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The envelope distortion analysis was done using a modulation frequency of 1 kHz, but this time higher modulation frequencies were included. It has been well-established since at least 1984 that increased modulation frequency causes increased AM envelope distortion in a class E amplifier. This is because, when the output tuned circuit is adjusted below resonance, which it is for class E operation, the upper sideband has lower amplitude than the lower sideband, and the phase shift of the upper sideband is more negative than the phase shift of the lower sideband. Increasing the modulation frequency causes the difference between the amplitudes and phase shifts of the upper and lower sidebands to increase. The difference between the amplitude and phase shifts of the upper and lower sidebands is what causes envelope distortion, and higher modulation frequency causes greater difference between amplitude and phase shift of the sidebands, and therefore greater envelope distortion
