Calculation of nonlinear amplifier third order intermodulation products of two unequal angle modulated carriers by Shimbo method

The effect of amplitude nonlinearity and AM-PM conversion as inherent characteristic of a nonlinear amplifier as found in a satellite travelling wave transponder results in intermodulation products at the amplifier, notably third order IM products. The calculation of intermodulation from two angle modulated carriers using Bessel expansion as proposed by Shimbo is presented. The calculation results shows that for a typical nonlinearity, even for 10 dB input backoff for each carrier, the carrrier-to-IM ratio is still smaller than 20 dB. The use of Shimbo method is also applicable in the case of unequal carriers.