I've seen the term "tone" used a lot in RF literature, but haven't found any helpful explanation of what exactly a tone is. I am looking at tone in context of IMD; i.e. the fundamental input tones. What exactly is a "tone" in this context?

Tone, in this context, typically refers to a sinusoidal signal at a single frequency. For example, if I'm doing a "two-tone" power measurement to check for intermodulation distortion ("IP3 measurements"), then I'll be supplying to my DUT two CW sine waves, one at \$f_a\$ and another at \$f_b\$. A "one-tone" measurement, by contrast, would be a measurement where I'm supplying only a single sine wave, say, at \$f_a\$ only.

In general a "tone" is a single pure frequency component. That is a fundamental or any one of its individual harmonics, or the harmonics of your carrier frequency.

Some circuits (for example ∑∆ converters) can introduce their own oscillations in the signal chain, that is "tones" that are related to the sampling frequency (not harmonics, as these would be lower in frequency).

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