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

From: Marcus D. Leech
Subject: Re: [Discuss-gnuradio] PSD in dBm/Hz
Date: Fri, 06 Mar 2015 10:33:03 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.2.0

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
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.


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
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.

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