Re: [Discuss-gnuradio] usrp_spectrum_sense and average dBm power levels

[Martin Braun (CEL)]

On Fri, Jul 06, 2012 at 01:24:12PM -0400, address@hidden wrote:
> uhd_fft *is not in any way* showing real-world dBm values -- it's merely
> showing FFT magnitude values in dB -- but it's dB relative to a
> pretty-arbitrary scaling.
> 
> Different sample rates result from different decimation values in the FPGA,
> which amounts to different amounts of *filtering* between the ADC and the
> towards-the-host data stream.
> 
> In order to get actual dBm values, you need to calibrate, at various
> frequencies, adn if you're going to be running at various sample rates, you'll
> have to calibrate for that sample-rate as well.  And the calibration will vary
> from card-to-card due to inherent variability in analog gain components.
> 
> The reason your $10K spectrum analyser shows actual dBm values is that it has
> been calibrated in the factory prior to shipping to you, and it has to be
> recalibrated every couple of years.

Marcus, you're way too helpful.
One could think you're Canadian :)

Eric, your question pops up a lot here and is thus on the gnuradio.org
FAQ:
http://gnuradio.org/redmine/projects/gnuradio/wiki/FAQ#How-do-I-know-the-exact-voltagepower-of-my-received-input-signal

The wiki gets a lot of flak for being crap, but there's actually a good
amount of useful information on there. Please check it out.

MB
-- 
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