Re: [Discuss-gnuradio] USRP problem ? used for Digital beacon receiver‏

[Josh Blum]

> Did you mean the tune frequency in the basic Rx same with the frequency that i 
> can set in the usrp source block ?
>
> So if the same i set the frequency in the usrp source block(from my understand 
> is nco frequency ) equal 1.5Mhz
>
> and the tones of the FFT sink seem like the product of the multiplication of 
> the input signal(1.5Mhz) and nco signal(1.5Mhz)
>
> and showed only the input signal - nco signal ' s  frequency response and i 
> think it's correct.

Yes, if the tune frequency in the usrp source is 1.5 MHz, then the DDC 
will shift input by -1.5 MHz. Therefore, the tone from the signal 
generator should be at zero Hz (baseband). However, life is not perfect, 
and you will have a small offset which is the sinusoid you see in the 
scope sink.

> and
>   i have another question that is the signal that come from usrp source
> that show in the scope sink show the I and Q signal but if i connect the
>   usrp source with the FFT sink it's show the frequncy response only one
> graph So i'm not sure what was the FFT sink show(I and Q , just I or Q
> only , or I +Q )

The FFT is complex, and uses I and Q as complex numbers. Notice that the 
FFT plot is not symmetric.

The scope plots a real signal vs time, and therefore it uses I and Q as 
2 real channels.

-josh