Thank you pancho_hideboo
I got your point.
I think in both cases of direct up-conversion and IF up-conversion the mixer operation is the same. However, in the direct up-conversion mixer we are interested in the whole up-converted signal. However, in the IF up-conversion case, we are interested in only one side band of the signal (either the upper side band or the lower side band).
This can be shown from the following mathematical relations:
The IF up-conversion
IF signal = m(t)*cos(w_IF*t)
The transmitted signal = IF signal *cos(wc*t) = m(t)*cos(w_IF*t)*cos(wc*t)
= 0.5*m(t)*(cos((wc+w_IF)t)+cos((wc-w_IF)t)))
which has a spectrum of --> 0.25*(M(w-(wc+w_IF))+M(w+(wc+w_IF))+M(w-(wc-w_IF))+M(w-(wc-w_IF)))
we are interested in the signal with spectrum (for example): 0.25*((M(w-(wc+w_IF))+M(w+(wc+w_IF))
or in time domain = 0.5*m(t)*cos((wc+w_IF))
The direct up-conversion
Baseband signal = m(t)
The transmitted signal = m(t)*cos(wc*t)
which has a spectrum of --> 0.5*M(w+wc)+0.5*M(w-wc)
However, we are interested in the hole signal which is =m(t)*cos(wc*t)
So,apparently there is a factor of 2.
Please, pancho_hideboo, I am confused could you explain it more to me.
Also, If the spectre is calculating the IF conversion case. How can I calculate the direct up-conversion case??
Thanks for your time inadvance