5. Digital modulation strategy

For a long time, broadcast transmitters were amplitude modulated by controlling the supply voltage to the final stage (figure 19 ). This classic modulation method, mainly used in tube equipment under the name of anode control modulation, has very good linearity and stability, but limited efficiency.

Because of the class C of the RF final stage, η ₂ = 0.9 is usually achieved. But the efficiency η ₁ is due to the class B of the AF stages limited to just 0.5.

The typical efficiency of all amplification stages is given by : ${\eta }_{\text{g}}=\frac{3{\eta }_1{\eta }_2}{2{\eta }_1+1}$