[Discuss-gnuradio] Fwd: Re: grc file -- display problem

[Mrs. Parvathi Piduparthi]

pls reply to below mail

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Subject: Re: [Discuss-gnuradio] grc file -- display problem
From: "Mrs.  Parvathi Piduparthi" <address@hidden>
To: address@hidden, address@hidden
Date: 20-03-2019 21:49




dear  mr marcus

Thanks for the reply . kindly see below

1.  The version of gnu radio is 3.7.10.1 as attached screenshot shows and let us know is it the x axis displaying as 'dB' is due to this version.

2. My intention was as to based on what input level it is  displaying output as -8.83dB in previous mail attached output . I could not understand the mistake you are mentioning. kindly explain again. 

3. that means with ' QT GUI frequency sink' we can be able to get the signal and need to write a python block. As we are very beginners to gnu radio and new to python block writing , request you to kindly give some examples for the same. We would basically want to record continuous  data of signal which is fed to the usrp n210 which we are  processing by means of  gnu radio.

thanks & regards
Parvathi

On Friday, 15-03-2019 at 15:54 Müller, Marcus (CEL) wrote:
> Hi Parvathi!
>
> > 1.  why the screen shot is displaying x-axis and y-axis as dB only
> i.e.  2.03dB, -8.83dB.
>
> Displaying the frequency (x-axis) in dB is most definitely a bug. Which
> version of GNU Radio are you using (`gnuradio-config-info --version`
> has the answer)? We've fixed that a while ago.
>
> Displaying the y-axis in dB is a pretty common thing to do for PSD
> estimates.
>
> > 2.   How to interpret the magnitude of the output and on what basis
>
> Not quite sure what you're asking here – that plot is a power density
> estimate over frequency. The dB are relative to "full scale".
>
> You're making mistakes when configuring the Qt GUI frequency sink –
> your center frequency is simply what the center of your x-axis labeling
> is shifted to; since you're dealing with baseband only, it needs to be
> zero.
>
> > 3. We would basically want to write the peak of the signal into a
> file to record continuously and request you to give an example for the
> same. 
>
> That sounds like you want a spectral estimator, not a visualization!
> So, you wouldn't do that with the Qt GUI frequency sink – which is just
> something to look at, not something to generate data.
>
> The operation done by the Qt GUI frequency sink is really just
>
> stream to vector (length=1024) ->
> FFT (length=1024) (including windowing) ->
> complex to magnitude^2 ->
> convert to logarithmic scale
>
> You could do the same, write a small Python block to find the bin with
> the maximum value, and write that bin's index to a file. Should be
> pretty easy, if you know Python!
>
> Generally, from a scientific point of view, it's questionable whether
> this FFT-based route is an appropriate estimator for whatever you want
> to do – in the end, you're quantizing your frequency in to FFT-length
> number of steps, and you get horrendous leakage effects if a frequency
> isn't exactly N·f_sample/L_FFT for some integer N.
>
> But: the frequency estimator suitable for your problem can not be
> advised on without knowing which problem you're trying to solve, or
> which question to answer in the end.
>
> Best regards,
> Marcus
>
> On Fri, 2019-03-15 at 09:48 +0530, Mrs.  Parvathi Piduparthi wrote:
> > hello
> > 
> > We have installed gnu radio and working with ubuntu .  Attached is the grc file and its output which basically gets a signal from source and tries to display the fft  of the same as a spectrum. W.r. to these , kindly clarify the following
> > 
> > 1.  why the screen shot is displaying x-axis and y-axis as dB only i.e.  2.03dB, -8.83dB. 
> > 
> > 2.   How to interpret the magnitude of the output and on what basis
> > 
> > 3. We would basically want to write the peak of the signal into a file to record continuously and request you to give an example for the same. 
> > 
> > thanks &  regards
> > Parvathi 
> > Scientist/Engineer 
> > National Atmoshperic Research Laboratory , Department of Space
> > Gadanki, Tirupathi
> > Andhra Pradesh 
> > India
> > 
> > Ph. 91 877 2500583
> > _______________________________________________
> > Discuss-gnuradio mailing list
> > address@hidden
> > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio

Screenshot from 2019-03-15 16-13-16.png
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