Figure of merit and limit of short-term stability in passive hydrogen maser

Frequency stability of passive frequency standard is associated with the figure of merit of a quantum discriminator. The method of the discriminator figure of merit measuring and the lowest achievable short-term frequency stability in the passive hydrogen maser are presented. Results of theoretical and experimental research are compared. The result of investigation shows that with increase in a excitation signal power, spectral line shape can be considered as the sum of several lines.