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,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.
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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.
-- Marcus Leech Principal Investigator Shirleys Bay Radio Astronomy Consortium http://www.sbrac.org