This concise paper analyzes the transfer of amplitude modulation from one sinusoidal carrier to phase modulation of other carriers passing through the same memoryless nonlinear device. It gives exact expressions for the AM-PM transfer coefficient, which is one part of the mechanism that leads to intelligible crosstalk, and the expressions are valid for sinusoidal carriers in the presence of Gaussian noise through devices with known amplitude and phase characteristic. Two particular input signal combinations (two carriers with a given level difference, and a large number of small carriers) are considered in more detail, and the expressions are applied to a typical TWT characteristic.

The intermodulation analysis presented in this paper is based on the important observation that any output component of a memoryless nonlinear device can be expressed as an integral of the product of two functions, one being the single carrier amplitude characteristic of the nonlinear device and the other being a function of the statistical parameters of the input signal. The analysis can also be extended to nonlinear devices with AM-PM conversion by expressing the amplitude characteristic as a complex function. The signal-dependent function is given analytically for two simple but important types of signals, for two sinusoidal carriers of equal level, and for Gaussian noise, and it is demonstrated that good agreement has been obtained when the method has been applied to microwave devices like high powerX-band klystrons and traveling wave tubes. It is also shown that the method is convenient for analysis of intermodulation in cascaded nonlinear elements for which the individual single carrier amplitude characteristics are known.