In the SMPTE IM (Intermodulation) distortion test using 60 Hz and 7000 Hz signals, it is normally assumed that the IM distortion products form amplitude modulation (AM) sidebands, which are commonly measured with an envelope AM detector. However, with other IM test signals the output of the AM detector is minimized or close to zero, while instead producing FM (Frequency Modulation) distortion. This paper investigates testing for phase and frequency modulation from cross-modulation and intermodulation distortions in audio amplifiers. The cross modulation test signal includes a 3 kHz tone and a high frequency amplitude modulation signal. Alternatively the test signal may include the 3 kHz signal and two high frequency tones. Amplifiers with feedback are tested.
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