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

From: Kevin Reid
Subject: Re: [Discuss-gnuradio] QT Freqency Sink vs. Log Power FFT values
Date: Sun, 6 Aug 2017 16:21:21 -0700

On Sun, Aug 6, 2017 at 1:57 PM, John Ackermann N8UR <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 output?

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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