Re: [Discuss-gnuradio] Re: wrong dB

[Stefan Brüns]

On Tuesday 03 March 2009 18:16:36 Markus Feldmann wrote:
> Martin Braun schrieb:
> > 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.
>
> However there should be a reference point ?
> So if you are right, which is the reference number ?
>  From this to my Number should be a difference of 60dB.

The reference point is 0dbFS - FS meaning Full Scale, -32768...32767. The 
signal power you need to get this readings from the ADC, or more exactly from 
the FPGA depends on the daughterboard, PGA setting on the daughterboard, 
decimation ...

You have to calibrate your USRP to get from the sampling values to input power 
in Watts.

> > 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.
>
> As i know at this moment and as my results says that is not correct.
> You need a Throttle, otherwise your application
> is busy. Maybe you can substitute the throttle, but thats it.
> The USRP transmits as much data as it can to your computer.

The USRP outputs 64MS per second, divided by the decimation value, so it is 
rate limited. Maybe your computer is not able to keep up with that, so you 
have the _feeling_ the USRP is not rate limited, but thats just wrong.

Stefan

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