On 7/28/2021 1:12 PM, Kai-KE4PT via groups.io wrote:

Of course an FM signal may exhibit side bands in the frequency domain [think of a square wave in the frequency domain]! However we are talking here about the time domain,  [think of a square wave in the time domain] were the signal is constant envelope.

Time and frequency are the inverse of each other, and the two domains are simply "different ways of looking" at the same phenomena. Those of us working in pro audio in the last decades of the 20th century benefited greatly from the mentoring of the late Richard Heyser. Dick's day gig was at JPL, where he worked on communications, but at the time of his death, he was President-Elect of the Audio Engineering Society. Among his patents was Time Delay Spectrometry, which revolutionized Pro Audio. A study of his work is quite worthwhile.

More important, QRM is seen and heard in the frequency domain. Key clicks and splatter are intermod products resulting from amplitude non-linearity in the RF chain (remember that what we call CW is 100% modulation of a carrier by a rectangular waveform).

73, Jim K9YC