need to know about upconverters

Hi all
I need to know about digital upconverters ( both base band to RF and IF to RF upconversion) . I want to know about the principles of it's operation and the effects of this part (in a digital transmitter) on the other parts , and also the effects of the other parts ( e.g. power amplifier ) on upconverter. I study about DVB-T transmitters and I focused on this part.

Why should we convert it from BB to IF and then to RF , Why not directly from BB to RF.in the second case we can have fully digital upconverters in the configuration of the trasmitter and it would be more reliable ( by using digital filters ) and may be more economical! . but there is another question , why most of the DVB_t transmitter manufacturers use another kind of configuration ( BB to IF and then If to RF) ? Is it because of technological constraints or something else?

as use see I have lots of questions in this area and I'm looking for a kind gentle person to get understood.

If you are one , just let me know to leave you no time to scratch your head

have a nice time
Ramin