Re: [Discuss-gnuradio] wrong dB

[Martin Braun]

On Tue, Mar 03, 2009 at 09:49:02AM +0000, feldmaus wrote:
> i think i have a misunderstanding, but i don't know where it is.

Here it is:

> I tried out an example named <usrp_am_mw_rcv.py> with
> a Hardware Signal Generator connected to my USRP.
> My Signal Generator generates a sin Signal with 7.0711e-3V effective
> at f=1.05e6 Hz.
> 
> So my power is,
> P = U*I = U^2/R, with I = U/R
> P = (7.0711e-3_V)^2/50_Ohm
> P = 1e-6 W
> 
> Now we convert this into dB,
> P_dB = 10 * log10 (P/P_0), with P_0 = 1W
> p_dB = -60_dB

Here, you're comparing with 1W. You could have randomly chosen something
else, such as 1mW, yielding yet another relation.

> But my measured Power is -11,933_dB so what is my Problem ?
> Any Hints ?

Here, you're comparing with something else. There's a lot of stuff
between the coax connector and your software, so you can't trivially
translate sample values into Volts. Calibrating a USRP/GNU Radio setup
to actually measure physical units is not easy. That's why it uses 'dB'
and not 'dBW' or 'dBm', it's merely a relation.

Cheers,
MB

-- 
Dipl.-Ing. Martin Braun           Phone: +49-(0)721-608 3790
Institut fuer Nachrichtentechnik  Fax:   +49-(0)721-608 6071
Universitaet Karlsruhe (TH)       http://www.int.uni-karlsruhe.de/

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