# FFT plot unit **URL:** \ **Category:** GNU Radio\ **Created:** [August 3, 2015, 3:28pm UTC](https://www.ruby-forum.com/t/fft-plot-unit/242859 "2015-08-03T15:28:27Z")\ **Posts on this page:** 8\ **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, 3:28pm UTC](https://www.ruby-forum.com/t/fft-plot-unit/242859/1 "2015-08-03T15:28:27Z")
I was able to gather results, and I am really confused with it. I generated a -30 dB signal based on the fft plot shown and transmitted it using a usrp. My spectrum analyzer received a signal at -50 dBm (-80 dB) and my receiver which also uses a usrp received the signal and plotted it at -30 dB. My question is, what is the unit of the fft plot? Is it dB 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, 3:37pm UTC](https://www.ruby-forum.com/t/fft-plot-unit/242859/2 "2015-08-03T15:37:13Z")
Hi, > I was able to gather results, and I am really confused with it. I generated > a -30 dB signal based on the fft plot shown and transmitted it using a usrp. > My spectrum analyzer received a signal at -50 dBm (-80 dB) and my receiver > which also uses a usrp received the signal and plotted it at -30 dB. My > question is, what is the unit of the fft plot? Is it dB or dBm? They’re dBFS or dB Full Scale … So they’re just relative to the “full scale” range defined in GR as -1.0 … 1.0 USRP are not calibrated instruments, so you can’t map this to dBm or any absolute power measurement. Only relative measurements are valid. Cheers, Sylvain ---
**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, 3:39pm UTC](https://www.ruby-forum.com/t/fft-plot-unit/242859/3 "2015-08-03T15:39:33Z")
Also … that’s just plain wrong : “-50 dBm (-80 dB)” dB is an unit for the ratio of two things. That’s it. dBm is the ration relative to a 1 mW power. You probably meant to say -50 dBm ( -80 dBW ) … and the ‘W’ is important, it says it’s relative to 1 W. Cheers, Sylvain ---
**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, 6:25pm UTC](https://www.ruby-forum.com/t/fft-plot-unit/242859/4 "2015-08-03T18:25:29Z")
This pops up a lot, and hence earned a spot on the FAQ a while back: [http://gnuradio.org/redmine/projects/gnuradio/wiki/FAQ#How-do-I-know-the-exact-voltagepower-of-my-received-input-signal](http://gnuradio.org/redmine/projects/gnuradio/wiki/FAQ#How-do-I-know-the-exact-voltagepower-of-my-received-input-signal) …although that section could surely be expanded. M ---
**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, 6:05pm UTC](https://www.ruby-forum.com/t/fft-plot-unit/242859/5 "2015-08-03T18:05:37Z")
On 08/03/2015 09:36 AM, Sylvain M. wrote: > In order for Gnu Radio to display calibrated power units, it would need to have a very vigorously-defined interface to each and every piece of hardware so that power levels displayed in the FFTs are in calibrated convenient power units, like dBm. This in turn, would require that every piece of hardware that connects to Gnu Radio be vigorously calibrated over their entire operating parameter space, including sample-rate, tuning frequency, and gain setting. This isn’t, as you might imagine, practical. So, what Gnu Radio receives are digitized voltage samples that are mostly-linearly-proportional to the voltage received at the antenna terminals of the device. These are in turn, for purposes of convenience and generality, converted into a floating-point number in the range {-1.0,+1.0} within a flow-graph. But without calibration on the part of the end-user, they are “unitless”, and you have to determine the proportionality in the context of your own application. ---
**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, 6:28pm UTC](https://www.ruby-forum.com/t/fft-plot-unit/242859/6 "2015-08-03T18:28:59Z")
On 08/03/2015 12:24 PM, Martin B. wrote: > This pops up a lot, and hence earned a spot on the FAQ a while back: [http://gnuradio.org/redmine/projects/gnuradio/wiki/FAQ#How-do-I-know-the-exact-voltagepower-of-my-received-input-signal](http://gnuradio.org/redmine/projects/gnuradio/wiki/FAQ#How-do-I-know-the-exact-voltagepower-of-my-received-input-signal) > …although that section could surely be expanded. > > M > I just did so 🙂 ---
**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, 6:44pm UTC](https://www.ruby-forum.com/t/fft-plot-unit/242859/7 "2015-08-03T18:44:11Z")
On 08/03/2015 12:35 PM, John Ackermann N8UR wrote: > It might be helpful to clarify that since this is a voltage ratio, > it’s 20log rather than the 10log used for power (e.g., doubling > voltage is 6dB, doubling power is 3dB), so the scaling will look > different than a typical spectrum analyzer. (It would be nice if the > instrumentation blocks could have an option to select voltage, or > power into 50 or 75 ohms…) > Yes, and we need 50 ohm and 75 ohm terminator blocks in GRC… 🙂 🙂 But, seriously, one can always prefix any instrumentation block with a scaling function in the ax + b form with a multiplier and adder to achieve whatever pleasant-and-convenient scaling one wants. ---
**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, 6:36pm UTC](https://www.ruby-forum.com/t/fft-plot-unit/242859/8 "2015-08-03T18:36:35Z")
It might be helpful to clarify that since this is a voltage ratio, it’s 20log rather than the 10log used for power (e.g., doubling voltage is 6dB, doubling power is 3dB), so the scaling will look different than a typical spectrum analyzer. (It would be nice if the instrumentation blocks could have an option to select voltage, or power into 50 or 75 ohms…)