  In this paper analysis is given to find the third order intermodulation power
given sinusoids are fed into a nonlinear device. A simple expression of the
third order intermodulation power is given for the case that the center
frequencies of the input sinusoids are equally spaced. Further, if the powers
of the signals are equal, the expression becomes a closed form expression. The
analysis will be helpful for communication system engineering in estimating the
adjacent channel interference due to nonlinearity. Numerical results are
presented for various values of (number of input channels). Though the analysis
assumes the input signals to be sinusoids without phase modulation, the third
order intermodulation power estimate serves as a good estimate for link budget
computation purpose. For the case that the center frequencies of the input
sinusoids are not spaced equally, the analysis can still highly likely be
applied if we insert pseudo channels in between the real channels so that all
(real and pseudo) channels are spaced equally (or approximately equally for
approximation). In this case, the pseudo channel powers are set to zero so that
the interference powers due to the pseudo channels will not be included in the
analysis. In other words, the analysis is highly likely applicable without the
constraint of the input channel center frequencies being equally likely.
Simulations are also provided for the case that the input sinusoids are QPSK
modulated.
