I was going through radio transmission, and looking at a handout from Analog Devices on Heterodyne structure. Here is layout from this handout.
What I understand that in "MOD" (refer the layout above) level the actual signal is mixed with the carrier frequency, so that my baseband signal is moved to carrier frequency on which we will do the transmission. In that case why we need two mixers after the "MOD" level to further upconvert the frequency.
Example: to transmit at 200 MHz, my carrier frequency would already be 200MHz inside the "MOD" stage. Then why need to further upconvert this frequency via these mixers.
I have thought on it a lot, but still could not figure a satisfactory answer.
Your guidance will be greatly appreciated.
During "MOD" stage, we do add the signal with some sort of sine function, say $$sin(2\pi f_ct)$$.
In that case is this $$f_c$$ the carrier frequency (200 MHz for my example), clearly it can't be, it has to be much much lower than that, and then we use the mixer to move it to 200MHz. Then during modulation stage, why do we call the $$f_c$$ as carrier frequency, aren't we suuposed to transmit the signal at this frequency??