Roy Lewallen wrote:
It's not clear to me whether you're proposing an alternative definition
for linearity or for superposition.

Lest anyone think that the addition of an offset term (the b in y=mx+b)
violating superposition is merely of academic interest, analyze a
direct conversion receiver. At my shack I have a Softrock 40, which
tunes about thirty KHz of the 40 meter band. At the center of the
spectrum (which corresponds to the mixer converting to very low
frequencies and DC) is a 1/f bump that the designers have attributed to
mixer 1/f noise. But it is not noise at all. When tuned in, it is
full of strong signals and strange sounds. These are distortion
products, and are caused mainly by the b term in the detector used on
the receiver.

They wipe out about 3% of the receivable band.

Sometimes the b term is benign, but not in this case.

73,
Glenn Dixon AC7ZN