Owen Duffy wrote:


The most widely accepted test for linearity (Vout/Vin) of an RF PA is the
'two tone test', where the drive is a complex waveform (the sum of two
equal amplitude sine waves quite close in frequency) and at least some of
the distortion products due to third order and fifth order etc transfer
terms appears in-band in the output after all output filtering, and where
they can be reliably compared in amplitude to the desired signals. A
Class C RF PA will not appear to be linear under such a test at any drive
level.

Actually, in modern systems with very complex signals, there are more
meaningful tests like noise power ratio with a notch that look for
spectral regrowth. The two tone test has the advantage of being
moderately easy to perform for middling performance amplifiers/devices.
But if you're looking for very high performance, such things as
generating the two tones without one generator interfering with the
other get to be challenging.



I suspect that the issue of transfer linearity is a red herring to your
proposition about the Thevenin equivalent of an RF PA, but if you do
depend on arguing that the transfer characteristic of a Class C RF PA is
linear, I think you are on shaky ground.

I don't know that the concept of a Thevenin equivalent (a linear circuit
theory concept) really has applicability to "box level" models, except
over a very restricted range, where one can wave one's hands and ignore
the nonlinearities as irrelevant to the question at issue. Sure, over a
restricted dynamic range and bandwidth and restricted class of input
signals, a Class C (or class E or Class F or E/F1, or a fancy EER
system) can be adequately modeled as a linear ideal amplifier.


The real question is what is the value of that model. If the model
provides conceptual understanding of some underlying problem, great. For
instance, it might help with a link budget. If the model helps design a
better amplifier, great. The model might allow prediction of behavior;
so that you can, for instance, detect a fault by the difference between
model and actual observation, as Richard mentioned with the harmonic
energy detector.




Owen