he should use a window function which
    is designed to give the same amplitude response for any frequency, be
    it bin-centered of not. a 'flattop' window (google for it) is
    recommended in that case - it has 0.02dB flatness.

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