# RE: USRP pr oblem ? us ed for Dig ital beaco n receiver â€

Thank you very much Josh

but i have another question for more please

What frequency did you tune the basic RX? Perhaps you are seeing an
offset between signal generator and DDC. Do the tones the FFT sink look
to be in the correct positions?

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.

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 )

thank you very much for yours advice

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

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