Re: [Discuss-gnuradio] Tutorial 2.4.5 The Singing Sine Wave

[Marcus D. Leech]

On 08/14/2015 04:52 PM, Michael Thelen DK4MT wrote:

Hi,

I have two questions to the above mentioned Tutorial chapter, since I
realized two differences, even though I think I set it up the same.

a) To get an acceptable view in the Waterfall graph I had to adjust the
frequency to a fourth of the suggested value in the tutorial picture.
Please compare in my attachment Flowgraph_2-4-5.png, where the value is
marked with a red arrow. Why could that be?

b) How could it be managed to set the X-axis of the Waterfall graph from
-50 to 50 in the picture of the tutorial? In the documentation is stated
that this is related to the bandwidth value of the GUI Waterfall item.
So I could only change this by setting the bandwidth to 100k. But than
the maximum frequency is shown as 40kHz instead of 20kHz and the
triangle pattern stays the same. This does not seem correct, as the
frequency reading would be wrong. Please look at Waterfall_2-4-5.png.
Again marked with red arrows.

Refers to:
http://gnuradio.org/redmine/projects/gnuradio/wiki/Guided_Tutorial_GRC
-- Scroll to 2.4.5

Best regards
Mike

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The "bandwidth" setting on the waterfall is just to allow it compute the X-axis legend along the bottom, which in your attached pics shows -24kHz to +24kHz,
  although the last actual numeric marker in each direction is +/-20kHz.

Again, if you set the bandwidth to 100kHz in the waterfall, that will dutifully calculate the markers to show +/-50kHz.  But setting the "bandwidth" in the
  waterfall does *nothing* about the actual delivered sample rate.  It's just a hint to the plotter about the bandwidth of your signal.  Nearly the entirety
  of Gnu Radio doesn't really "know" anything about sample rates--they are an artifice that only becomes "real" at the edges with actual hardware, and
  also as a convenience when calculating filter parameters.