Re: [Discuss-gnuradio] PSD in dBm/Hz

[Marcus D. Leech]

On 03/06/2015 10:12 AM, Marcus Müller wrote:
> 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
>
> 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:
>
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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.