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Re: [Discuss-gnuradio] how to get dBm value from usrp_spectrum_sense.py

From: Martin Braun
Subject: Re: [Discuss-gnuradio] how to get dBm value from usrp_spectrum_sense.py
Date: Thu, 17 Jun 2010 16:33:08 +0200
User-agent: Mutt/1.5.20 (2009-06-14)

On Thu, Jun 17, 2010 at 06:03:46PM +0800, lishan_wh wrote:
> I wonder how can I get the dBm value by using usrp_spectrum_sense.py.
> should I do it as follows?
> fft = gr.fft_vcc()
> c2mag = gr.complex_to_mag_squared()   (or without squared?)
> log = gr.nlog10_ff()
> stats = gr.bin_statistics_f()
> self.connect(self.u,s2v,fft,c2mag,log,stats)
> in main function, I defined 
> m = parse_msg(tb.msgq.delete_head())
> Is mean(m.data) the dBm value? If it isn't, how can I get the it?

Hi lishan_wh,

the Rx chain (before your flow graph) consists of a lot of analog stuff
(including amplifiers) and then your signal is converted to nothing but
numbers during the A/D conversion.
So, if you mag-square the incoming signal, you get some kind of power,
but differing from the absolute power by an unknown factor. You need to
calibrate your whole system before you can do actual power measurements.


Karlsruhe Institute of Technology (KIT)
Communications Engineering Lab (CEL)

Dipl.-Ing. Martin Braun
Research Associate

Kaiserstra├če 12
Building 05.01
76131 Karlsruhe

Phone: +49 721 608-3790
Fax: +49 721 608-6071

KIT -- University of the State of Baden-W├╝rttemberg and
National Laboratory of the Helmholtz Association

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