Quadrature modulation/demodulation

Thread Starter


Joined Oct 31, 2011
Hi everybody,
I'm reading about digital modulation/demodulation (in particular on Lee-Messerschmitt's Digital Communications) and I have this picture (RX.jpeg attached). These three general structures are equivalent: so, at the end of each structure, I should have the same spectrum.
I tried to think what can happen to the spectrum of a received signal in the third case (analytic bandpass filter) and I tried to design some spectra shapes ( like they do @pag.13 of this article http://www.eumus.edu.uy/eme/cursos/d...re.Signals.pdf ) .

In the case with a complex filter I found those spectra pictured in the attached figure RXchain.jpg, but if I multiply the Q-path signal by j and then sum to the I-path signal (like they do in the paper of Lyons) I DON'T obtain the same signal that I have from the structure with two mixers+low pass filter.

So, here my questions:
1) First of all, why the spectrum of quadrature modulated signal have the two sidebands asymmetric? How do they create such signals?
(I have found this paper http://www.google.it/url?sa=t&rct=j&...AjJtkA&cad=rja where in Fig. 1(a) they represent some baseband spectra QPSK but I don't know if they are correct or related to my problem..)
2) Why the receiver chain with the complex filter doesn't give the same result as the other two structures?
How should I treat the transfer function of a complex filter? I need a 'visual' help, like Lyons do.

Thank you if you can help me or suggest me some book that can explain!