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

[Michael Thelen DK4MT]

Am 15.08.2015 um 01:55 schrieb 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.
> 
> 
Hi,
yes ok that is clear so far. And this his how it is described in the
documentation tab. The question arose, because in the tutorial I cannot
explain how the adjustment and the result go together in the pictures.
If you look at the flowgraph in the tutorial the bandwidth is set to
48k, but the X-axis labels in the Waterfall GUI go from -50kHz to 50kHz
implying that the bandwidth might have been set to 100k. I am just
trying to get the basics right.

BR
Mike