The statistical properties of the logarithm of the amplitude ratio of neighboring information symbols coded by the two-level APSK and DAPSK signals, widely used in various discrete information transmission systems, are investigated. It is shown that the probabilistic properties of this magnitude do not depend on the absolute signal and noise levels, but are determined by the signal-to-noise ratio only. Expressions for the probability densities of the logarithm of the amplitude ratio for different levels of neighboring symbols are determined, and the method for their calculation is developed. A statistical simulation of the operation of the APSK signal demodulator confirms the calculated relations. The research results can be used to estimate noise immunity and measure the signal-to-noise ratio in the working communication channels through which two-level APSK signals propagate, without interfering with the transmitted signals. In addition, they can be used in the development of DAPSK signal demodulators. #CSOC1120.