# Re: GRC FFT Plot

**URL:** <https://www.ruby-forum.com/t/re-grc-fft-plot/242879>\
**Category:** GNU Radio\
**Created:** [August 4, 2015, 5:32pm UTC](https://www.ruby-forum.com/t/re-grc-fft-plot/242879 "2015-08-04T17:32:21Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![Sadaf\_N](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/858c86/32.png) [@Sadaf\_N](https://www.ruby-forum.com/u/Sadaf_N)\
**Post date:** [August 4, 2015, 5:32pm UTC](https://www.ruby-forum.com/t/re-grc-fft-plot/242879/1 "2015-08-04T17:32:21Z")

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Actually, you were right at first madengr. My transmitter is USRP1 with  
WBX  
50-2200 while my receiver is USRP N200 with TVRX2 and spectrum analyzer.  
Can you give me suggestions on how I can match the amplitude on both the  
spectrum analyzer and the USRP’s? Attached is a screenshot of my  
experiment  
on our lab.

Using just the GRC Signal Source and Noise Source, I was able to display  
the desired power amplitude in dBW with less than 10 dB loss by varying  
the  
amplitude of each block. For the Signal Source block, the amplitude was  
6.31 exp -8 (-144 dBW). For the Noise Source block, the amplitude was 2  
exp  
-8 (-154 dBW). GRC FFT produces -137 dB and -159 dB respectively.  
Therefore  
I can say that these dB values in the FFT plot are in dBW.

Best,

\*Gerome Jan M. Llames \*  
Engineering Research and Development for Technology (ERDT) Scholar  
University of San Carlos - Technological Campus  
Nasipit Talamban, Cebu City, Philippines, 6000  
Mobile: +639271525124  
Email: removed\_email\_address@domain.invalid

“Design is not just what it looks like and feels like. Design is how it  
works.” - Steve Jobs

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<div class="post-metadata">

**Author:** ![Gerome\_Jan\_L](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/ee59a6/32.png) [@Gerome\_Jan\_L](https://www.ruby-forum.com/u/Gerome_Jan_L)\
**Post date:** [August 4, 2015, 7:26pm UTC](https://www.ruby-forum.com/t/re-grc-fft-plot/242879/2 "2015-08-04T19:26:52Z")

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Same procedure I outlined before. You need to feed a CW tone (via  
cable)  
directly into the USRP and spectrum analyzer, compare spectrum  
amplitudes,  
and multiply the USRP samples by 10\*\*(offset\_dB/20.0). Noise or a  
modulated  
signal makes things more difficult since spectrum analyzer must be put  
in  
noise marker mode. Just use a CW tone.  
Lou

Jan wrote

> the  
> amplitude of each block. For the Signal Source block, the amplitude was  
> 6.31 exp -8 (-144 dBW). For the Noise Source block, the amplitude was 2  
> exp  
> -8 (-154 dBW). GRC FFT produces -137 dB and -159 dB respectively.  
> Therefore  
> I can say that these dB values in the FFT plot are in dBW.
> 
> Best,
> 
> \*Gerome Jan M. Llames \*

–  
View this message in context:  
[http://gnuradio.4.n7.nabble.com/GRC-FFT-Plot-tp55210p55258.html](http://gnuradio.4.n7.nabble.com/GRC-FFT-Plot-tp55210p55258.html)  
Sent from the GnuRadio mailing list archive at [Nabble.com](http://Nabble.com).