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Re: [Discuss-gnuradio] QT Freqency Sink vs. Log Power FFT values

From: John Ackermann N8UR
Subject: Re: [Discuss-gnuradio] QT Freqency Sink vs. Log Power FFT values
Date: Mon, 7 Aug 2017 11:16:45 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1


Interesting that the default value shown in the box is "2" not "2.0" -- that would have given the clue.

I tried setting the scaling value to 4.0 and that seems to align very nicely with the Frequency Sink results.

On 08/06/2017 07:21 PM, Kevin Reid wrote:
On Sun, Aug 6, 2017 at 1:57 PM, John Ackermann N8UR <address@hidden <mailto:address@hidden>> wrote:

    I also have a QT Frequency Sink as eye candy, and notice that its
    display is about 6dB different than the Log Power FFT output.  I can
    think of a number of reasons why that could be, but my simple
    question is... is there an appropriate way to adjust one or the
    other to align their readings?  (Later I'll send the log power
    results into a calibration routine to convert to dBm based on
    measured results.)

    Also while I'm at it, there's a "Reference Scale" parameter in the
    Log Power FFT, set by default to "2".  Is there a way to use this
    parameter to align the two readings?  I tried changing it to "1" but
    that threw an error at runtime.

I took a look at logpwrfft's code and the first thing that is done to that number is ref_scale/2, so you need to pass in a float (1.0) rather than an integer to not get it truncated to zero. Sounds like something to fix.

    Also, there doesn't appear to be any window applied (or at least
    apparent) in the Log Power FFT block.  Is there a way to window the

The GRC definition for the block doesn't let you set it but there actually is a parameter win which defaults to Blackman-Harris.

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