Direct quadrature modulation
, as shown in Figure 4, is a common form of direct conversion used in the transmit path.
FM signal generation is an all-analog process; wideband FM signals may be produced by quadrature modulation
followed by frequency multiplication
Telephone designers chose to remove quadrature modulation
and demodulation functions from the baseband ASICs and place them in the analog BF/IF chips to alleviate mixed-signal isolation issues and allow for lower power CMOS designs.