# GRC FFT Plot **URL:** \ **Category:** GNU Radio\ **Created:** [August 3, 2015, 4:19pm UTC](https://www.ruby-forum.com/t/grc-fft-plot/242860 "2015-08-03T16:19:52Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![Rafa\_F](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/958977/32.png) [@Rafa\_F](https://www.ruby-forum.com/u/Rafa_F)\ **Post date:** [August 3, 2015, 4:19pm UTC](https://www.ruby-forum.com/t/grc-fft-plot/242860/1 "2015-08-03T16:19:52Z")
Hi guys, I transmitted this -30 dB signal (Image 1) using a usrp: The spectrum analyzer displays -50 dBm (Image 2) as its reading. My receiver end picked up the signal with the same peak power at -30 dB (Image 3). I’m confused. How do I convert the dB power in the GRC to the received signal of the spectrum analyzer in dBm? How do I convert the Power (dB) of the FFT to dBW or dBm? 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 ---
**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 3, 2015, 4:28pm UTC](https://www.ruby-forum.com/t/grc-fft-plot/242860/2 "2015-08-03T16:28:03Z")
> I transmitted this -30 dB signal (Image 1) using a usrp: The spectrum > analyzer displays -50 dBm (Image 2) as its reading. My receiver end picked > up the signal with the same peak power at -30 dB (Image 3). I’m confused. > How do I convert the dB power in the GRC to the received signal of the > spectrum analyzer in dBm? How do I convert the Power (dB) of the FFT to dBW > or dBm? Just remove 20 … ---
**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 3, 2015, 6:33pm UTC](https://www.ruby-forum.com/t/grc-fft-plot/242860/3 "2015-08-03T18:33:40Z")
1. Send in a CW tone from a signal generator of known power (dBm) 2. Read the level from GR FFT (dB) 3. Figure out the amount you need to offset (dB) 4. Multiply your samples by a constant value of 10\*\*(offset\_dB/20.0) 5. The GR FFT will now be calibrated to an absolute power (dBm). Note if you change any settings, technically you would have to go through this again. Lou Jan wrote > Best, > > \*Gerome Jan M. Llames \* – View this message in context: [http://gnuradio.4.n7.nabble.com/GRC-FFT-Plot-tp55210p55217.html](http://gnuradio.4.n7.nabble.com/GRC-FFT-Plot-tp55210p55217.html) Sent from the GnuRadio mailing list archive at [Nabble.com](http://Nabble.com). ---
**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 3, 2015, 6:43pm UTC](https://www.ruby-forum.com/t/grc-fft-plot/242860/4 "2015-08-03T18:43:01Z")
I misread you question and thought you were receiving with the USRP. Same procedure, just don’t let the transmitted signal go past +/- 1.0 or you will clip the DAC. In practice keep it backed off 6 dB or more to allow for crest factor. Lou madengr wrote > > spectrum > > analyzer in dBm? How do I convert the Power (dB) of the FFT to dBW or > > dBm? > > > > Best, > > > > \*Gerome Jan M. Llames \* – View this message in context: [http://gnuradio.4.n7.nabble.com/GRC-FFT-Plot-tp55210p55220.html](http://gnuradio.4.n7.nabble.com/GRC-FFT-Plot-tp55210p55220.html) Sent from the GnuRadio mailing list archive at [Nabble.com](http://Nabble.com).