U.S. Pat. No. 488,255 proposes a system for transmitting a complex signal comprising a data transmission signal component and an AM broadcast signal component. The transmitters using the method of Kahn, in particular, that described by patent U.S. Pat. No. 488,255 for the transmission of an AM broadcast signal, comprise separate means of processing of the envelope and of the phase and a multiplier of the envelope signal and of the phase signal at the output.
In digital modulation, the phase channel is not as in AM a pure RF frequency but a phase modulated RF frequency. As the envelope signal and the phase signal are processed by different and independent means, the delay between these two signals, envelope and phase, must be accurately tailored to obtain an RF output signal of high quality.
A loop allows automatic elimination and dynamic adjustment of the delay between the envelope signal and the phase signal.
The technique generally used in loops is to compare in the time domain the initial baseband signal with the demodulated transmitted RF signal. To do this, the delay between the two signals, initial and demodulated transmitted, is evaluated accurately. Then, the envelope and phase parts, in digital mode, of the two signals, initial and demodulated transmitted, are compared to determine the delay between envelope and phase.