a two - channel rf digital transmitter . the digital rf transmitter comprises a first and second digital modulator for receiving and modulating a first and second digital base - band signal from a first and second channel , a first and second local oscillator for generating a first and second digital carrier signal , a first and second mixer for receiving the first and second digital base - band and carrier signal and implementing multiplication of the first and second digital base - band and carrier signal to generate a first and second transmission signal , a first and second filter for band - pass filtering the first and second transmission signal respectively , and an adder for summing the filtered first and second transmission signal to generate a summation signal .