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Re: [Discuss-gnuradio] QT Freq Sink Display not centering correctly?

From: Kevin Reid
Subject: Re: [Discuss-gnuradio] QT Freq Sink Display not centering correctly?
Date: Mon, 18 May 2015 08:37:26 -0700

On May 15, 2015, at 12:07, Jason Noble <address@hidden> wrote:

> So, I first noticed this problem with my main GNURadio install (3.7.5 on 
> Debian Jessie) but I've reproduced it on a brand new live USB.
> Basically I'm looking at two frequency sinks. One of them centered at 0 and 
> one centered at 16kHz. I have a single signal source input to both sinks, 
> starting at 4kHz but adjustable up to 32kHz. My expectation is that the 
> initial 4kHz signal will display at the far left of the 16kHz-centered 
> sink....but it doesn't. It displays as a 20kHz signal instead, to right of 
> center.
> I'm pretty sure that's not WAD...and if it is, can someone explain why?

The sink always displays the content of the signal you give it. The purpose of 
the center frequency setting is to adjust the frequency scale displayed in the 
widget to match the original frequencies of a downconverted signal.

The simplest use of this is to set it to the same frequency as your receiving 
hardware is tuned to, so that the frequency scale displays true RF-spectrum 
frequencies rather than 0 ± bandwidth.

If you want to actually shift the way the frequency content of the signal is 
displayed, you could use the rotator_cc block before the sink.

Kevin Reid                                  <http://switchb.org/kpreid/>

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