Direct I/Q modulation or upconversion?

[sophiante]

Semiconductor manufacturers sometimes suggests circuit arhcitecture for their IC's. In the case of I-Q modulators one can see two approaches:

1. Direct conversion where the quadrature IC modulates a VCO on the output frequency.
2. An intermidiate frequency VCO in the range 50-250MHz is modulated and then upconverted to output frequency.

Number 1 is obviously much simpler (less filtering) - that is why I suspect it to have some serious drawbacks!

Is there a short answer to why these two approaches both are used?

Jan

 

[mahdithdn]

First approach is direct but with some problems. it's main problem is that filter designing in RF is complicated (e.g. 14GHz in satellite Ku band communication) because you can't reach high Q factor.
Second approach is a little complicated (two level approach) but with many advantages, First of all, is standardization. IF standard frequency are 70MHz, 140MHz, .... so a manufacturers can design a generic Upconverter, Modulator, Filter, etc. Second you can reach higher Q factor than RF.
regards