Bill Turner wrote:

Correct of course, but as I understand it, the only complication is
that there are harmonics present. It is still a case of add and
subtract, isn't it?

The original question only mentioned the *four* frequencies present in
the output, ignoring the harmonics.

Unless you can show me otherwise, I stand by my original observation.


The math does show otherwise.

When we talk about "square-law" and "third-order", we're actually buying
into a whole package deal of math-based concepts. Logically, the deal is
that we can't use those words *meaningfully* unless we also accept what
the math tells us, namely:

1. Each order of distortion is independent of all the other orders. It
generates its own individual package of output frequencies.

2. Frequencies that are in the same-order package *must* all be
generated together (you can't have one of them without having all the
others too).

3. Frequencies that are in different-order packages are totally separate
and unconnected.


2f1 and 2f2 are part of the package of 2nd-order products, along with
(f1 + f1) and (f1 - f2)... there are four 2nd-order output frequencies,
no more and no less.

3f1, 3f2, (2f1 + f2), (2f1 - f2), (f1 + 2f1) and (f2 - 2f1) are all part
of the 3rd-order package... there are six 3rd-order output frequencies,
no more and no less.

A perfect square-law mixer produces only 2nd-order products. 2f1 and 2f2
are present at the output, but they do not "go round again" and mix with
the input signals to produce (2f1 + f2) etc. Those 3rd-order products
arise *entirely and exclusively* from 3rd-order distortion.

That conclusion follows by strict, non-negotiable mathematical logic
from the fundamental definition of what "order of distortion" means.



As others have said, from the practical engineering point of view, the
way you envision mixing products being produced is "purely academic".
But that "purely academic" debate is exactly what we're involved in
here... so here, it makes all the difference in the world.




--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek