[Amps] A tale of two IMs What happens?

[Dr. David Kirkby]

Roger wrote:

> My background in this topic is a bit weak, but as I see it:
> All we can deal with are analogies as the real world is dynamic and any 
> testing such as the two tone is a bit outdated. OTOH how would we 
> simulate the equivalent real world  where the signal is voice  which is 
> almost random and contains many  frequencies at once that are constantly 
> changing along with changes in the voltages on the tube?  I'd think even 
> small changes would show up as some form of modulation, superimposed on 
> the intended signal which would be a source for  additional  IM.   CATV 
> is at least a signal containing dynamic components and the results are 
> based on real world conditions albeit a bit different than the ones we 
> are working with.
> 
> 73
> 
> Roger (K8RI)

I believe analytical techniques could be used to put absolute limits on this. 
Perhaps if I've got a bit of time, and inclination, I'll have a go at this. It 
can't be rocket science, though my maths is not that great.



As for what might be typical with voice, Monte Carlo numerical methods are 
probably suitable for this. There are tons of models of the human voice. Or one 
could record a QSO and feed that data into a Monte Carlo model, to see what are 
typical values. It's probably easier than making real-time measurements, though 
that might be possible with a DSP to digitize the output of an amp.

I think an analytical model is probably doable without too much effort, and 
would allow one to put absolute limits on the values, which so far nobody has 
produce what I personally consider any convincing evidence to prove.

I think the problem with the CATV analogy is that the aim of the amp is not to 
increase the signal, but to overcome losses. So there I could imagine things get 
much worst, as the IM products get produced at each stage, but the signal never 
gets any bigger, so the IMD would deteriorate rapidly as more amps are added.


Dave