Evaluation of Influence of the Radio Receiving Devices Heterodynes Functioning Quality on the Quadrature Amplitude Modulation Digital Signals Reception Noise Immunity

In digital transmission lines with quadrature amplitude modulation of signals the information is contained in the change in the amplitude and phase transmission of the signal. Therefore, in comparison with other types of modulation, there are more stringent requirements for the level of phase noise of the signal at the input of the QAM-signal demodulator, leading to a decrease in the reliability of reception. This article will consider the requirements for short-term frequency stability of synthesizers of transmission lines, which use signals of multi-position quadrature-amplitude modulation in conjunction with quasi-coherent reception of these signals. The carrier is extracted on the receiving side directly from the received QAM-signal using a carrier recovery device by demodulating the received signal and then its subsequent narrow-band filtration. As a narrow-band tunable filter, an auto-tuning frequency switching system is used. With the improvement in the filtering quality of the demodulated signal, the phase fluctuations of the restored carrier associated with the action of additive noise are reduced, the energy losses of the applied quasi-coherent reception are reduced in comparison with the ideal one, which requires the complete absence of the phase error of the selected reference signal. The approximation of the noise immunity of real coherent QAM-signal demodulators to the theoretical one limits the phase noise of the signal at the input of the demodulator associated with the imperfection of the frequency synthesizers on the transmitting and receiving sides of digital communication lines.