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After calculating digital I and Q data from the baseband message signal, there are two methods of converting this data into an analog RF signal. The first method involves converting I and Q data into analog signals, then feeding them into a quadrature encoder. There, they control the amplitudes of two oscillators, operating 90 degrees out of phase. The output of these oscillators is summed, resulting in an RF signal with the appropriate amplitude, phase, and frequency.
The next method of converting digital I and Q data to analog RF performs the oscillator scaling and summing in the digital domain. That is, digital sinusoids with a phase difference of 90 degrees are scaled by the digital I and Q values, then added together. These digital sinusoids are of a lower frequency than the analog oscillators in the IQ modulation scheme, but still at a significantly higher frequency than the message signal. A digital to analog converter (DAC), which operates at a much higher frequency than the DAC used in IQ modulation, converts the resulting digital waveform to low frequency analog RF. Finally, an analog IF to RF upconverter uses several stages of mixing and filtering to shift the analog RF signal to the desired RF frequency.
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