Re: [Discuss-gnuradio] Re: wrong dB

[Martin Braun]

On Tue, Mar 03, 2009 at 02:52:44PM +0000, feldmaus wrote:
> Martin Braun <braun <at> int.uni-karlsruhe.de> writes:
> > 
> > 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.
> > 
> You are right. There is many stuff between USRP and my Generator, but
> the difference between -11.933dB(W or m) to -60dB is very big.
> So there should be a misunderstanding or big problem ?

Don't confuse dB and dBW. You want to see -60dBW, but you see -11.9dB.
That's because your justing looking at numbers, not voltages.

> I also tried it on my own python program with the following
> modules:
> [USRP Source]-->[Compl_To_Mag]-->[Throttle]-->[FFT-Sink]
> 
> Is that ok to analyze the magnitude and freq of my sin signal ?

Only their relations, i.e. 'there is twice as much energy around f1 than
there is around f2'. Take a look at the FFT sink code and you'll find
averaging etc. GNU Radio does not know the precise antenna voltage.
Besides, there is a FFT sink for complex signals and you won't need a
throttle here, as the sampling rate is limited by the USRP.

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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