Forum: GNU Radio Re: sending single tone Am modulated using usrp1+wbx

on 2013-01-08 04:04
```I will write my theory about usrp working and some math in Am
modulation,
because I want to be sure before continuing:

If I want to modulate in DSB amplitude a carrier of saying 1GHz ,with a
m(t)
signal (in this case a simple 1kHz cosine tone) the carrier is sourcing
by
USRP module, and I have to set up the freq of usrp in 1GHz and based on
Leon
Couch Book the complex envelope for DSB AM should   be I=Ac(1+m(t))
Q=0.
(In my case Ac=1).

This is equivalent to  (1+m(t))(e^0)      eq.1

I and Q are a complex signal that I connect to the usrp sink.

Internally the usrp  multiplies   eq.1 by e^(2*pi*1GHz*t) resulting
(1+m(t))e^(2*pi*1GHz*t)     eq.2

Eq2. travels by the air to the receiver.

In the USRP receiver, Eq. 2 is multiplied by  e^-(2*pi*1GHz*t) resulting
(1+m(t))(e^0)  whose I and Q components are   1+m(t)   and 0
respectively ,
and I can obtain the magnitude with a complex to magnitude block, this
magnitude is 1+m(t).

This my theory and I want to be sure if this is correct.

Thanks.```
on 2013-01-08 05:03
Attachment: complex_am_xmit.grc.png (79,3 KB)
```>
> to magnitude block, this magnitude is 1+m(t).
>
> This my"theory"and I want to be sure if this is correct.......
>
> Thanks.
>
>
> _______________________________________________
So, here's a more complicated example, that factors in additive noise,
which is the actual, realistic scenario.  The Q term is never actually
really zero,
only "conceptually" zero.  The real world is *never* as neat and tidy
as your simulations.  Also this has a -1.5kHz frequency offset, which is
also
realistic.

complex-am```