# PSD in dBm/Hz **URL:** \ **Category:** GNU Radio\ **Created:** [March 5, 2015, 8:45am UTC](https://www.ruby-forum.com/t/psd-in-dbm-hz/234449 "2015-03-05T08:45:37Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![addis\_a](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c4cdca/32.png) [@addis\_a](https://www.ruby-forum.com/u/addis_a)\ **Post date:** [March 5, 2015, 8:45am UTC](https://www.ruby-forum.com/t/psd-in-dbm-hz/234449/1 "2015-03-05T08:45:37Z")
Dear Sir, I am using USRP to receive signal, and then plot the signal using “WX GUI FFT Sink”. I hope to get the vertical axis as “Power Spectral Density in dBm/Hz”. Is there any way to do that? Regards. ---
**Author:** ![activecat\_k](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c57346/32.png) [@activecat\_k](https://www.ruby-forum.com/u/activecat_k)\ **Post date:** [March 5, 2015, 4:20pm UTC](https://www.ruby-forum.com/t/psd-in-dbm-hz/234449/2 "2015-03-05T16:20:02Z")
Do you mean adding a text label on the vertical axis, or scaling the spectrum? For the latter, you would have to feed the USRP a known (calibrated) noise source that is well above the USRP noise floor such as an HP 346 noise source. Then read the value from the spectrum and just re-scale the samples to your known PSD. If you change the FFT bin size, decimation, tuning, etc, you may need to re-scale. I suppose you could also feed the USRP with a CW tone of known power to figure out it’s gain, then terminate the input with 50 ohms (i.e. -174 dBm/Hz) and measure the power (which would be -174 dB/Hz + NF + gain). In either case, you need a known power source to convert the dB to dBm, and noise to fill your bandwidth to get the 1/Hz. Lou KD4HSO activecat wrote > I am using USRP to receive signal, and then plot the signal using “WX GUI > FFT Sink”. > I hope to get the vertical axis as “Power Spectral Density in dBm/Hz”. > Is there any way to do that? – View this message in context: [http://gnuradio.4.n7.nabble.com/PSD-in-dBm-Hz-tp52572p52578.html](http://gnuradio.4.n7.nabble.com/PSD-in-dBm-Hz-tp52572p52578.html) Sent from the GnuRadio mailing list archive at [Nabble.com](http://Nabble.com). ---
**Author:** ![activecat\_k](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c57346/32.png) [@activecat\_k](https://www.ruby-forum.com/u/activecat_k)\ **Post date:** [March 5, 2015, 4:23pm UTC](https://www.ruby-forum.com/t/psd-in-dbm-hz/234449/3 "2015-03-05T16:23:18Z")
Activecat, this is actually on our FAQ: [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) This is a very difficult problem to solve and it requires expensive lab equipment. Regards, Martin ---
**Author:** ![activecat\_k](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c57346/32.png) [@activecat\_k](https://www.ruby-forum.com/u/activecat_k)\ **Post date:** [March 6, 2015, 2:56pm UTC](https://www.ruby-forum.com/t/psd-in-dbm-hz/234449/4 "2015-03-06T14:56:46Z")
Dear Sirs, Thanks for the answers. Let me further elaborate. I could get what I want using Matlab, via below steps: 1). Using GRC flowgraph, connect the USRP Source to a File Sink. 2). Copy the file (says, test02.dat) to a PC running Matlab. 3). Using Matlab, run these commands: a). c = read\_complex\_binary(‘test02.dat’) b). figure; periodogram( c(1:100000), [], [], 10e6 ) # samp\_rate is 10e6 In this case I am able to get a PSD plot. The question is, how to produce the PSD plot using only gnuradio (without Matlab) ? Thank you very much. ---
**Author:** ![activecat\_k](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c57346/32.png) [@activecat\_k](https://www.ruby-forum.com/u/activecat_k)\ **Post date:** [March 6, 2015, 4:18pm UTC](https://www.ruby-forum.com/t/psd-in-dbm-hz/234449/5 "2015-03-06T16:18:09Z")
Equivalent function here, which could be encapsulated into a GR block if you don’t need speed: [http://docs.scipy.org/doc/scipy-dev/reference/generated/scipy.signal.periodogram.html](http://docs.scipy.org/doc/scipy-dev/reference/generated/scipy.signal.periodogram.html) I suppose it boils down to the difference between an FFT just normalized to bandwidth, and the FFT of the autocorrelation; the latter is the true definition of PSD. I learned this once 20 years ago (power signals vs. energy signals) but I don’t remember anymore. Maybe someone can chime in. Lou activecat wrote > samp\_rate is 10e6 > > In this case I am able to get a PSD plot. > The question is, how to produce the PSD plot using only gnuradio (without > Matlab) ? > > Thank you very much. – View this message in context: [http://gnuradio.4.n7.nabble.com/PSD-in-dBm-Hz-tp52572p52596.html](http://gnuradio.4.n7.nabble.com/PSD-in-dBm-Hz-tp52572p52596.html) Sent from the GnuRadio mailing list archive at [Nabble.com](http://Nabble.com). ---
**Author:** ![activecat\_k](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c57346/32.png) [@activecat\_k](https://www.ruby-forum.com/u/activecat_k)\ **Post date:** [March 6, 2015, 4:14pm UTC](https://www.ruby-forum.com/t/psd-in-dbm-hz/234449/6 "2015-03-06T16:14:02Z")
Hi Activecat, I still don’t understand; do you really just want a offline data periodogram? In that case, have a look at the python “matplotlib” library examples. Also, Martin’s and Lou’s statements still stand: just by labeling an axis dBm doesn’t make the data actually represent that physical entity. Greetings, Marcus ---
**Author:** ![activecat\_k](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c57346/32.png) [@activecat\_k](https://www.ruby-forum.com/u/activecat_k)\ **Post date:** [March 6, 2015, 4:34pm UTC](https://www.ruby-forum.com/t/psd-in-dbm-hz/234449/7 "2015-03-06T16:34:35Z")
The gr-specest toolbox has all sorts of spectrum estimation algorithms, but they also don’t calibrate your device. A periodogram would be in there, called ‘welch’. M ---
**Author:** ![activecat\_k](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c57346/32.png) [@activecat\_k](https://www.ruby-forum.com/u/activecat_k)\ **Post date:** [March 6, 2015, 4:34pm UTC](https://www.ruby-forum.com/t/psd-in-dbm-hz/234449/8 "2015-03-06T16:34:35Z")
On 03/06/2015 10:12 AM, Marcus M. wrote: > On 03/06/2015 02:54 PM, Activecat wrote: > > > Thanks for the answers. Let me further elaborate. > > > > I could get what I want using Matlab, via below steps: > > * * * > > Discuss-gnuradio mailing list > [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) > [Discuss-gnuradio Info Page](https://lists.gnu.org/mailman/listinfo/discuss-gnuradio) > So, I think what is wanted is some version of: signal—\>window–\>FFT—\>scaling-of-some-sort(both linear and non-linear)—\>average—\>display GR can do all of those things, and the FFT “instrumentation” widgets are a version of the above. But as several have pointed out, and I’m about to repeat, without _calibration_ you only know what the results mean in a _relative_ sense. The instantaneous voltages as seen by the ADC have been manipulated by all the analog “goo” in front of the ADC, including amounts of gain that aren’t precisely known. Further, the ADC _output_ is filtered (via decimation) before you see it, which should be a largely-linear operation, but you don’t know the exact gain of that transform. So, the only way to get precise numbers in dBm/Hz is to use laboratory calibration measurements so that you can refer this to “at the plane of the antenna connector”. That’s the only reasonable way, and it’s the way that laboratory grade things like spectrum analyzers deliver fairly-precise numbers–not because of mathematical “tricks”, but because they are _calibrated, regularly_ against calibration standards.